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1. Computer Science E 1 Understanding Computers and the Internet Harvard Extension School So let s see what the implication is I m going to open Move2 scratch And that again is in the Lecture 11 directory which is downloadable from the URL that s on all of the slides Move2 scratch is here I ll leave the program alone But just appreciate if you would the redundancy in the bird s script And now notice the feature I ve just sort of revealed by clicking around Right now the bird is highlighted which means I see the bird s scripts or in this case script If I click the cat I see the cat s script And so here we have the notion of two different threads and two different sprites But it s really no different from before I just have multiple sprites And you can create new sprites simply by clicking the appropriate icons here Like the cat icon click it you ll get a new cat which is the default costume for a sprite And then you can change the costume by playing around for instance up here Again a lot of the program is designed to be very intuitive And we re only going to give you a sense of some of the features because part of the fun in it is sort of exploring exactly what can you do and how can do it But it s the programmatic constructs that tonight are the most compelling for us Let s go ahead and run this I ll make it full screen Click the green flag Notice the bird started in the left of the screen Now he s bou
2. That s the interesting part right It s clearly this way in the phonebook But I get to now tear the phonebook in half yet again So in just two steps what fraction is this remaining problem of the original problem It s a quarter and that s a powerful thing Because think about the original algorithm in the original phonebook After just two steps how many pages into the phonebook would I be STUDENT inaudible response DAVID MALAN Yeah on page 2 page 3 whatever But still that leaves pretty much the whole damn phonebook And this is a powerful thing And what this algorithm suggests is something that s called a logarithmic running time If you think back to algebra you might have recalled logarithmic graphs which rather than looking like this instead look something more like this where they very very very gradually grow eventually Whereas the linear plot keeps on going up 00 10 00 And remember if this is the number of pages on our x axis and this is the amount of time 4 that the whole algorithm is taking which one in the long run is clearly the superior the more efficient the more intelligent algorithm Well if your metric for success is speed presumably the one that s logarithmic And you might say that that s a log graph and this is a linear just to slap the labels on it Well consider the power of this really If we have a thousand page phonebook and we use a linear search algorithm botrrowing term
3. Go o o0 a a I right much to the chagrin perhaps of his roommates So you can do some neat things And we re only scratching the surface of some of the features But it really is easy to get into Finally you have the so called Stage at top right This is where your sprites actually do something and can interact with each other can move around When I clicked the easel before all that did was make the Stage full screen just because it s a little more fun to see your program fill the screen But the dimensions of these rectangles are exactly the same Finally you have down here the Stage If you recall in my Oscartime game there was always that Sesame Street lamppost on the screen That wasn t a sprite That was just the Stage like a wallpaper behind the scenes So that s what you can use the Stage for to just lay out a graphic for instance So how do we get started Well I m actually going to show you this first Hellol scratch is just a program What is a program It s one or mote scripts Well because we ve opened Hello1 scratch we see in our Scripts area the script called Hello1 scratch Notice that this sprite is highlighted down here So that means that that script or those scripts if there are many belong to this sprite And this is how you ll see you can make different sprites do different things In a moment we ll have an example of a cat chasing a bird two sprites with their own scripts so that they operate
4. Other sites might simply have been using some off the shelf software Apache Microsoft IAS Internet Authentication Service and they just turned on the Cookie option which means by default every Web page you visit is being sent along with a cookie They re just not using it It s just the default setting which doesn t mean it s good or bad It just happens to be there The short answer is it depends But if you go through your Windows folder in C Documents and Settings STUDENT inaudible response DAVID MALAN You ll see dozens of cookies from almost every Website you visited And most of those sites are probably not using them again in a way that you would notice Though it s becoming increasingly common it s not really something to worry about In years past there have been more bugs in browsers that have resulted in cookies as we said a few weeks ago being a potential security hole And theoretically they still are It s not something I would lose sleep over There are so many more important threats viruses worms spywate that pose a much more imminent threat Cookies I think are far more useful than they are dangerous Though I might for instance turn off the third party cookies as I demo d in class one day just because But that s about it 16 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School All right so how can we now begin to approach programming a bit m
5. independently And you ll see what occurs This program here begins with the when green flag clicked puzzle piece And as I said before to start a program you click the green button or in this case the green flag and that starts your program So the means via which you get a program to start running is you start your script with that when green flag clicked icon Anything below that gets executed subsequently This block here say hello world is what s called a statement When we had Dawne down here doing her socks example and we said put on sock open sock drawer those were statements They re not conditions they re not loops they re not variables they re statements Do something So most of these puzzle pieces such as say something is a statement and it s telling the sprite in this case to do something to state something Well let s go ahead and run this program You re about to be incredibly underwhelmed by our first program I m programming already right So not too bad Well let s see if we can t build on this and do some more interesting things Well here s just a snapshot of some of the statements Scratch supports 35 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School Note on screen slide 12 You should be able very quickly to infer from what is written on a puzzle piece if it s a statement if it s a condition if it s a loop Because if
6. But it s essentially somewhat to the left and somewhat down as we ll see Then it points it in the direction 45 degrees So it s angled in a certain direction And then it forever does this if touching the edge bounce Okay And I realize this is a bit of a bug since I wasn t thinking when I did this It s sort of redundant to say if touching edge and then if on edge bounce So we ll fix this in a moment But that s simply not necessary Scratch now supports this new blue block that we described earlier if on edge bounce But if not touching cat move 3 steps Well let s see what that really means Let s turn our attention to the cat s script for a moment The cat is going to start notice in a different location but still kind of close 160 160 this time It s going to point in what direction A random direction from 91 to 179 which I just sort of chose arbitrarily so that it would start at an angle rather than going straight up or straight across And then it forever does this if it s touching the bird play the roar sound and then stop the script You re all done Otherwise point towards the bird and move 1 step Given this definition which of these sprites cat or bird is moving slower STUDENT inaudible response DAVID MALAN All right so the cat seems to be moving slower because he only takes one step each iteration whereas the bird seems to take three steps 46 of 53
7. Note on screen slide 11 So the first file and these are files you can play with if you download them for the project is one called Hello1 scratch So in each of these examples is the name of the file that implements the program you see on the screen To download Scratch as your own Problem Set 8 shows you can access it via this URL which isn t public per se but the Media Lab has been kind enough to offer you E 1 and you the E 1 podcast subscribers access to the software via this link And by all means I would encourage you those of you with kids in particular if you want to have your kids for once help you with your homework if you haven t already this might be the assignment to do so because they too might get a kick out of it 33 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School So how do we use Scratch Well I load the software by double clicking the Scratch exe or whatever icon on my desktop And the problem set tells you how to get to that point Just going to be a folder say on your desktop Mac or PC is supported even though I m using a Mac No I m using a PC What you do to open a project like that is to simply click the Open option at top If you haven t made any changes to the program you just say no save no changes And then you ll get the default folder that Scratch comes with Scratch comes with a whole bunch of projects that MIT teaching fe
8. Stage which is like our wallpaper in this case And again some of the details here like how do I put a wallpaper on the stage we ll leave to you to explore to figure out a la our Google Earth example Or again next week s Sections on Wednesday and Saturday will introduce Scratch in a hands on sense with you in front of computers Let s make this full screen Click the green flag And this game if you read through the soutce happens to be played such that with the arrow keys I can move left and right And then if I hit forward I can start punching myself And the more I punch myself I change colors getting greener sicker and sicker 49 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School Notice that I m saying things Turning even greener a little blue now I can move over Hit him with the left Hit him with the right The left He s moving fast So there s clearly some randomization tight There s some picking of random numbers Almost So this is a little Internet speak At Dan s request I got a little hipper with this example And it says It s not a typo I got How do we pronounce Pwned I got pwned which P on your keyboard is very close to the letter O So in Internet speak saying pwned is sort of the cool way of saying I got owned which is like saying I got beat up or beaten or knocked out in this case So hopefully now I m all that coo
9. brute force attack Because yeah someone can fifteen minutes later try 15 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School another ten passwords but they can t try ten passwords per second And you ve pretty much choked off an attack like that So what would happen with something like Amazon Odds are unless you really put some time into it and were clever about how you were banging on a Website probably the traffic would not be noticed Unless you tried logging in or doing something that is fundamentally related to security odds are the site would break in some way or it would just behave and give you an error message which isn t necessarily breaking but it s Amazon s way of just saying Eh something went wrong It s not a big deal Here s an error message STUDENT inaudible response DAVID MALAN How much do you have to know to use cookies on a Website STUDENT inaudible response DAVID MALAN Mm hm STUDENT inaudible response DAVID MALAN Yes so to summarize Many Websites out of the box these days just by default use cookies in some way They put cookies on your computer but they don t seem to use them in a way that you would notice That is for a couple of reasons They might very well be using cookies but in a way that you don t notice And if you have cookies turned off they are smart enough to figure out how to still interact with you in some way
10. code it should make more sense simply has our cat counting from one up to infinity 01 30 10 Now not terribly interesting in this context but I clearly was using a counter in Oscartime for what feature STUDENT inaudible response DAVID MALAN Counting the score Every time a piece of trash was successfully dropped in I d it I changed counter by one What about the example at right Hello9 scratch What does this do now STUDENT inaudible response DAVID MALAN Excellent And so that s the value add here Essentially this is the same example as before where I picked a random number and meowed only with 50 percent probability This program by using the variable and storing the random number in that variable called Wom number in this case could have called it x I called it number instead it says the number 43 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School just so that we can get some reinforcement as to why the cat meowed or didn t It s just showing us the number So in Hello9 scratch notice that we have this right here Hello9 scratch I ll hit Play Six is bigger than five The next one is oh and this doesn t even have a loop So it looks like I have to play this one again and again Let s hit it again Nine is not less than six cat sprite meows There we go One through five was chosen cat sprite meows One through f
11. green flag at top left And if we want to stop it we click the red light instead So with that said I m going to hit the green flag And as I recall Dawne will simply use the Up Down keys to move the soccer player up and down on the field And I think you can hit Left or Right when you want to kick the ball And meanwhile the goal s going to be moving Soccer game vendor Hot dogs hot dogs beer hot dogs soccer game audience cheers DAVID MALAN So what happens if ah let me see It s not the keys Oh there you go Click the guy So just click this in here DAWNE Oh DAVID MALAN Yeah So you can t Turns out you can t move up and down but you can So the cursor s on the guy So just click the button when you want to shoot And the idea is The goal moves The guy stays in place Oh and the keys don t do anything I completely set that up wrong Right yeah there you go Okay Soccer game vendor Hot dogs hot dogs beer hot dogs 29 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School soccer game audience cheers DAVID MALAN Yeah okay So just click when you re ready to kick No no keys no keys Just this button DAWNE Oh DAVID MALAN Just that button Sorry Game s even simpler than I thought You just hit one button Soccer game vendor Hot dogs hot dogs beer hot dogs soccer game audience cheers Soccer game announcer Go o o a a l Soc
12. it s a loop you ll see something like the word while or forever something that intuitively is a loop And that s the beauty of Scratch It s not arcane commands It s puzzle pieces shaped logically to fit where they should and also label as you might expect intuitively So say Hello is one statement wait 1 second is another statement play sound meow might be another statement How can we incorporate such things as these Well here are two new examples hello2 scratch and hello3 scratch written in my sort of shorthand notation up there Note on screen slide 13 This is referring to two different files left and right hello2 scratch and hello3 scratch Take a wild guess What is the script on the left going to do once we execute it STUDENT inaudible response DAVID MALAN Say hello for one second or really say hello three times pausing for one second in between Well let s actually build this I could just open hello2 scratch which you can do at home if you d like But let s just reconstruct this Well I m going to move over here I m going to say New And it s just going to give me an empty everything All right so now I need to go to let s see Control which is where all the major Control structures are And to make a program let s click and drag all right when flag clicked We could drop it anywhere but sort of in terms of neatness let s start at the top left and work our way down The ne
13. probably flip closer to the back because I ve just opened to the L section which is roughly in the middle And you would expect that 26 letters we re roughly in the middle So this is the Lamps page So I clearly need to move to the right Well let s perhaps apply that same approach and dive into the middle of this right hand side But consider for a moment what I m about to do You told me to open to the middle of this book Because I ve opened to Ls and I m looking for R what can you tell me about this chunk of pages in my left hand STUDENT inaudible response DAVID MALAN Forget it And let s literally be dramatic here Forget it I ve just chipped away at this problem such that I ve thrown half of it away because my answer is clearly not in there if I m looking for Rs Well if I apply this same logic here s the L on my left and the Zs are presumably on the right Let s dive in roughly in the middle All right we re trying to do this Well that s great Restaurants And we re done So that s okay So now let s say I m on Restaurants and I am currently on I m going to cheat slightly because we re in the advertising section not the main listings and I am at the restaurants that begin with the letter B currently So where do I need to go next 4 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School STUDENT inaudible response DAVID MALAN All right so I can again
14. problem albeit with a computer is can I do better than that can I do fundamentally better right It s one thing to just say you know what Spend an extra few hundred dollars and throw a 2 GHz CPU at it instead of a 1 GHz CPU But that s not really an interesting solution Doesn t require any intellect and doesn t fundamentally change the na vet or the brilliance of your algorithm All you re doing is throwing hardware at it And that s fine but there s really no intelligence There s nothing interesting about that approach 3 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School So I put forth to you now the question if I have this phonebook in the real world and I wanted to solve that same problem the looking up of a phone number fundamentally more efficiently could I do better than starting at the left and working my way all the way through to the right until I find the listing I m looking for STUDENT inaudible response DAVID MALAN I could do what STUDENT inaudible response DAVID MALAN All right so I could start in the middle And frankly if you think about it if you ve even used the Yellow Pages in recent years what with the Web and all you would probably start roughly in the middle And then what would you do looking for instance for restaurants STUDENT inaudible response DAVID MALAN Sorry STUDENT inaudible response DAVID MALAN So
15. right I m sliding it under the baby All right And STUDENT inaudible response DAVID MALAN Pull the straps STUDENT inaudible response DAVID MALAN Pull the bottom tab up around the top STUDENT inaudible response DAVID MALAN On both sides Oh good and STUDENT inaudible response 13 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School DAVID MALAN Gently dress the baby All right I think that deserves a round of applause for someone All right students applaud All right so excellent So the lesson hopefully eventually came across that even when we try to tell another human how to do something or what to do even we make assumptions That s perfectly reasonable in a society But when you start to deal with what fundamentally is a dumb device but granted a device that s much faster than you at doing certain things you can t make these assumptions And you have to think through what could possibly go wrong What situations might arise that this program needs to know about preemptively Make this more real Think about going to a Website Well a lot of Websites have forms you fill out like when you buy something from Amazon Well among the things a programmer for Amazon has to think about is how do I handle a user s buying an item Well typically to buy an item you add something to your shopping cart and then eventually you provide your fi
16. twenty students Okay I ll take twenty All right all take twenty So that s not bad So there s clearly a tradeoff sometimes with the complexity of algorithms daresay And this isn t quite fair to compare to my algorithm Because whereas I arguably am just one CPU one brain so to speak or lack thereof counting you you are arguably are eighteen or three or twenty CPUs 9 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School So really you could say that the way you executed this algorithm would be an example of like a multiprocessor algorithm a parallel algorithm in which multiple brains are each doing something at once And we ve certainly talked about that idea in terms of dual core processors being able to do two things at once This is an extreme case This is like a computer having twenty some odd processors but the idea is the same 00 20 00 In theory by throwing hardware at a problem sometimes you can just double the speed But by throwing CPUs at his problem I m able to fundamentally change the running time of the algorithm So the benefits therefore really depend on how you re using that hardware and if you re just turning a dial and speeding things up or if you re fundamentally changing the solution And that s the fun part when you get to design more clever better algorithms for solving problems And again to come back to Google that in effect i
17. you guys have six steps to go through or three steps on each pass But the spirit is certainly the same And only because that was really wrong can you humor me once more Let s try to get this right and see If we get eighteen then there will be a big surprise somehow at some point 8 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School All right so if you ll humor me Stand up once more And actually we can take something away What is a bug in a program STUDENT inaudible response DAVID MALAN Something that doesn t work the way you planned So case in point STUDENT Lack of direction DAVID MALAN Yes Lack of direction Oh programmer error STUDENT inaudible response DAVID MALAN Well yeah I mean add your number to that person s number STUDENT inaudible response DAVID MALAN Ah all right So buggy hardware All right So Step 2 Everybody s the number one Step 3 Find a buddy Okay Add your number Everyone s number two One of you sit down And repeat There s three of you That s okay Just two of you pair off and we ll get rid of one of you STUDENT inaudible response DAVID MALAN What you have twelve STUDENT inaudible response DAVID MALAN How many do you guys have STUDENT inaudible response DAVID MALAN So you have four That s sixteen students How many do you have STUDENT inaudible response DAVID MALAN See that s
18. yourself nuts biting off too much initially And plus this is a very representative process when programming When people sit down to write real programs in Java C C they don t sit down and write the program and then go test it Baby steps Write part of it that s very easy to debug to find mistakes in to refine Then move on to something else And we ll leave it to you to decide what parts of your program make sense to bite off first But part of the challenge part of the fun in the process will be figuring out on your own How do I go about implementing my algorithms How do I go about implementing my art animation or game So good luck with that You have almost a month to do it We will see you next week for Exam 2 and a little something special end 01 51 48 53 of 53
19. A programmer is a software developer might be a software engineer A lot of these terms are synonyms for the most part depending on where one works and what one calls him or herself So let s consider a problem Here is a phonebook with roughly 1 000 pages in it If I wanted to look up a phone number in this phonebook and I knew the name of the company how would I likely go about solving that problem Physically literally what do I do Here s the phonebook STUDENT inaudible response DAVID MALAN Turn to the business section Let s assume the whole thing s the business section It s pretty much all yellow pages All right STUDENT inaudible response DAVID MALAN All right so let s look alphabetically by name Give me a company name that s likely to be in here STUDENT inaudible response DAVID MALAN Leo s Diner all right So I m going to open to the business directory which starts with the As All right A is clearly too early so I m going to turn the page going to turn the page I m going to fast forward lest we get bored too soon tonight And eventually I m going to get to the Ls Oh just passed the Escort section apparently I noticed Now we re in the Health section the Hotel section Insurance Lawyers so we re at least in the Ls A lot of lawyers STUDENT inaudible response DAVID MALAN Oh we re going to We ll go to restaurants right So we re not I m an idiot is the takeaway here There is
20. ALAN Perfect It s going to meow three times and it s going to pause two seconds in between each Let s go ahead and just open this one And again would not be hard to reconstruct it manually This is hello3 scratch And just for kicks I m going to expand it to full screen But the effect is the same Click the green flag now cat sprite meows three times pausing in between My first program with sounds And how do I do sound Well as you might have glimpsed there s a Sound panel here It comes with a bunch of default sounds but you can also import sounds And I ll defer to your own exploration as to how to do that But that s how first instance I imported that Oscar the Grouch song And you can import sounds like the meow But the meow happens to come with Scratch 01 20 00 Well what else can we do Well in programming in most languages many languages there s this other notion which we didn t quite discuss earlier that of a Boolean expression 37 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School Note on screen slide 14 A Boolean expression which is sort of inexorably linked with the notion of a condition is an expression that is either true or false named after literally a Mr Boole right And Boolean expressions have this close relationship as an aside with the notion of zero and one true and false And true and false is how we ll spin them tonight
21. Anything dirty or dingy or dusty DAVID MALAN It s a good score so far Oscar Anything ragged or rotten or rusty Yes I love trash Now look at the rest of this junk DAVID MALAN Big finale Oscar I ve a clock that won t work And an old telephone A broken umbrella a rusty trombone And I am delighted to call them my own I love them because they re trash Oh I love trash Anything dirty or dingy or dusty Anything ragged or rotten or rusty Yes I love I love I love trash song ends DAVID MALAN All right A big round of applause Never seen a volunteer leave so quickly Okay That s very good 41 Now granted In retrospect I realize maybe I should have made the song a little a little shorter But it is synchronized with the words he s singing And that s a very good score What you ll see in Problem Set 8 is that you re invited to play Oscartime yourself And for a point try to come up with your own high score And you ll see if you look around at the program we won t do this tonight that this program is the result of implementing looks like nine sprites as they re called So let s go ahead and dive into Scratch And realize that though we re introducing you to this graphical language that allows you again to make animations artwork games and so forth it s 32 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School the constructs that t
22. Computer Science E 1 Understanding Computers and the Internet Harvard Extension School Transcript Lecture 11 Programming Hour 1 00 00 00 DAVID MALAN Welcome to Computer Science E 1 My name is David Malan This is Lecture 11 Programming We are nearing the end and as we do the topics become a little more of a segue to perhaps additional courses in computer science Recall after Exam 1 we passed out that handout Life After E 1 which offers you some pointers in which directions you might go after this course Programming is certainly among them And programming even though you won t exit this course a programmer per se by the end of tonight you should have a sense of what programmers do what programming is all about And daresay a better understanding of why some things go wrong in your own computers why your computer crashes why your computer hangs ultimately as a function of mistakes that humans have made Speaking of mistakes what is correct about this advertisement It s terribly small I know So you can go out on a limb and take a guess What did they get right in Bed Bath amp Beyond s winter issue of their Christmas specials and such STUDENT inaudible response DAVID MALAN Yeah they finally got the URL right www bedbathandbeyond Recall on Exam 1 we had a photograph that I had taken in all my photographic skills of a window display of Bed Bath amp Beyond and it was erroneous in that it had
23. Okay And so again terribly few additional blocks But again the program s beginning to get a little more interesting right Slap a wallpaper on there maybe another sprite and we begin to have real interactivity real animation a real game What about Count scratch at far left 42 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School Note on screen slide 19 What does this program seem to do STUDENT inaudible response DAVID MALAN Keeps counting and it keeps saying the current count Well let s just slap some jargon on here set counter to 1 What is counter probably What kind of construct is that STUDENT inaudible response DAVID MALAN A timer but let s use the programming speak from tonight This is an example of I didn t call it x y or z I called it counter but what is it STUDENT inaudible response DAVID MALAN It s a variable And I m initializing the variable not to zero like we did with socks_on_feet but rather to one And then forever I m saying the current value of the counter and that s Scratch s way of allowing you to do that by tucking the variable inside of the say block I m going to say it for one second I m going to wait one second I m going to change the counter by one aka and then I m going to repeat forever and ever If I open up count scratch what we have here is now a program that if you look back now at the
24. These are some of the examples of Boolean expressions that Scratch supports Again constructs like these are found in other languages Here is in the domain of Scratch what exists touching mouse pointer Think about what that means Either a sprite is touching the mouse pointer the cursor or it s not It s either true or false yes or no on or off right The same ideas recurring again and again mouse down Well either the mouse is down true or it s not false lt ess than Is the thing on the left less than the thing on the right So we clearly can manipulate numbers with Scratch and another Boolean expression Is this Boolean expression and this one true If so the whole thing is true If one or both of them is vof true then the whole thing is false So it s sort of an intuitive notion of what it means to be something and something Well let s deploy these To deploy them we need to build on the notion of a condition right Because Boolean expressions in and of themselves not interesting But if we use them to govern what we re going to do then they become a feature Well here are some examples of other Control structures if conditions Note on screen slide 15 And notice that similar to what we had in our own sock changing example we had this notion of if or if else right We could either do this if something is true else do that If you wanted to have three conditions with Scratch y
25. a restaurant but instead had asked Can you find me a local zoor which might be a stretch if there s even a zoo category that certainly would be at the very end of this phonebook So if we wanted to just wave our hands slightly and say you know what In the very worst case the number of steps that that algorithm that procedure might take is maximally going to be what in the case of this thousand page phonebook STUDENT inaudible response DAVID MALAN A thousand right It s a pretty naive algorithm but it s pretty darn correct No matter what no matter how long it takes and no matter how bored we get we know that if we did this brute force approach so to speak starting at the beginning and then just looking alphabetically we would eventually find our answer if it s present But that algorithm insofar as I was sort of walking a line starting at one end and working my way to the other end is what you might call an algorithm with a linear running time right If you were to plot you think back to algebra if you ve got that x y plot Anything that s a straight line was a linear plot And that essentially implied that as you increase the number of pages in the phonebook well you increase the number of maximal steps that you might have in that algorithm So in the worst case a thousand page phonebook might take a thousand some odd steps The question before a programmer before a software developer solving a similar
26. a we ve mentioned Pearl is another popular one C is another one Just be familiar with these kinds of terms and just know that these are alternative languages sometimes that make software development easier sometimes that make it harder sometimes that enable the programmer to do things that he or she couldn t do in other languages But for the most part one s choice of languages Python is another popular one boils down partly to what infrastructure you already have in place that is what is your company invested in and therefore what language do you need to use to continue using that same infrastructure two what do you know in the first place It s a lot faster to develop in something you know rather than go learn something new and then develop in it three where you want to run the software and you can come up with a half a dozen other arguments I m sure The short answer is it just depends on where you re coming from and what you want to do with the software what language you use So in years past we have used a language called JavaScript which should be on this list as well because you see it similarly often as well JavaScript which is not the same as Java they just kind of stole the first part of the name from it JavaScript is a language that Web browsers execute And it is a language that you can do things in Web pages like error check fields so that you ensure before the user even clicks Submit that they ve pro
27. ainly those three And those are constructs that despite their simplicity prove to be very powerful Well let s now make things more real That was pseudocode sort of an amalgam of some programming laneuage like syntax but also some actual English What is this here on the display Note on screen slide 7 What s this STUDENT inaudible response DAVID MALAN XHTML right This is Extensible HyperText Markup Language Well there we go It seems that we ve already seen a language before But just to be clear we haven t seen a programming language before XHTML and some might try to debate this is a markup language which means it s good for the kinds of things you ve begun using it for making Web pages marking up text presenting movies and sounds and images and so forth It s not a programming language Because in XHMTL itself there s really no notion of branches or conditions Rather it s all statements Do this do this do this And by that I mean make this bold make this green make this big make this centered XHTML is about marking up a document But a programming language fundamentally let s you do anything It allows you to manipulate in the computer in a way that allows you to solve problems whether that s finding a phone number searching some database or simply allowing someone to check out with a shopping cart Well what s a real programming language This is an example of a language called what Anyone kn
28. an ampersand and not the word and So some crazy demos today We have a phonebook it would seem We have a small child We have some digital demos So let s dive right in So programming is ultimately about implementing algorithms and solving problems An algorithm is just a procedure it s a way of doing things And it s in algorithms that a programmer gets to exercise a bit of discretion a bit of intelligence gets to think about the design of a program And perhaps before we dive in too deep let s just put some words on this What is a program Should be a softball right What s a program Yeah STUDENT inaudible response DAVID MALAN Good So it s just a set of instructions to a computer And we interact with programs by way of double clicking them usually But what is that Word exe on your hard drive Well it s just a collection of bits It s a file But those bits are laid out in a way that the specific CPU that s inside your computer understands And at the end of the day we ve said that those bits simply comprise instructions Do some addition Do some subtraction Do some printing 1 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School But ultimately a program is just a set of instructions So a programmer therefore is just a person who actually writes those instructions and writes them in a whole bunch of different languages some of which we will glimpse tonight
29. an you have variables per sprite it turns out and if you decide to explore this you can also have what are called global variables which are simply variables that not only can one sprite use but all of your sprites can use And in this way can your sprite sort of interact and share information In fact one of the ways that Oscartime works is that effectively when you do drag a piece of trash to the trashcan that sprite signals to Oscar that it is now in the trashcan and therefore Oscar should increment his own variable for the score So you can have some neat interactions And I will say that Oscartime is sort of a complexity that we don t expect for your Problem Set 8 It s meant to give you a sense of what you could do with Scratch if you spent eight hours on a Friday night listening to that damn song on repeat for at least six of those hours Can t stand that song anymore not that it was on my playlist earlier But here s another example which I pretty much borrowed from one of the programs that an MIT teaching fellow made It comes with Scratch Note on screen slide 23 I simply changed the name changed the icon and a couple of the lines of code But if you open up this at your leisure perhaps if you find yourself particularly frustrated at some point soon you can open David scratch I ll make this full screen This actually just take note has three sprites a left glove a right glove and me And it s got a
30. at location 20 Next time start the trash at location 40 Next time start the trash at location 55 And it just gives a bit of variability to the program which tend to make games and other types of programs more interesting So at left we ve nested a few blocks and it looks like if it is true that Scratch picks a random number from one to ten and that random number is less than six that is it s one to five then it s going to play a sound the meow In other words because there are five values that are less then six pretty with much 50 percent probability do we meow 50 percent of the time we won t meow And we can sort of vet this by just trying it Let s quickly open hello I think this is uh what did we say five Yup hello5 scratch is in Lecture 11 as with everything else And let s go ahead and play What s going to happen when I click Play STUDENT inaudible response DAVID MALAN Excellent It might it might not It looks like it picked a number that was not less than six cat sprite meows This time it did Didn t cat sprite meows Did Didn t Didn t Didn t cat sprite meows Did 40 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School Now you do this 100 times you ll get roughly 50 yeses 50 nos Do it a thousand times you ll get roughly 500 yeses 500 nos and so forth assuming that the random number generation is accurate which hop
31. bering in code how many socks are on Dawne s feet So we don t have to keep checking for instance All right Line 8 says look for matching sock All right if you find a matching sock then Did we find a matching sock DAWNE Yes DAVID MALAN We did find a matching sock We re going to go to Line 10 put on matching sock Excellent Line 11 socks_on_feet Line 12 close sock drawer All right and now just to be clear we don t want to execute Lines 13 or 14 or 15 because what we began to do in Line 9 was an example of a branch or a condition a sort of fork in the road If Line 9 was True then we did the indented Lines 10 11 12 beneath it Else had Dawne not found a matching sock the idea would be that we would instead go to the else line the other fork in the road the other direction and we would instead execute the two indented lines on Lines 14 and 15 So now we find ourselves down where Line 16 it doesn t apply because notice Line 16 is associated with which other line based on the indentation STUDENT inaudible response DAVID MALAN Line 5 right But Dawne did find a sock So we follow that first big branch and not this latter one So now we re at the end of the program But remember how we got here What was the line of code that launched us into those Lines 3 through 17 STUDENT inaudible response DAVID MALAN So Line 2 recall right That was the loop that Eric said
32. bunch of other people screwed up before you So there are not three people here Something went wrong along the way But let s think about what was happening on each step and see if we can salvage this demonstration and see where we were going with it So Step 1 you all stood up and had the number one In Step 2 you all sat down No half of well practically half of you sat down With that in mind how many iterations of this algorithm did you all go through by the end of the demonstration In other words how many times could half of the classtoom sit down half sit down half sit down Well we have roughly I counted Well let s see 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 I count eighteen students So we re just only slightly off by fifteen So eighteen students You can divide that by two and you get nine students standing Divide that in two and you get like four students standing giving or take then two students then one student So in theory it should have only taken four passes through this algorithm until there was just one of you left Contrast this now with my teacher like approach to taking attendance 1 2 3 4 5 6 7 8 and so forth Mine is clearly what we would call what STUDENT inaudible response DAVID MALAN A linear algorithm Yours in spirit was a Logarithmic algorithm Yes there s more steps It s sort of like apples and oranges because this is one step whereas
33. cer game vendor Hot dogs hot dogs beer hot dogs soccer game audience cheers DAVID MALAN Give us one more Soccer game announcer Go o o a a l DAVID MALAN All right Very well done Nicely done applause And actually let me keep Dawne here for just moment Because I have feeling I won t get a volunteer What I m going to do is I m going to go to the Open menu up top I m then going to say no don t save the current project because we haven t made any changes I m going to go into our Lecture 11 folder which you will be walked through in Problem Set 8 as to how to install that Lecture 11 folder if you want to play with these demos And I m going to load up one other program that was written by a former student a former student when he was in a class at MIT So this is the first program I wrote in Scratch What you re about to see and hear is a song that I heard for six hours straight while writing this program And you ll know what I mean in a second Do we have any volunteer who would like to knock Dawne off her throne down here or otherwise Dawne will get to play Oscartime Yes All right come on down Thank you to Dawne We have a replacement You have to be comfortable appearing on camera needless to say 30 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School Oscartime is also a program written in Scratch It is all about watching as the trash is about
34. cosmic rays striking your computer was human error Even if you ve never met that person before they at least contributed to the software So let s try something that always gets me into a bit of hot water here We are going because of the pain that this doll sometimes suffers So we are going to change a baby s diaper We are going to implement an algorithm that you are going to craft verbally I will be the computer the dumb computer that only does what he is told And our goal in this exercise is to change this baby s diaper And clearly the stakes are high since we re changing a baby s diaper And I as a few of you saw 10 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School who got here earlier I have never done this before It took me ten minutes just to get the first diaper on the child So I need step 1 from a father from a mother from someone with more intuition about this than I How do I begin to change a baby s diaper STUDENT inaudible response DAVID MALAN Place the baby on a safe surface So this is where I would start getting a little reckless But I ll be gentle at first All right Safe surface Step 2 Someone else STUDENT inaudible response DAVID MALAN Take the clothes off Unsnap the Let s roll back Step 2 So it s step 2 Turn over I did turn over the baby STUDENT inaudible response DAVID MALAN Got to be precise Tell me
35. creen slide 9 Looks very similar And this is not meant to give you a sense of how to write C but just a snapshot of what it looks like This too would be compiled down to patterns of zeros and ones like this And believe it or not I did take the time to actually compile that code into these zeros and ones which are literally the zeros and ones that a compiler would output on I think an x86 based Linux computer So these aren t me typing randomly at the keyboard 010001 These are the real zeros and ones that are physically in that exe file or the file on say a Linux box that s double clickable as well Finally one other example This is an example of Take a guess Note on screen slide 10 STUDENT inaudible response 25 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School DAVID MALAN That s Java yes So this is an example of Java which is another quite popular language It is not compiled in quite the same way but the spirit is the same Java for what it s worth is compiled into something called bytecodes not into this stuff that previously I didn t slap a label on but was called object code a minor distinction that s not an important takeaway for now But the neat thing about Java is that because it s compiled into something that s slightly different fundamentally from what we just looked at for C and C compiled into bytecodes and not object code t
36. e something 6 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School that s more than obvious And even with some slight changes even by taking a few more minutes up front and thinking how can I do this algorithm how can I write this program better you can imagine the savings that someone like that gets per unit of time And even we though it might have taken us slightly longer as it did maybe a couple of minutes to get to the discussion of our logarithmic algorithm consider if we use that algorithm day in and day out be it in a digital world or this physical world just how many thousands of steps you ultimately save And that s a powerful thing This logarithmic running time We can see it in another way And I need you to humor me where we all partake in a demo Note on screen slide 4 If you would I need you all to stand up in place You don t have to come down But go ahead and stand up And we re going to execute an algorithm together In this algorithm we re going to follow Step 1 which is Stand up Nicely done Step 2 Assign yourself the number 1 So think to yourself I am the number 1 Step 3 Find someone else that s standing up In other words find a buddy Say hello if you ve never yet met A little awkward if you re across the room Don t be left out And Step 4 Add your number to that person s number A sanity check Who does not have the number tw
37. e drawer The same darn sock And so the program though you might think this sort of corner case is a little silly it is incredibly representative of what happens all of the time in programming failing to consider what might be a rate but feasible scenario And just as we ve described it an infinite loop suggests something that goes on ad infinitum forever Well have you ever been sitting in front of a program your computer and it just seems to again hane not freeze but it slows to a crawl And it looks like based on the light in the front of your computer it s doing something but it s never giving you control back 22 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School Well that might be an example potentially of one of these infinite loops The program because of a mistake the programmer made somehow got stuck in a cycle because he or she did not consider whatever the user might have just typed in or where the user might have just clicked or what data might be available what socks might be available to that program The takeaway here then is one there s these fundamental constructs in programming You see them in silly things like algorithms written in pseudocode for putting on socks But among those constructs are variables loops and these things called branches And almost every language in which one would tend to write software has among its constructs cert
38. ectively converts bytecodes to whatever the zeros and ones are that each of these CPUs requires software to be written in So originally Java was touted as this wonderful language because finally you could improve development time and efficiency and earnings because you could just write software once and run it everywhere And Java was touted as a wonderful language for the Web because you could download these so called Java applets in your Web browser And then no matter what kind of computer Mac or PC you were running you could run this software in your Web browser Well Java applets if you ve ever pulled one up are typically very slow and not terribly fun to interact with And so you really never saw this happening But Java is in fact being used quite a bit these days for server side development A lot of big very high capacity Websites tend to be written in something called Java or Java Enterprise Edition is just a fancier way of saying Java with a whole bunch of special libraries extra add ons to it 26 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School Whereas most software that you might install on your PC these days tends to be written still in C or a language called C But for the most part those are perhaps the most popular languages C C Java and then there are other languages used say on Websites besides Java PHP is another one C C PHP Jav
39. efully it is And it tends to be in programs Randomization incidentally is used a lot in cryptography And it would be a very bad thing in ctyptography if your random number generator always generated the same number right It s not that hard to guess then what a secret key might be All right how about a loop Let s now introduce the same construct we explored in a different domain earlier Note on screen slide 17 Scratch has such constructs as these forever Turns out sometimes an infinite loop is a useful thing if you really want the program to do something forever For instance I always wanted trash to fall again and again But it turns out there are blocks statements like stop script that you could put inside of a forever loop so that the loop does actually stop even though it says at top do this forever Or you can say do something some number of times repeat 10 times in this case How might we integrate these Well here are three new examples Note on screen slide 18 At far left is hello6 scratch What s this thing going to do in a sentence STUDENT inaudible response DAVID MALAN Yup Unlike the last example which meowed I think three times and waited a couple of seconds in between this thing is going to meow forever and only pause two seconds in between But it s not going to stop until we hit the red stop sign in Scratch I won t even bother playing that because it s pretty easy to j
40. he implication is that Java programs can be run on any CPU So essentially what Sun Microsystems the inventors of Java did was they designed a language that rather than compiling source code directly into zeros and ones they rather imposed an intermediate step So that Java code gets compiled into something called bytecode And on Macs on PCs on Linux computers and even on other operating systems even some of your cell phones these days there is what s called a Java Virtual Machine a JVM for short And so in the world of Java what happens is a compiler for Java translates the source code that programmers write into something called bytecodes which looks similar but it s not quite zeros and ones it s closer to the original source code Those bytecodes are portable so to speak They can be run on any platform provided there exists what s called a Virtual Machine for that platform Long story short all a Virtual Machine or JVM is is a program literally that understands bytecodes and executes them on behalf of the CPU The implication is that underneath in this sort of layered idea that we re making you can have an x86 CPU or you might have you know in yesteryear a Power PC s CPU or you might have something that s from other domains a Spark CPU which is another CPU more for corporate use all together The point though is that the code itself can be run anywhere Because effectively people have written software that eff
41. his language Scratch supports and it s the fact that you are in fact programming with it that are the real takeaways We just have the fortune this year because of MIT s Media Lab specifically the Lifelong Kindergarten Group over there They ve given us access to the software You ll be able to download it from the Web yourself install it on your computers and begin to explore programming in what we think is a very fun programming environment So with that said this program here is the Scratch version of the program in the previous three slides Note on screen slide 11 So we saw a C program a C program and a Java program each of which implemented a terribly simple program the first program ever written if you will called helloWorld which simply prints to the screen the words Hello world But recall the kinds of syntax that you saw in these programs You needed to write not the zeros and ones but these parentheses and the fairly cryptic looking syntax public static void main String argv Note on screen slide 10 I mean just to get yourself into the simplest of programs one that prints to the screen Hello world there s a lot of overhead There s a lot of distraction Similarly was the C version similarly difficult to acclimate to perhaps on first pass And the C version wasn t much better at all 01 10 00 Fast forward now to Scratch that same exact program becomes a two piece puzzle so to speak
42. ive Two cat sprite meows Nine Same program but now a bit of feedback What about this guy Note on screen slide 19 Well now finally we have a bit of animation right And with only introduction of a couple more blocks First let me open Movel scratch This is going to empower our sprite to start moving around quite literally And in moment we ll introduce a second sprite Here I ll make this full screen and I ll click Play So now simple animation but again only takes a few blocks cat sprite roars We have a cat that s moving around and every time it hits the edge not only do we play a sound it also reflects like a pool ball would off the side of a pool table and keeps going All right how did we do this Well this is just this program here forever do the following Mi ee banging continues 44 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School Okay that s going to annoying banging stops if forever do the following If you re touching the edge well what intuitively do we want the cat to do Play the sound called bang which I also took out of Scratch s arsenal of sounds if on edge bounce which this is sort of a cheat on Scratch s part It s sort of a condition and a statement combined But that s okay It s sort of a little faster to implement with blocks like that Then it s going to turn 15 degrees and move 2 step
43. ler for having changed the text to I got pwned Anyhow Google pwned or look it up on Wikipedia for the history of such things Well this we won t spend much time on just because it begins to get a little scary But realize that that whole game if it can be called that was the result of just what one two three four five scripts three of which are terribly short one of which is pretty long But again if you read through it perhaps after tonight you ll see that again it s just implementing the logic of the game move David back and forth somewhat randomly And if he ever gets hit with one of the other sprites that is one sprite is touching another which is one of the blocks under the Sensing category well then change David s color change the color really of David s costume My face is just a costume on what might otherwise look like a cat Well there s this one other feature of Scratch and really of a lot of programming languages in general which is this ability to signal from one thread to another And threads can talk to one another effectively either by using some global variable such that one changes a variable and the other watches that variable as in our muting example Or you can send a message you can trigger an event throw an event that another sprite or another script listens for or waits for So this is a very simple example Marco scratch that demonstrates the use of what s called event hand
44. ling And this is common in most programming languages that interact with users I m going to open Marco scratch And notice that all this program does is the following I hit the spacebar and then she responds Polo So notice let s play it again The moment I hit the spacebar the boy says Marco Hands off the keyboard The girl later says Polo Now we could certainly just use a timer of sorts right Wait two seconds and then say Polo But notice that these two characters are two different sprites each of which has a different script I m going to go back to the PowerPoint so we can see them simultaneously Note on screen slide 24 50 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School It s the same stuff but notice that the boy simply waits forever until I press the spacebar That is forever do this If the spacebar is pressed say Marco for two seconds That s pretty obvious But then broadcast what s called an event This is sort of like a message a whisper if you will between sprites Meanwhile the girl is designed not to start acting when you click the green flag but when she receives event that is when she hears the event whispered to her And when she receives that event she says Polo for two seconds And we offer this as a very simple example but one that hopefully will offer you an ability to solve some problem perhaps that you might come ac
45. llows and faculty and also kids in these computer clubhouses have contributed to the project So what you get out of the box for free is a whole bunch of demo games and artworks and animations which we ll leave to you to explore We ll focus on a few that convey some of the basic constructs we want to introduce tonight But realize that if you just flip through these folders for instance games there s a whole bunch of options in here of varying difficulty many them written by young children some of them written by older MIT students But the point is that it wasn t hard for the most part for anyone to dive into this language and start producing some really neat stuff on day one We ll spend most of our time in this Lecture 11 folder tonight I m going to open the file called Hello1 And as you ll see we have the following layout On the left of the screen is essentially all of the puzzle pieces that we have at our disposal They re categorized into categories like Motion related puzzle pieces and Looks related puzzle pieces Control related puzzle pieces We ll highlight some of the blocks in each of those categories But for the most part things are fairly intuitively categorized And this is the sort of language that you don t need a user s manual for really even though the problem set does point you at some online references for ramping up It s really the sort of thing that you learn honestly by playing around And if intuitive
46. lly after one night of class and a few hours of playing around on their own they were able to implement things like this soccer game where you try to score goals this baseball game here this sort of fairytale which tells an animated story about some I think knight strolling up through a castle area this sort of fun musical thing as I recall with a dog and a dinosaur and some graffiti on the back wall this which I ll demonstrate probably later which is another fairytale of a fairy princess and a knight and so forth Again literally students who had never programmed before And the reason we introduce Scratch is because it allows you the students the neophytes arguably to hit the ground running immediately And that s not something you can do otherwise without say a full semester of programming 01 00 30 So with that said let s take a five minute break and return Hour 1 DAVID MALAN We te back REI DIAZ So this week Section s going to be on CSS or Cascading Style Sheets We ll use them to prettify the XHTML stuff that we worked on last week This Saturday will be the exam review and we ll cover everything from the last exam up until today s lecture in preparation for the second midterm 28 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School Also this Saturday will be Section at its normal time and Workshop will be on Macromedia Flash the animation of
47. ly you think there probably exists a condition a branch via which I can do this or do that well there probably is under the Control category for controlling your program so to speak In the middle of the screen we have the Scripts area This is where we will write our programs aka scripts by simply dragging puzzle pieces and letting them lock together Down here we have our Sprites area A sprite is simply a character that you can manipulate The first character we re going to manipulate is that cat on the screen which is the default sprite But the neat thing about Scratch is that you can put costumes on sprites So what you saw to be Oscar the Grouch was sort of like a cat dressed up as Oscar the Grouch But really a sprite isn t necessarily a cat That s just the default costume You can make your own icons be it in Photoshop or in a built in graphical editor a simple one like MS Paint But you can import graphics too if you find something on the Web that you re allowed to and would like to import So you can do some neat things 34 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School And the soccer player for instance That student as you probably gathered I think hand drew the soccer field And you can kind of tell But in the end it worked out pretty well And I think that too was his voice late at night Scratch also lets you record sound So he late at night was
48. mber related block So these are all under Numbers and also they re color coded which helps to give you a little mental cue I want the less than lt one Notice that even though this is quite larger than the hole everything is shaped to be similar in structure So notice if I hover over that it gets highlighted If I let go it jumps right in there and it grows quickly to fill it It s the shape that s important Finally I can just type in these white boxes 1 and 2 And then what I wanted to do is play a sound I want to play the sound meow So I m going say play sound meow Go ahead and click the green flag cat sprite meows Right Pretty deterministic Let s change it if 2 lt 1 what s going to happen STUDENT inaudible response DAVID MALAN Nothing Completely uninteresting But confirms what our intuition might say What about at right What does this program do STUDENT inaudible response 39 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School DAVID MALAN Good So this is a neat feature of Scratch It allows for what are pseudorandom numbers or random numbers This is a powerful thing if you want your game to be interesting right Oscartime not so interesting if the trash always fell in the same location right Even a child would probably get pretty bored with that So even Oscartime uses some random numbers to say start the trash
49. n her drawer and she executes this program Describe for me the symptoms that would occur if Dawne only had in her drawer for whatever reason one sock STUDENT inaudible response DAVID MALAN Exactly So we have in that scenario it results in an infinite loop a loop that seems to go on forever Why Well if she has just one sock in her drawer well then we do in fact in Line 15 on the first iteration of this loop find a sock Note on screen 5 if you find a sock then And so Dawne puts it on 21 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School Note on screen 6 put on sock She increments the variable Note on screen 7 socks_on_feet She looks for a matching sock Note on screen 8 look for matching sock But this time in this scenario she doesn t find one Note on screen 13 else And so we execute these So Dawne removes the first sock from foot Note on screen 14 remove first sock from foot decrements the variable minus minus Note on screen 15 socks_on_feet And then where does she go back to Well back to line 2 while socks_on_feet 2 At this point in the story is socks_on_feet equal to 2 or not equal to 2 STUDENT inaudible response DAVID MALAN It s not equal to 2 because it started at zero She incremented it to one decremented it again so it s back at zero and so the loop repeats This time in Line 5 what does she find in th
50. n some way worst case your software breaks in a security sense such that because you didn t consider some scenario someone s able to type in not just their phone number but a line of say SQL code which is a database language 14 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School And because you were sloppy when writing your code if someone happens to write programming code where their phone number goes because you didn t think that someone might dare try something like that the result when they click Submit is that your entire database is deleted Because what they typed in for their phone number is something like writes on board delete from database And this is just an example of a query that one might type in the world of database programming 00 30 00 But if you don t check that someone is actually inputting a valid phone number and instead allow them to type crazy stuff like that you re putting yourself at risk So not only might the program break for the user the program might also break in the backend in a very expensive or certainly unpredictable way These are very common situations And we talked about hacking and so forth in our Security lectures Step 1 in a lot of hacking is just banging on a computer or on a server and trying to pass it bogus input really big input no input at all and just see what happens Because usually if by providing a compu
51. ncing as before Cat keeps cat sprite roars pointing toward him And as soon the cat touches the bird the cat does that roar Same thing again Notice if I wanted to make this slightly more interesting I could have the bird start in a random location too cat sprite roars And he caught him again Well what mote can we do Hello10 scratch Note on screen slide 22 47 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School Well this is kind of a neat way to show you how to use variables to maintain what s called state in a program Variables allow you to keep track of sort of the state of your world This program quite simply is one that has a cat meowing but it allows me to mute the cat or re enable sound just by clicking the spacebar So let s put this into context first Let me open Hello10 scratch in Lecture 11 the last of our Hello examples I m going to click the green flag cat sprite meows There we go green flag cat sprite meows Okay but notice on your own soutce code if you want to look down there s that if condition that said if space pressed change the value of that variable I m going to hit the spacebar And it goes quiet because I ve changed the variable called muted this time to one And notice that the part of the loop in this program that says play meow yes or no first says what s the value of muted Does it equal o
52. ne Then don t play anything If it equals zero do play something Let s hit it again cat sprite meows Now it comes back on So the program was still running but every time the loop iterated notice that if muted 0 is zero that is it s not muted play the sound But if it is equal to something other than zero for instance one than it doesn t play anything 01 40 02 But how is this happening It seems to have two different scripts Well threads are useful So that even conceptually you can divide your program into different sort of modules one of which handles a cettain task the other of which handles the other task So at left these two scripts are both associated with the same cat sprite So this sprite just has two scripts 48 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School The one at the left just meows forever every couple of seconds but only if the variable called muted is zero Meanwhile the thread at right simply spends its entire life listening or watching for whether the user hits the spacebar And the moment the user hits the spacebar because that script at right is doing this forever it s going to change at that moment in time the value of muted to 1 and that is going to have an effect on the script at left because the script at left is using that same puzzle piece that same variable And so what s neat about Scratch is that not only c
53. nerator pick a number between one and ten and only 50 percent of the time does the princess actually come back to the knight So that s kind of a cute little trick One other program that I ll demonstrate submitted by another student This I would say was sort of on the extreme end of Wow can t believe a student implemented this so quickly This is one of those um oh what s the name of the popular arcade game that looks like Street Fighter There s another one that s very popular STUDENT inaudible response DAVID MALAN Sorty STUDENT inaudible response DAVID MALAN Mortal Combat this reminds me of So I m going to hit Play up here Okay Press C for control so I have some instructions Notice I can move around I can jump by hitting Up And then if I hit the spacebar notice I can strike the demon with lightning But I have a sort of light force You ll notice at bottom left I m really getting pwned here Oh and I m almost Oh I have failed My life force is gone So this too was particularly impressive This is a student who had a good number of scripts a good number of sprites But what you have to appreciate especially if you re somewhat daunted by the complexity of some of these programs for instance this one even some of the ones in Scratch s sample directory no one sits down and writes these things all at once The first thing for instance I did with Oscartime was one I pulled up Pho
54. not a section called Leo So why don t we go to the Restaurant section Good takeaway for tonight All right and so here in the Restaurant section the section itself is alphabetized presumably And so now I can do a similar process starting at the restaurants starting with A and sort of work my way from left to right through the alphabet until I hit L then Le then Leo and hopefully Leo s Diner would then be in this phonebook 2 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School So if this phonebook has say 1 024 pages which it actually happens to roughly how would you describe the running time or the duration of that algorithm as I just executed it which again to be clear was to open to the A section and make a few mistakes but then eventually work our way to the R section for Restaurants and then home in on Leo s Diner How would you quantify the amount of steps that that would take if for instance we say that one step is the act of me turning one page STUDENT inaudible response DAVID MALAN Good So you d have to know pretty much how many pages are there between the start of the book and the letter R Well in the worst case maybe you wouldn t have asked me about Leo s Diner but rather Zoe s down on Mass Ave So starting with Z that restaurant listing if it s in here is going to be at the end of the Restaurant section If you hadn t even asked me about
55. o STUDENT inaudible response DAVID MALAN All right one of you might not if we have an odd number But hopefully all of you have the number two Now is the race to sit down Number 5 One of you in each pair should sit down Never saw some people sit down so fast Step 6 If you re still standing go back to Step 3 And let me ask that those of you still standing repeat this algorithm Go ahead as directed And by repeat I mean find someone else add your numbers Don t look at me I don t count I m not standing per se Keep on counting All right we have two Kind of standing yes Still standing Just you guys So all right so you guys are presumably on You just finished Step 3 so add your numbers together Okay so one of you gets to sits down One of you Whoa All right what s the total number you have in your mind STUDENT inaudible response 7 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School DAVID MALAN You would have just added Eric s number to whatever number you had a second ago STUDENT inaudible response DAVID MALAN Three You may sit because this is about to embarrass all of us I think There are three students in the classroom room right now based on this algorithm If I wasn t clear this is an algorithm for counting students And I will offer that it was correct and don t worry if you got three because it means a whole
56. oad over the Internet When you buy something and the box says maybe it s a game PCs only that means because even though the program though it would be thousands tens of thousands of lines long not just six or so lines long the programmers wrote stuff reminiscent of this most likely 00 50 00 But they used a compiler that was specific to x86 based PCs the result of which is that the zeros and ones that they then burned to a CD or a DVD for you to buy from the store are patterns of zeros and ones that only say Intel or AMD CPUs understand A Mac would not understand those same patterns of zeros and ones If a company instead wanted to produce that software for a Mac they d probably write something similar to this though there s enough differences between Macs and PCs and so forth that you can t just take the same source code usually and run it through a different compiler and get a Mac based CD There s a lot more work typically that goes into it Such that even for a while I think Microsoft had separate divisions one of which made Microsoft Word for the Mac one of which made Microsoft Word for the PC They re that different operating systems and CPUs But the point is that before you run software and can double click that icon it s been converted from something like that source code to zeros and ones that we don t understand but the computers do This is an example of the language that was cited C Note on s
57. or integer and main are common keywords that you might see But a computer does not understand this A programmer understands this A computer understands only what alphabet STUDENT inaudible response DAVID MALAN Binary or zeros and ones And so before you can actually run a program you typically have to do what s called compile it which literally means to take your source code and convert it effectively into patterns of zeros and ones that a computer does understand So when you are a programmer and you re writing Windows software you ll probably use a program like Visual Studio or Visual C These are just programs that Microsoft wrote that allow other people to themselves write software And among Visual Studio s features is the ability to make this red arrow happen to convert what a programmer has written into the zeros and ones that an x86 CPU an Intel CPU understands 24 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School Similarly might you write code very much like this on a Mac or on a Linux computer But if you want that program that source code to be translated into zeros and ones that a Power PC s CPU understands or some Motorola CPU well you need to use a compiler that s specific to that CPU So this is why when you buy shrink wrapped software off the shelves even though that s becoming decreasingly common with how much software you can buy and downl
58. ore procedurally Dare I ask for a volunteer This volunteer doesn t need to speak but they do at the risk of terrifying need to sit here Anyone at all Nonspeaking role This is a tough one I know Anyone Oh and you have to take your shoes off STUDENT inaudible response DAVID MALAN Anyone Dawne Yes Ah Dawne is a trouper in this class If we could applause All right Dawne is saving one of you from coming down and taking your shoes off So in order to transition from this very high level discussion of programming let s introduce something that s specifically known as pseudocode Typically when writing a program certainly for the first time one won t just start by writing Java code or C which are common languages They ll rather sketch out ideas on paper a framework an outline if you will just like a high school essay might be outlined in English or shorthand Well pseudocode is something that typically looks very much like English but it also has a mix of some programming syntax some shortcuts that we ll highlight in the moments that follow which really get across the idea of the algorithm you want to implement without letting you get bogged down in some of the details the syntactical details of the language which might take a little more getting used to Everybody understands English though Everyone should hopefully understand pseudocode You re looking very awkward or at least amused So um we ll
59. ou can nest things Because these blocks in addition to snapping together if they re logically consistent they ll also grow to fit each other so that you can fit as many statements inside of them as you want So even though this starts to indent a little misleadingly this is Scratch s way of allowing you to do if something s true do this else if something else is true do that else if something else altogether is true do this other thing And you can nest these things as many times as you want Well let s use these In hello4 scratch and hello5 scratch we have these two examples 38 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School Note on screen slide 16 On the left tell me in a sentence what will the program at the left do It s almost silly right Because we ve hard coded in something here STUDENT inaudible response DAVID MALAN Perfect It s silly because 1 is always less than 2 so it s sort of a weird use of an expression But that s fine The behavior of this program is very deterministic It always meows Well let s quickly whip this one up rather than opening it I m going to make a new project Get that Control structure when the green flag is clicked I need to then grab an if construct So I m going to drag this from down here Notice you can scroll down to some others if and notice it will be snap right into place Now I need that Nu
60. ow Note on screen slide 8 STUDENT inaudible response 23 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School DAVID MALAN Not Java What s another you ve heard of STUDENT inaudible response DAVID MALAN Not Basic What s another STUDENT inaudible response DAVID MALAN Not C What s one more STUDENT inaudible response DAVID MALAN Doesn t exist Well not really Well think about it Not JavaScript but another popular one So there s not really a C There s C and then har har har C was the next version of C and you know what plus plus means It means increment So this is an example of a language called C It was one of the earliest high level languages which simply means this was among the earliest languages that people could program computers without dealing with punch cards without writing out zeros and ones themselves without writing out very esoteric instructions that only a computer s CPU would understand C was among the first languages that allowed you to sort of write almost at that pseudocode level where you can sort of write as you might speak but granted you use a bit more arcane syntax than we did for our socks example This is an example of source code Whenever you write a program in a language like C it tends to look like this Parentheses are common Squiggly braces are common Words like print and exit and int f
61. page phonebook if you apply a logarithmic algorithm and that makes it sound scary right All I m doing is common sense really You can search that whole phonebook assuming it s alphabetized in simply ten steps And contrast that obviously with the linear algorithm just as correct just as obvious an algorithm perhaps but so much more expensive 100 times more expensive Put it even more in perspective Suppose that the phonebook actually had one million pages and you applied a linear algorithm Worst case how many steps is a linear algorithm going to take A million right Easy one What about the logarithmic one the divide and conquer approach STUDENT inaudible response DAVID MALAN Not that bad This begs the question how many times can you divide a million by two Twenty 2 is the same thing as 2 2 which We just said that 2 is 1 024 So 1 024 1 024 1 000 1 000 gives you roughly a million If that math went over your head it s okay You can work it out later But the point is twenty steps to search a million page phonebook Now consider the relevance to today s world right Millions billions of Web pages on the Internet Google and search engines like that search it Clearly Google and the like are not using linear search algorithms And daresay they re not even using just these sorts of logarithmic type search engines But the point is they are presumably doing something that s more than naiv
62. retty much have And just Okay Notice I ve never let go of the baby All right Step 7 STUDENT inaudible response DAVID MALAN Gently put the baby back down All right that s not bad STUDENT inaudible response DAVID MALAN Without moving the baby remove the diaper Carefully STUDENT inaudible response DAVID MALAN Something tells me this was not intelligent But it s not done Step 9 STUDENT inaudible response DAVID MALAN Throw diaper All right Throw diaper away into All right step STUDENT inaudible response DAVID MALAN Pick up new diaper All right we re moving along nearing the end 12 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School STUDENT inaudible response DAVID MALAN Prepare diaper We re calling another function all together What does it mean to prepare a diaper STUDENT inaudible response DAVID MALAN Open it up All right Done STUDENT inaudible response DAVID MALAN All right Gently wash the baby Let s assume that just happened Next STUDENT inaudible response DAVID MALAN On baby Under baby All right and actually I m going to ask The things the straps Where do these go STUDENT inaudible response DAVID MALAN I m not even joking around Front or back STUDENT Back DAVID MALAN Back All right All right Whoops STUDENT inaudible response DAVID MALAN All
63. ross in your own implementation of your own project for Problem Set 8 as you know How can I get two sprites to talk to one another Well you might be able to do so by way of this event handling You ve already seen Oscartime In Problem Set 8 what you ll be tasked with is in addition to debugging a little program and teaching us the staff how to make peanut butter and jelly sandwiches you will quite simply be challenged to implement something in Scratch And you saw a glimpse of the gallery from our Summer students what they implemented I promised to show one of those as well We looked at the soccer game which was very interesting in a quite impressive program that that student submitted I wanted to give you an example of an animation as well that s not interactive but sort of tells a story This too is going to be in this same directory that we ve been playing with I m going to make it full screen and I m going to hit Play There s not much sound That s a bug right We re not at the end yet But it s okay fairytale animation plays You can make sprites zoom in as this student happened to do that is grow in size Dragon sprite appeared at left dragon sprite roars Knight appears at right princess sprite screams 51 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School One of the neat things is that she used a She used a pseudorandom number ge
64. rst name and last name your street address your credit card number and so forth You fill out a form But immediately should occur to you the fact that what if a user messes up or forgets a field or chooses not to fill out a field That s sort of an event you have to handle And error checking is a huge part of writing software certainly software that has to interact with humans Because where I was going with that was this Suppose that the user forgets his zip code or forgets the three or four digit secret code associated with his or her credit card number clicks Submit Well you probably don t want to go ahead and forge ahead and say All right your item s en route to you in seven to ten business days Well no you want to check that the input was correct You want to go back and check allow the user another chance to input that Moreover if someone types in alphabetical characters to their phone number well that might seem kind of stupid right Who would do that But someone might Might be an accident They might be trying to hack your site figure out are there bugs that I can exploit to take advantage of some security hole in your system You ve got to handle that too And one of the hardest parts or certainly most time consuming or frustrating parts about writing software that interacts with humans is dealing with every possible case Because the downside of not doing so is that either one your software breaks i
65. rvard Extension School Here s your opportunity to shine Pretend to open a sock drawer And we ll need you to take the Oh are these socks or are they like uh hosiery DAWNE Yeah DAVID MALAN Okay that s going to be awkward Yes no so we re going to pretend that Dawne has socks to put on That s okay we ll take what we got So Dawne has just pretended to open sock drawer Step 4 look for sock Your moment to shine All right if you find a sock then Here s a decision point Did you find a sock DAWNE Yes DAVID MALAN She found a sock so Line 5 is as they would say True Notice the indentation We do now everything that s indented below Line 5 Line 6 says put on sock Excellent socks_on_feet What could that be meaning STUDENT inaudible response DAVID MALAN Both feet Not quite This too is a programming shorthand notation that you ll often see STUDENT inaudible response DAVID MALAN Completed action Kind of STUDENT inaudible response DAVID MALAN Incrementing Excellent So it s a good inference socks_on_feet was our variable In many programming languages when you write a variable and then it just means add 1 to that value That s all 00 40 00 19 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School So socks_on_feet our variable has been incremented And this is our programmatic construct for remem
66. s which is effectively like two pixels both at the same time else if the cat is not touching the edge it s going to move just one step With that said if I wanted to speed up this cat which this is the same script what number could I change to speed up the cat s movement STUDENT inaudible response DAVID MALAN Yeah so let s change this 1 to 5 which means he s taking five steps at once Hit Play again and now he s really moving banging continues as cat hits wall Again simple change All right well let s introduce a second sprite Note on screen slide 19 All right now it starts to get fun if you don t just have a cat moving around But let s have for instance like a cat chasing the bird Well let s look at the bird first The bird is at your left And this is going to be one script belonging to one sprite the other script will belong to the other sprite So they operate sort of independently And at this point we can introduce the notion of a thread A thread is a common construct in many operating systems and really programs And a thread refers to the ability of a program to do two things at once If a program is multithreaded it can do two things at once So really we can think of these scripts as threads And if a program has two or more scripts both of which start running when you click the green flag it s as though your program has two or more threads because by definition they execute sim
67. s here from computer science worst case Remind me How many steps might a linear algorithm take to look up a phone number in a thousand page phonebook STUDENT inaudible response DAVID MALAN A thousand steps Easy right So you look on the x axis to a thousand maybe it s here and a thousand on the y axis ends up being here right A thousand pages a thousand steps a thousand units of time Well now let s talk in binary just because the math will work out a little cleaner Suppose the phonebook actually has 1 024 pages just so we have a nice power of two And now I apply not my linear algorithm but my much more clever logarithmic one the one that does if you recall divide and conquer split the problem in two throw half away split the remaining problem in two throw the other half way 5 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School That begs the question If you ve got 1 024 pages how many times can you split that value in two in two in two until you re left with just one page STUDENT inaudible response DAVID MALAN Good E 1 answer Heck of a lot less Let s put a number on it because this time it s impressive How many times can you divide 1 024 by the value 2 STUDENT inaudible response DAVID MALAN Not quite Too many STUDENT inaudible response DAVID MALAN Almost eight That would give us 256 Ten Literally you can search a 1 024
68. s makes sense right Because if we re putting on Dawne s socks for the first time in the day she presumably doesn t have socks on yet And in fact if you wouldn t mind removing the shoes and socks And then we ll be perfectly set to go in just a moment So it would make sense that socks_on_feet at the first thing in the day is equal to zero And so it s certainly not equal to two So in Step 1 we declared a variable and initialized it to zero socks_on_feet 0 Step 2 we begin what s called a loop which is a fundamental programming construct I know this is difficult It s a longwinded lecture isn t it DAWNE No I m okay I m fine DAVID MALAN Okay so Step 2 we launch into a loop which is quite intuitively a process that s going to repeat hopefully just two times Because she has two feet we ll hopefully have two socks and we ll be done And you ll be out of this awkward moment Step 3 So notice the indentation So this too is sort of a detail of many programming languages where to convey the idea that this next line should only happen if the previous line was true or applicable we indented to sort of show that now the implication is that everything from line 3 to line 17 should proceed to execute And Chris as we do this if you wouldn t mind spending time on the board too that would be great So Step 3 open sock drawer 18 of 53 Computer Science E 1 Understanding Computers and the Internet Ha
69. s what they did up front Right They solved the search problem better And efficiency was certainly among the challenges they faced All right so where could we be going with this Well it s one thing to talk about things in terms of algorithms and sort of spoon feeding algorithms to you It s another thing to begin writing programs yourself implementing algorithms yourself because precision becomes ever more important And by precision in this context I mean being so precise and essentially acknowledging that that computer no matter how fast or expensive it is it s just a machine and it will only do what you tell it to do And therefore if you don t tell it to do something it s not just going to figure out what you meant or infer from other instructions you ve given it It s going to do nothing or do something incorrectly or just not work at all And so in programming as you ll see as you work on your Problem Set 8 being precise and being specific and being thorough becomes ever more important Because the downside of not being so is that your software just doesn t work How many of you have ever been running some program on your computer and all of sudden it hangs it freezes it slows to a crawl Well that s because somebody somewhere along the way screwed up It s not the computer s fault The computer is doing what it s told And so that error must fundamentally you know 99 times out of 100 if we ignore hardware issues and
70. sort of launched this whole process into motion So whenever you have a loop that means that eventually just like with our self counting algorithm for students you go back to wherever the line of code was that started that loop Well that was Line 2 And Line 2 says while socks_on_feet 2 20 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School Is socks_on_feet equal to 2 at this point or not equal to 2 STUDENT inaudible response DAVID MALAN It is equal to 2 because we incremented it twice which meant she has two socks on her feet So that means because there s nothing else in this program now the program is complete A round of applause if we could for Dawne for humoring us applause Excellent And now let s see Dawne is welcome to return to her seat if she d prefer DAWNE inaudible question DAVID MALAN There doesn t need to be It s okay Thank you Is there anything wrong with this program Can we debug it It looks like it executed perfectly You all recited the lines perfectly Dawne executed the lines perfectly It didn t look like we needed any there were any assumptions that led us astray So is there any potential mistake or bug as they say in this program that would cause unintended behavior STUDENT inaudible response DAVID MALAN Excellent You ve just offered a so called corner case What if she only has Dawne one sock i
71. take either Here s an algorithm with which Dawne will be able to put on hopefully her socks Note on screen slide 6 We ve numbered the lines for the sake of discussion but a lot of programming environments will show you line numbers for your own sense of discussion with other people perhaps Line 1 says let socks_on_feet 0 And notice I ve used underscores instead of spaces to sort of imply that socks_on_feet is some self contained entity And that s going to be called a variable Right In algebra there s variables x y z In programming you can call variables x y z 17 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School Variables simply hold values whether it s a number or a word or a letter or a sentence But it s more useful in programming typically to name your variables not x y or z but rather something that s a little more instructive I figure we ll both be on camera rather than just one of us here So Step 2 while socks_on_feet 2 What do you think that s getting at What does that mean STUDENT inaudible response DAVID MALAN Perfect So how do we begin this next step in our algorithm this next action Well do so by realizing that socks_on_feet does not equal is programming shorthand for not equals because there s no key on the keyboard that s an equals sign with a line through it So that means not equals 2 And thi
72. ter Science E 1 Understanding Computers and the Internet Harvard Extension School A lot of the semicolons and the parentheses and the squiggly braces we ve seen already tonight in these other programming languages you don t have it in Scratch Rather you do exactly the sorts of ideas that we ve been talking about tonight But you do it by way of puzzle pieces that only snap together if it logically makes sense for them to do so We used this for the first time this past summer in a Harvard Summer School course with a similar group of diverse students from high school age to college age to retired age And we got some fantastic projects out of this literally after the students having just sat down with Scratch after a bit of introduction from me and then taking the ball and running with it For instance just to put this into perspective among the types of submissions we got this past summer I m just going to whisk us away briefly to the Website for that course which was Computer Science S 1 Great Ideas in Computer Science We had a little gallery on the course s Website You don t have to quite remember how to get back here But just to give you a sense this page is full of screen shots of the types of animations or games or programs that students this past summer wrote that give you a sense of sort of the playfulness of the language But mind you these were students many of whom had never once programmed before But litera
73. ter with bogus input you can compel it to crash that s usually a sign of some bug running in that computer s software And whenever there s a bug there s a potential for exploiting it and taking over the machine accessing the machine deleting data from the machine It really varies And so a lot of hacking in the beginning is just futzing around see what breaks And once you see Ah something broke then home in on that and see in what way is it breaking Incredibly important precision and thinking these things through Yes STUDENT inaudible response DAVID MALAN Good question So legality and so forth If you started banging on Amazon s site just to see what would happen could someone come knocking on your door and you know do something about it I think the short answer is that it depends A well written Website should detect that you re banging on it and for instance making too many queries of it too quickly And ideally that Website should respond by just ignoring you or locking you out And some Websites do this Even FAS If you forget your FAS password and you try guessing every password you ve used in recent years but you try too many guesses a system like FAS like many good systems will say You know what You re locked out for fifteen minutes because we are suspicious of this behavior Now that doesn t permanently lock you out It might annoy you But the upside is that now the system is protected against a
74. the horses and stuff like that Perhaps not that advanced but we ll show you how to cteate some basic Flash animations and other neat stuff DAVID MALAN Awesome thanks And one of the goals for Eugenia s Flash Workshop will be beyond just empowering you to make these animations is perhaps to do it with an eye toward your Final Project If you would like potentially to incorporate whatever Flash you produce in the Workshop you re by all means welcome to incorporate it into your Final Project And after Exam 2 next week like last time we ll do a little special something With that said I need a volunteer who s good at kicking goals Or bad Bad is even funnier So come on down if you re bad too You get to interact with one of the games submitted by one of our students this past summer It s a penalty goal kicking contest of sorts Anyone at all More fun I m very good at it It s more fun if one of you who ve never tried it before plays Dawne will come up Dawne is on the payroll now Meet your new teaching fellow Dawne Come on down What I m going to do is currently just to give you a sense and you ll get a tour of sorts of the Scratch program But long story short I ve fired up the software I opened the student s program and I ll show you how to do that in a moment And then I simply clicked this little easel icon down here which makes the program full screen And Scratch is very nice in that to run a program you click the
75. to fall from the sky And you simply using the touchpad and mouse need to pick up the trash by clicking it and drop it into Oscar s trashcan With that said It s about a two minute game The limit will be clear based on the song And here we go STUDENT Just click right here Oscartime music starts playing DAVID MALAN Yup you re going to move the mouse around STUDENT Okay Oscar singing Oh I love trash DAVID MALAN And just click the trash and drag it to the trashcan Oscar Anything dirty or dingy or dusty DAVID MALAN Now it s on the ground so now you can It s easier now It gets easier Yup Click on the tra Yup there you go Oscar Oh I love trash DAVID MALAN Oh you have to use the left button STUDENT Oh okay DAVID MALAN There you go All right Whoo Oscar and the laces are torn A gift from my mother the day I was born DAVID MALAN There you go there you go Oscar I love it because it s trash Oh I love trash Anything dirty or dingy or dusty Anything ragged or rotten or rusty Yes I love trash Here s some more rotten stuff 31 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School I have here some newspaper thirteen months old I wrapped fish inside it it s smelly and cold But I wouldn t trade it for a big pot of gold I love it because it s trash Yes I love trash DAVID MALAN It s almost over Oscar
76. toshop and I made the icon And I downloaded a picture of Oscar from the Web and so forth Did the easy stuff right It s not quite programming yet 01 50 00 But then I simply said All right what s the first step of this program I minimally need to get trash to start falling So I found a costume that looked like a ball of trash and I simply implemented a sprite with a script that chose a random location at top was originally invisible then eventually made itself visible And then just started moving by moving one step waiting half a second a millisecond for instance then moving again 52 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School And once I was comfortable that All right I got the trash falling Now let me try to introduce the soundtrack with another sprite and have the song start playing And now let me go back to that original sprite and introduce a wait timer And I looked at my watch and I said All right I want the trash to start falling after twelve seconds So I added a wait statement inside of this block for the trash and said All right don t start falling immediately Wait twelve seconds And now I had the song synchronized with the first piece of trash and so forth So when you approach your Scratch problem set Problem Set 8 don t try to bite off the whole thing at once baby steps not because it s necessarily hard but because you ll drive
77. ultaneously In the real world or in the real virtual world when you go to the File menu in Microsoft Word and say Print usually you immediately can get right back to typing even though it might take five minutes to print out your twenty page essay Well that s because Microsoft Word is multithreaded It has at least two threads one of which allows you to keep typing your essay the other thread allows it to go off and print 45 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School And that s a bit of a white lie because there s also what are called print spoolers built into printers and so forth these days But for the most part most every program these days that does a lot things seemingly at once is multithreaded So is Scratch multithreaded and you use threads by just writing multiple scripts So this one at left it s starting to get a little bigger But again still pretty self explanatory The Stage in Scratch is laid out like in Photoshop in terms of pixels And you ll be able to experiment either at home or in next week s Section with exactly where 0 0 is and where 100 0 is It s pretty much a Cartesian plane with x coordinates and y coordinates So what this left script for the bird is doing is it s moving the bird at the moment the green flag is clicked to location 150 150 So we ll see in a moment what that means for the bird where he appears
78. ust infer from the code What about the middle example hello7 scratch What is that going to do STUDENT inaudible response DAVID MALAN Good If the cursor s on top of the sprite then it will meow and otherwise it won t So already even with just introducing a couple of new constructs and even though this program is only six or so blocks long notice if I open hello7 scratch which is here notice that when I click the green flag nothing happens But now 41 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School cat sprite meows we sort of have cat sprite meow granted simple but a program that allows you to pet the cat And every time cat sprite meows you pet it it meows at you So already baby steps granted But we re already building more interactive programs again with terribly few blocks with terribly straightforward syntax What about hello8 scratch at far right STUDENT inaudible response DAVID MALAN It s always going to play a sound every two seconds but STUDENT inaudible response DAVID MALAN Good And I ll summarize by saying if you pet the sprite this time he does something a little different So I m going to hit Play cat sprite meows Just going to meow a lot every two seconds If you try to pet this cat though cat sprite roars All right Now get it goes back to meowing Pet it again cat sprite roars
79. vided their phone number or their name and last name and so forth It can check the format of their email address And it can do a whole bunch of other things Increasingly is JavaScript being used to make really interactive Websites The problem is it s an ugly looking language It is difficult to introduce to a class of ostensibly nonprogrammers in just an hour s time or so And you can t also do things that are terribly glamorous with it And certainly not after just being exposed to it once What we decided to do then this year is introduce you to something that is a whole lot of fun This is a programming language called Scratch And as I mentioned in an email last night Scratch is a language that s being developed currently by MIT s Media Lab down the road Scratch officially has been developed for after school computer clubhouses for students particularly in disadvantaged communities whereby these computer clubhouses offer kids an opportunity to congregate together to hang out in a fun friendly safe environment and so forth and actually do something intellectually stimulating to solve problems to implement games to experiment with art and so forth And so what Scratch is touted as is a programming language that makes it really easy and really fast to implement animations games art and other such fun projects And the neat thing about it is is that it s a graphical programming environment 27 of 53 Compu
80. what to do STUDENT inaudible response DAVID MALAN Find the snaps Okay I mean here too you re not being very Here there are Step 2 STUDENT inaudible response DAVID MALAN I want something along the lines of Be gentle with the baby or Lift the baby Look for snaps Anything STUDENT inaudible response DAVID MALAN You re making this too easy STUDENT inaudible response DAVID MALAN All right gently I ll take that All right gently STUDENT inaudible response DAVID MALAN The point clearly has not come across yet So we re going to keep doing this until it does STUDENT inaudible response 11 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School DAVID MALAN I like that I like that All right I ve found them on the back of the clothes STUDENT inaudible response DAVID MALAN None in between the legs Just on the back It s like a sleeper STUDENT inaudible response DAVID MALAN Without moving the baby Okay It s technically Velcro but that s okay The Velcro Okay done STUDENT inaudible response DAVID MALAN Okay not bad Without harming the baby gently remove the clothes to expose the diaper All right STUDENT inaudible response DAVID MALAN All right STUDENT inaudible response DAVID MALAN All right Without crushing the cranium All right You didn t say that the first time All right so now we p
81. xt thing I wanted to do is what I wanted to say something Well we don t have to make precisely that program So let s go to Looks which somewhat counterintuitively is where say is because saying in this domain means showing something change the look So let s do this say hello for something In fact let s make this a little more interesting What do you want him to say STUDENT inaudible response 36 of 53 Computer Science E 1 Understanding Computers and the Internet Harvard Extension School DAVID MALAN I survived E 1 And for let s say 1 second All right next And notice too Though I dragged it to the area I dragged it close enough that you got this white line which says Hey this will fit here And as soon as you let go they snap together like true puzzle pieces All right let s go over to Control And you ll get better at knowing where these things are with practice Let s wait 1 second And then let s go back to Looks And let s say something different now Let s say say Okay I didn t for 2 seconds and then we ll leave it at that All right let s go ahead and run this Not too bad right Simple but incredibly easy to build Let s look back at this example Note on screen slide 13 The example at right is going to do what Infer from the puzzle pieces alone In one sentence what is this program at right going to do hello3 scratch STUDENT inaudible response DAVID M

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