NetLogo 3.1 User Manual: Tutorial #3: Procedures
Contents
1. Now that you ve seen the uphill algorithm work in the model let s go through the new primitives involved If you haven t run the model yet since writing move to local max give it a try There are three new primitives here uphill of and patch ahead uphill elevation finds the heading to the patch with the highest value of elevation in the patches in a one patch radius of the turtle Then through the use of the command set heading the turtle sets its heading to that direction elevation of patch ahead 1 has each turtle look at the variable elevation in the patch on which the turtle would be if it went forward 1 If the test reports true the turtle moves itself forward 1 The test is necessary because if the turtle is already on the peak we don t want it to move off it Go ahead and type that in but before you test it out by pressing the go button ask yourself this question what do you think will happen Try and predict how a turtle will move where it will go and how long it ll take to get there When you re all set press the button and see for yourself Surprised Try to understand why the turtles converge to their peaks so quickly Maybe you don t believe the algorithm we ve chosen works correctly There s a simple procedure you can make to test it write a procedure recolor patches so that it says to recolor patches ask patches Sel SE elevation pycor pcolor scale color green elevation m2. Forever Commands setup Display name Action key Force view update after each run Checking this box produces smoother animation but may make the button run more slowly 4 Press OK to dismiss the dialog box Now you have a button called setup It will execute the procedure setup when pressed which once we define it will do just that set up the NetLogo world At this point both the new button and the Interface tab have turned red That s because there is no procedure called setup If you want to see the actual error message click on the button Finterface Information Procedures j abe Button ES Slider its Switch a Chooser P amp S Mon 24 gt gt Mm gt 398 cat 30 setup Now switch to the Procedures Tab and create the setup procedure shown below Notice that the lines are indented different amounts A lot of people find it very helpful to indent their code in a way at least similar to how it s done here It helps them keep track of where they re at inside of a procedure and makes what they write easier for others to read as well Interface Information _ Procedures i SS r LP Find j of Check i Procedures w to setup ca crt 100 ask turtles fd random screen edge x end One line at a time to setup begins defining a procedure named setup ca is short for clear all you can also spell it out if you
3. elevation lt lowest 100 set pcolor black ct ct end initializes the turtles to setup turtles crt number ask turtles if shade of green color set color red fd random max pxcor end RUN TIME PROCEDURES 7 main program control to go set turtles moved false move to local max do plots if not turtles moved stop end 7 each turtle goes to the highest elevation in a radius of on to move to local max ask turtles set heading uphill elevation if elevation of patch ahead 1 gt elevation fd 1 set turtles moved true end to do plots set current plot Turtles at Peaks plot count turtles with elevation gt highest 100 end
4. end Be careful of the spacing around the In Tutorial 2 we used red 2 with spaces in order to subtract two numbers but here we want move turtles without spaces The combines move and turtles into one word Line by line ask turtles commands says that each turtle should execute the commands in the brackets set heading random 360 is another command that uses a reporter First each turtle picks a random integer between 0 and 359 random doesn t include the number you give it as a possible result Then the turtle sets its heading to the number it picked Heading is measured in degrees clockwise around the circle starting with 0 degrees at twelve o clock north fd 1 Each turtle moves forward one step in the new direction it just set its heading to Why couldn t we have just written that in go We could but during the course of building your project it s likely that you ll add many other parts We d like to keep go as simple as possible so that it is easy to understand Eventually it could include many other things you want to have happen as the model runs such as calculating something or plotting the results Each of these sub procedures could have its own name The go button you made in the Interface tab is a forever button meaning that it will continually execute its code until you shut it off by clicking on it again After you have pressed setup once to create the turtles press the go button
5. them If a turtle starts off in one corner of the world on a hill and there s a mountain ina different corner the turtle will never find the mountain To find the mountain the turtle would have to go down off the hill first but in our model turtles only move up Notice that the individual turtles don t use highest anywhere The turtles just look at elevations close to them and go the highest point they can see Before trying something else it d be nice if we could have some other more precise method for evaluating the turtles performance Fortunately NetLogo allows us to plot data as we go along To make plotting work we ll need to create a plot in the Interface tab and set some settings in it Then we ll add one more procedure to the Procedures tab which will update the plot for us Let s do the Procedures tab part first Change go to call the new procedure we re about to add to go move to local max do plots end Now add the new procedure What we re plotting is the number of turtles who ve reached our peak zone within 1 of the highest elevation at some given time to do plots set current plot Turtles at Peaks plot count turtles with elevation gt highest 100 end Note that we use the plot primitive to add the next point to a plot but before doing that we need to tell NetLogo which plot we want since later our model might have more than one plot Thus we re plotting the number of turtles wit
6. want This command will blank out the view initialize any variables you might have to 0 and remove all turtles Basically it wipes the slate clean for a new run of the project crt 100 will then create 100 turtles crt is short for create turtles If the turtles didn t move after this command is given each of these turtles would begin on the center patch at location 0 0 You would only see what looks like one turtle they d all be on top of each other lots of turtles can share the same patch Only the last turtle to arrive on the patch would be visible Each of these newly created turtles has its own color its own heading All of the turtles are evenly distributed around the circle ask turtles tells each turtle to execute independently the instructions inside the brackets Note that crt is not inside the brackets If the agent observer turtle or patch is not specified using ask the observer runs it Here the observer runs the ask but the turtles run the commands inside the ask fd random max pxcor is a command that also uses reporters Reporters as opposed to commands are instructions that report a result Each turtle will first run the reporter random max pxcor which will report a random integer at least 0 but less than max pxcor the largest patch coordinate in the x direction It then takes this number and goes fd short for forward that number of steps in the direction of its heading The steps are the same
7. Tutorial 3 Procedures NetLogo 3 1 User Manual In Tutorial 2 you learned how to use command centers and agent monitors to inspect and modify agents and make them do things Now you re ready to learn about the real heart of a NetLogo Model the Procedures tab This tutorial leads you through the process of building a complete model built up stage by stage with every step explained along the way You ve already been exposed to the three types of agents you can give commands to in NetLogo turtles patches and the observer As you start to write your own procedures it ll be helpful to keep in mind how people usually think of these three different kinds of agents The turtles and patches usually don t use information about the whole world They mostly use information about what s close to them The observer on the other hand typically uses and accesses the whole world Also while patches can t move and often represent some sort of environment turtles can move around in the world Setup and Go To start a new model select New from the the File menu Then begin making your model by creating a once button called setup Here s how to make the button 1 Click on the button icon in the Toolbar 2 Click where you want the button to be in the empty white area of the Interface tab 3 When the dialog box for editing the properties of the button opens type set up in the box labeled Code 808s Button Agent s Observer HA _
8. Watch what happens Turn it off and you ll see that all turtles stop in their tracks We suggest you start experimenting with other turtle commands You might try typing turtles gt pendown into the Command Center and then pressing go Another thing to try is changing set heading random 360 to rt random 360 inside of move turtles rt is short for right turn Also you can try changing set heading random 360 to It random 45 inside of move turtles Type commands into the Command Center like set colorred or add them to setup go or move turtles Note that when you enter commands in the Command Center you must choose turtles gt patches gt or observer gt in the popup menu on the left depending on which agents are going to execute the commands You can also use the tab key which you might find more convenient than using the popup menu turtles gt commands is identical to observer gt ask turtles commands and patches gt commands is identical to O gt ask patches commands Play around It s easy and the results are immediate and visible one of NetLogo s many strengths Regardless the tutorial project continues Patches and Variables Now we ve got 100 turtles aimlessly moving around completely unaware of anything else around them Let s make things a little more interesting by giving these turtles a nice background against which to move Go back to the setup procedure We can rewrite it as follows patches own elevati
9. case the patch changes its color If the comparison reports false the patch skips over the commands inside the brackets These ifs cause all patches whose value of e evation is NEAR to the highest within about 1 for our values change their color to white and all patches whose values are NEAR to the lowest become black We want this so that they ll be easier to see You can make a couple of quick changes here if you wish they won t affect the rest of the model For example instead of saying set pcolor white and set pcolor black you can say set pcolor blue and set pcolor red or whatever other colors you may wish Also you can change the range of highest peaks and lowest peaks by changing the number 100 to some other number After this create two monitors in the Interface tab with the Toolbar You make them just like buttons and sliders using the monitor icon on the Toolbar Name one of them highest and the other one lowest The reporters you ll want in each of them happen to be highest and lowest as well If you want to learn more about reporters you can look them up in the NetLogo Programming Guide Now every time you click setup and redistribute the values of elevation you ll know exactly what the highest and lowest elevations are and where they can be found E E setup go m highest lowest j 0 0 Okay Finally we re ready to start hill climbing To rehash we ve got some turtles ra
10. ches In fact we even have an idea for a project here Notice that we called the patch variable elevation and that our landscape sort of looks topographical We re going to have our turtles do what is called hill climbing where every turtle seeks to find the highest elevation it can In order to do this we will learn how to write more complex instructions Go to the Command Center and type O gt show max values from patches elevation and show min values from patches elevation These two reporters will respectively search over all the patches to return to you the highest elevation and the lowest These commands work like this you can read about them in the NetLogo Primitives Dictionary Look up values from in the dictionary It shows values from AGENTSET expression and says it returns a list In this case it looks at the expression elevation for each agent in the agentset patches and returns all of these as a list of elevations Look up min in the dictionary It shows min ist and says it s a reporter So it takes the list of elevations and reports the smallest value Show displays this value in the command center We will use these reporters max values from patches elevation and min values from patches elevation in our model Just in case we end up needing the highest and lowest elevation in several places in our procedures let s make a shortcut We ll do a little extra work now so that if we n
11. d Setup turtles is exactly what we were doing in the old setup procedure After typing all of this in press the setup button back in the Interface tab Voila A lush NetLogo landscape complete with turtles and green patches appears After seeing the new setup work a few times you may find it helpful to read through the procedure definitions again Here s a way for you to see what diffuse does Return to the Procedures tab and use a semicolon to deactivate the diffuse command like this diffuse elevation 1 Semicolons are very useful in writing procedures They can be used as above to save you from having to delete code to try something else out and then having to rewrite them Also they can be used to add some explanatory text to your procedures A lot of people like to do this to make their procedures more readable to others Notice that all the text to the right of asemicolon becomes gray Press setup again looks different doesn t it This is because as mentioned above diffuse has each patch share its value of e evation with all its neighbors by having every patch reset its value of elevation to a new value that depends on the value of elevation all around it For further explanation of how diffuse works go to the Primitives Dictionary if you d like Also it may help to toy with the values being passed to it and see what happens We re now prepared to create some kind of dialog between the turtles and the pat
12. d executes the commands that number of times here five times Try it out and look at the landscape i e press setup and see what you think Then press go and watch the turtles behavior Remember that the lighter the patch the greater the elevation Obviously fewer peaks make for an improvement in the turtles performance On the other hand maybe you feel like this is cheating the turtles really aren t doing any better it s just that their problem was made easier True enough If you call repeat with an even higher number 20 or so you ll end up with only a handful of peaks as the values become more evenly distributed with every successive call Watch the values in the monitors In order to specify how smooth you want your world to be let s make it easier to try different values Maybe one time you ll want the turtles to try and solve a hard world and maybe another time you ll just want to look at an easy landscape So we ll make a global variable named smoothness Create a slider in the Interface tab and call it smoothness in the editing box The minimum can be 0 and the maximum can be 25 or so Then change your code to repeat smoothness diffuse elevation 1 Experiment with the turtles performance in different terrains We still haven t even begun to solve the problem of getting all the turtles to the highest elevation though So far we ve just been getting the turtles to the highest point that s near
13. eed these values later we ll have a shortcut to use First at the top of your code right after the patches own declaration declare two global variables as such globals highest the highest patch elevation lowest the lowest patch elevation Notice the use of semicolons here Although the names of the global variables are descriptive the semicolons allow us to describe the variables even more Global variables can be used by all the agents in the model In particular patches can use highest and lowest in the setup patches procedure We need to store the highest and lowest elevations in these global variables once and then everyone will have quick access to them after that Write to setup patches ask patches set elevation random 10000 diffuse elevation 1 ask patches set pcolor scale color green elevation 1000 9000 set highest max values from patches elevation set lowest min values from patches elevation ask patches if elevation gt highest 100 set pcolor white if elevation lt lowest 100 set pcolor black end Now we have saved the highest and lowest points in our terrain and displayed them graphically Look at the last two commands the if commands Each patch when it runs these commands compares its own value of e evation to our global variables highest and lowest If the comparison reports true the patch executes the commands inside the brackets In this
14. hin 100 units of our maximum elevation at some given point in time The plot command moves the current plot pen to the point that has x coordinate equal to 1 greater than the old x coordinate and y coordinate equal to the value given in the plot command in this case the number of turtles whose elevation is within 100 of highest Then the plot command draws a line from the current position of the plot pen to the last point it was on In order for set current plot Turtles at Peaks to work you ll have to adda plot to your model in the Interface tab then edit it so its name is Turtles at Peaks the exact same name used in the code Even one extra space will throw it off it must be exactly the same in both places Note that when you create the plot you can set the minimum and maximum values on the x and y axes and the color of the default plot pen pick any color you like You ll want to leave the Autoplot checkbox checked so that if anything you plot exceeds the minimum and maximum values for the axes the axes will automatically grow so you can see all the data Now reset the project and run it again You can now watch the plot be created as the model is running If you notice that your plot doesn t look exactly like the picture below try to think about why it doesn t look the same If you think it s because go remains pressed until you manually unpress it we ll fix that problem by the end of the tutorial Remember that we ke
15. in pycor max pycor end Press setup The model looks the same as it did before because recolor patches hasn t been run yet Instead of making a button that calls your testing procedure let s do something different Type observer gt recolor patches into the command center the procedure gets called Now when you press go see that the turtles all head for the highest elevation the top of the screen Another common tool to see what s going on is to write turtles gt pd in the Command Center Then each turtle traces its path with its color This will show you where the turtle has been Our turtles rapidly arrive at local maxima in our landscape Local maxima and minima abound in a randomly generated landscape like this one Our goal is to still get the turtles to find an optimal maximum which is one of the white patches Part of the problem is that our terrain is terribly lumpy Every patch picked a random elevation and then we diffused these values one time This really doesn t give us a continuous spread of elevation across the view as you might have noticed We can correct this problem to an arbitrary degree by diffusing more times Replace the line diffuse elevation 1 with repeat 5 diffuse elevation 1 The repeat command is another way for NetLogo to loop besides making a forever button which you already know how to do Repeat takes a number here 5 and some commands here the diffuse command an
16. ndomly spread out from the origin and we ve got a landscape of patches whose primary attribute is their elevation Lastly we have two kinds of tools to help us understand the patch landscape each patch has a color depending on its value of elevation and we have a pair of monitors telling us what the highest peak and lowest valley are What we need now is for the turtles to wander around each trying to get to the patch that has the highest elevation An Uphill Algorithm Let s try a simple algorithm first We ll assume three things 1 that the turtles cannot see ahead farther than just one patch 2 that each turtle can move only one square each turn and 3 that turtles are blissfully ignorant of each other Before we had a procedure move turtles like this to move turtles ask turtles set heading random 360 fd 1 end But now we don t want them to move randomly about We want each turtle to look at the elevation of each patch directly around it and move to the patch with the highest elevation If none of the patches around it have a higher elevation than the patch it is on it ll stay put This new procedure should replace move turtles inside of go Type in the following code and run it once or twice 7 each turtle goes to the highest elevation in a radius of on to move to local max ask turtles set heading uphill elevation if elevation of patch ahead 1 gt elevation fd 1 end
17. on to setup ca setup patches setup turtles end The line at the top patches own elevation declares that we have a variable for the patches called elevation Our setup patches procedure that we haven t defined yet will then use this variable We also still need to define setup turtles as well but for now here s how to define setup patches to setup patches ask patches set elevation random 10000 diffuse elevation 1 ask patches set pcolor scale color green elevation 1000 9000 end The setup patches procedure sets the elevation and color of every patch First each patch picks a random integer between 0 and 9999 and sets its elevation variable to that number We then use an observer primitive diffuse that smoothes out the distribution of this variable over the neighboring patches Remember that primitives are built in procedures in NetLogo as opposed to procedures that you define Scale color is a reporter that uses the different values of elevation to assign colors to the patches In this case we re assigning different shades of green to all the patches Don t worry about the numbers given to diffuse and scale color just yet The larger elevation is the lighter the shade of green Low values of e evation will result in darker shades The only part remaining in our new setup that is still undefined is setup turtles to setup turtles crt 100 ask turtles fd random max pxcor en
18. patch elevation turtles moved so we know when to stop the model e At the end of the go procedure add a test to see if any turtles have moved to go set turtles moved false move to local max do plots if not turtles moved stop end e In move to local max if a turtle moves set turtles moved to true to move to local max ask turtles set heading uphill elevation if elevation of patch ahead 1 gt elevation fd 1 set turtles moved true end Finally what rules can you think of that would help turtles escape from lower peaks and all get to the highest ones Try writing them What s Next So now you have a nice framework for exploring this problem of hill climbing using all sorts of NetLogo modeling features buttons sliders monitors plots and the view You ve even written a quick procedure to give the turtles something to do And that s where this tutorial leaves off If you d like to look at some more documentation about NetLogo the Interface Guide section of the manual walks you through every element of the NetLogo interface in order and explains its function For a detailed description and specifics about writing procedures refer to the NetLogo Programming Guide Also You can continue with this model if you d like experimenting with different variables and algorithms to see what works the best what makes the most turtles reach the peaks Alternatively you can look at othe
19. pt Autoplot on This allows the plot to readjust itself when it runs out of room Turtles at Peaks Pens 139 6 is 3 ai E 3 0 Time 33 8 You might try running the model several times under different settings i e different values of smoothness and watch how fast the plot converges to some value and what fraction of the turtles make it to the top You may want to even try the same settings several times Some More Details There are a few quirks you may already have noticed Here are some quick changes you can make First we have a green landscape a naturally green turtle is going to be hard to see In the ask turtles block in setup turtles you can say if shade of green color set color red Second instead of always using 100 turtles you can have a variable number of turtles Make a slider variable say number 00 Slider Global variable smoothness Minimum 0 Increment 0 1 Maximum 20 Value 8 Units optional Cancel OK Then inside of setup turtles instead of crt 100 you can type crt number How does using more or fewer turtles affect the success value displayed by the plot Third when all the turtles have found their local maxima wouldn t it be nice for the model to stop This requires a few lines of code e Add a global variable turtles moved to the globals list globals highest 7 Maximum patch elevation lowest 7 minimum
20. r models including the many models in the Code Examples section of the Models Library or even go ahead and build your own model You don t even have to model anything It can be pleasant just to watch patches and turtles forming patterns or whatever Hopefully you will have learned a few things both in terms of syntax and general methodology for model building The entire code that was created above is shown below Appendix Complete Code The complete model is also available in NetLogo s Models Library in the Code Examples section It s called Tutorial 3 patches own elevation 7 elevation of the patch globals highest 7 Maximum patch elevation lowest 7 Minimum patch elevation turtles moved so we know when to stop the model We also have two slider variables number and 7 smoothness number determines the number of turtles and smoothness determines how erratic terrain becomes during diffusion of elevation 7 resets everything to setup ca setup patches setup turtles end 7 creates a random landscape of patch elevations to setup patches ask patches set elevation random 10000 repeat smoothness diffuse elevation 1 ask patches set pcolor scale color green elevation 1000 9000 set highest max values from patches elevation set lowest min values from patches elevation ask patches if elevation gt highest 100 set pcolor white if
21. size as the patches end completes the definition of the setup procedure When you re done typing in the code switch to the Interface tab and press your setup button You will see the turtles quickly spread out in a rough cluster Notice the density distribution of the turtles in the view Press setup a couple more times and watch how the turtles arrangement changes Keep in mind that some turtles may be right on top of each other Can you think of other ways to randomly distribute the turtles around the world Note that if a turtle moves off the edge of the view it wraps that is comes in the other side this is the default behavior it can be modified see the Topology section of the Programming Guide for more information Make a forever button called go Again begin by creating a button but this time check the forever checkbox in the edit dialog setup 0 969 Button Agent s Observer B _ Forever Commands go Display name Action key v Force view update after each run Checking this box produces smoother animation but may make the button run more slowly Cancel E Then add its procedure to the Procedures tab to go move turtles end But what is move turtles Is it a primitive in other words built in to NetLogo like fd is No it s a procedure that you re about to write right after the go procedure to move turtles ask turtles set heading random 360 fd 1
Download Pdf Manuals
Related Search