NetLogo 4.0.2 User Manual: Tutorial #1: Models
Contents
1. It d be nice if we had a easier way to track the changes in the model behavior over time NetLogo allows us to plot data as we go along That will be our next step Plotting 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 e Change setup to call the new procedure do plots which we re about to add to setup clear all setup patches setup turtles do plots end e Also change go to call the do plots procedure to go move turtles eat grass reproduce check death regrow grass do plots end e Now add the new procedure What we re plotting will be be the number of turtles and the number of green patches versus time At each time step a single run through the go procedure these values are added to the plot to do plots set current plot Totals set current plot pen turtles plot count turtles file Applications NetLogo 204 0 2 docs tutorial3 Atml Page 11 of 16 NetLogo 4 0 2 User Manual Tutorial 3 Procedures 4 21 09 11 28 PM set current plot pen grass plot count patches with pcolor green end Note that we use the plot command to add the next point to a plot However before doing that we need to tell NetLogo two things First we need to specify what plot we will be using since later our model might have more than one plot2. and second we need to specify which pen we want to plot with we will be using two pens on this plot The plot command moves the current plot pen to the point that has an X coordinate equal to 1 greater than the previously plotted X coordinate and a Y coordinate equal to the value given in the plot command in the first case the number of turtles and in the second case the number of green patches As the pens move they each draw a line In order forset current plot Totals to work you ll have to add a plot to your model in the Interface tab then edit it so its name is the same name used in the procedures Even one extra space in the name will throw it off it must be exactly the same in both places Create a plot using the plot icon on the Toolbar and click on an open spot in the Interface Set its Name to Totals see image below Set the X axis label to time Set the Y axis label to total amp O fy f i f j Name Totals X axis label time X min 0 X max 10 Y axis label totals Ymin 0 Y max 10 M Autoplot Plot Pens Choose Pen to Edit default H Delete Create Color black Mode Line FA Interval 1 0 M Show in Legend Custom Color Next you will need to create two pens With the Plot dialog box still open press the Create button in the Plot dialog to create a new pen Enter the name of this pen as turtles and press OK in the Enter Pen Name dialog see image below Pr
3. When you open a model all the sliders and switches are on a default setting If you open a new model or exit the program your changed settings will not be saved unless you choose to save them Note in addition to sliders and switches some models have a third kind of setting called a chooser The Wolf Sheep Predation doesn t have any of these though Gathering Information Plots and Monitors A purpose to modeling is to gather data on a subject or topic that would be very difficult to do in a laboratory situation NetLogo has two main ways of displaying data to the user plots and monitors Plots The plot in Wolf Sheep Predation contains three lines sheep wolves and grass 4 The grass count is divided by four so it doesn t make the plot too tall The lines show what s happening in the model over time To see which line is which click on Pens in the upper right corner of the plot window to open the plot pens legend A key appears that indicates what each line is plotting In this case it s the population counts When a plot gets close to becoming filled up the horizontal axis increases in size and all of the data from before gets squeezed into a smaller space In this way more room is made for the plot to grow If you want to save the data from a plot to view or analyze it in another program you can use the Export Plot item on the File menu It saves this information to your computer in a format that can by read back by spre
4. and these two variables To be able to make more changes to the colors of turtles and patches or shall we say cars and backgrounds we need to gain a little insight into how NetLogo deals with colors In NetLogo all colors have a numeric value In all of the exercises we have been using the name of the color This is because NetLogo recognizes 16 different color names This does not mean that NetLogo only recognizes 16 colors There are many shades in between these colors that can be used too Here s a chart that shows the whole NetLogo color space black 0 white 9 9 gray 5 8 9 9 9 red 15 18 19 199 orange 25 20 21 28 29 299 brown 35 30 31 35 39 39 9 yellow 45 40 41l l 47 48 49 499 green 55 50 51 57 58 59 59 9 lime 65 60 6l 68 69 699 turquoise 75 70 71 78 79 79 9 cyan 85 80 81 8 amp 8 89 89 9 sky 95 90 91 98 99 999 blue 105 101 107 108 109 109 9 violet 115 110 111 112 117 118 119 119 9 magenta 125 120 121 122 1 128 129 129 9 pink 135 130 131 132 136 137 138 139 139 9 To get a color that doesn t have its own name you just refer to it by a number instead or by adding or subtracting a number from a name For example when you type set color red this does the same thing as if you had typed set color 15 And you can get a lighter or darker version of the same color by using a number that is a little larger or a little smaller as follows e Choose patches from the popup menu in the Command Center o
5. procedure has most of the same commands as the old setup procedure e Switch back to the Interface tab e Press the setup button Voila A lush NetLogo landscape complete with turtles and green patches appears After seeing the new setup procedure work a few times you may find it helpful to read through the procedure definitions again Turtle variables So we have some turtles running around on a landscape but they aren t doing anything with it Let s add some interaction between the turtles and the patches We ll make the turtles eat grass the green patches reproduce and die The grass will gradually grow back after it is eaten We ll need a way of controlling when a turtle reproduces and dies We ll determine that by keeping track of how much energy each turtle has To do that we need to add a new turtle variable You ve already seen built in turtle variables like color To make a new turtle variable we add a turt 1les own declaration at the top of the Procedures tab before all the procedures Call it energy turtles own energy to go move turtles eat grass end Let s use this newly defined variable energy to allow the turtles to eat file Applications NetLogo 204 0 2 docs tutorial3 html Page 5 of 16 NetLogo 4 0 2 User Manual Tutorial 3 Procedures 4 21 09 11 28 PM e Switch to the Procedures tab e Rewrite the go procedure as follows to go move turtles eat grass end e Adda new eat gras
6. sliders using the monitor icon on the Toolbar Let s make the first monitor now e Create a monitor using the monitor icon on the Toolbar and click on an open spot in the Interface A dialog box will appear e In the dialog box type count turtles see image below e Press the OK button to close the dialog box file Applications NetLogo 204 0 2 docs tutorial3 Atml Page 7 of 16 NetLogo 4 0 2 User Manual Tutorial 3 Procedures 4 21 09 11 28 PM Monitor Display name Decimal places 3 Font Size 11 full precision is 17 Cancel OR 7 turtles isan agentset the set of all turtles count tells us how many agents are in that set Let s make the second monitor now e Create a monitor using the monitor icon on the Toolbar and click on an open spot in the Interface A dialog box will appear In the Reporter section of the dialog box type count patches with pcolor green see image below In the Display name section of the dialog box type green patches Press the OK button to close the dialog box Monitor green patches 1225 kount patches with pcolor green Display name green patches Decimal places 3 Font Size 11 full precision is 17 a en A A Here we re using count again to see how many agents are in an agentset patches is the set of all the patches but we don t just want to know how many patches there are total we want to know ho
7. the code right in with the code itself You might use comments to help others understand your model or you might use them as notes to yourself In the Procedures tab comments are gray so your eyes can pick them out easily turtles own energy for keeping track of when the turtle is ready to reproduce and when it will die to setup clear all setup patches setup turtles do plots end to setup patches ask patches set pcolor green end to setup turtles create turtles number uses the value of the number slider to create turtles ask turtles setxy random xcor random ycor end to go if ticks gt 500 stop stop after 500 ticks move turtles eat grass reproduce check death regrow grass tick increase the tick counter by 1 each time through do plots end to move turtles ask turtles right random 360 file Applications NetLogo 204 0 2 docs tutorial3 Atml Page 15 of 16 NetLogo 4 0 2 User Manual Tutorial 3 Procedures forward 1 set energy energy end 1 to eat grass ask turtles if pcolor green set pcolor black the value of set energy energy ifelse show energy set label energy set label end to reproduce ask turtles e 1 e 7 4 21 09 11 28 PM when the turtle moves it looses one unit of energy ee AE A energy from grass slider is added to energy energy from grass is set to be the value of the energy is set to an empty text
8. y axis value Patch locations are designated in the same way as plotting a point e Open a patch monitor for any patch file Applications NetLogo 204 0 2 docs tutorial2 html Page 6 of 7 NetLogo 4 0 2 User Manual Tutorial 2 Commands 4 21 09 11 28 PM The monitor shows that for the patch in the picture its pxcor variable is 11 and its pycor variable is 4 If we go back to the analogy of the coordinate plane and wanted to plot this point the point would be found in the lower left quadrant of the coordinate plane where x 11 and y 4 To tell this particular patch to change color use its coordinates e Inthe bottom of the patch monitor enter set pcolor blue and press return Typing a command in a turtle or patch monitor addresses only that turtle or patch You can also talk to a single patch from the Command Center e Inthe Command Center enter ask patch 11 4 set pcolor green and press return What s Next At this point you may want to take some time to try out the techniques you ve learned on some of the other models in the Models Library In Tutorial 3 Procedures you can learn how to alter and extend existing models and build your own models file Applications NetLogo 204 0 2 docs tutorial2 html Page 7 of 7 NetLogo 4 0 2 User Manual Tutorial 3 Procedures 4 21 09 11 28 PM This tutorial leads you through the process of building a complete model built up stage by stage with every step ex
9. 25 maximum x coordinate for patches minimum y coordinate for patches 25 25 max pycor 25 Torus 51x 51 maximum y coordinate for patches vV World wraps horizontally vV World wraps vertically V j e W Patch size 9 Font size 14 measured in pixels of labels on agents vV Turtle shapes if unchecked turtles appear as squares Tick counter Mi Show tick counter Tick counter label ticks Cancel Apply OK What are the current settings for max pxcor min pxcor max pycor min pycor and Patch size e Press cancel to make this window go away without changing the settings e Place your mouse pointer next to but still outside of the view You will notice that the pointer turns into a crosshair e Hold down the mouse button and drag the crosshair over the view The view is now selected which you know because it is now surrounded by a gray border e Drag one of the square black handles The handles are found on the edges and at the corners of the view e Unselect the view by clicking anywhere in the white background of the Interface tab e Press the Settings button again and look at the settings What numbers changed What numbers didn t change The NetLogo world is a two dimensional grid of patches Patches are the individual squares in the grid In Wolf Sheep Predation when the grass switch is on the individual patches are easily seen because some of them are green while others a
10. 9 11 28 PM Page 2 of 16 NetLogo 4 0 2 User Manual Tutorial 3 Procedures 4 21 09 11 28 PM Press setup a couple more times and see how the arrangement of turtles is different each time Note that some turtles may be right on top of each other Think a bit about what you needed to do to make this happen You needed to make a button in the interface and make a procedure that the button uses The button only worked once you completed both of these separate steps In the remainder of this tutorial you will often have to complete two or more similar steps to add another feature to the model If something doesn t appear to work after you completed what you thought is the final step for that new feature continue to read ahead to see if there is still more to do After reading ahead for a couple of paragraphs you should then go back over the directions to see if there is any step you might have missed Making the go button Now make a button called go Follow the same steps you used to make the setup button except e For Commands enter go instead of setup e Check the forever checkbox in the edit dialog m4 gt gt ticks 0 setup Button 86 O9 Agent s Observer L M Forever Commands go Display name Action key Cancel The forever checkbox makes the button stay down once pressed so its commands run over and over again not just once e Then add a go procedure to the Procedures tab to go m
11. Logo 4 0 2 User Manual Tutorial 3 Procedures 4 21 09 11 28 PM When the switch is on you ll see the energy of each turtle go up each time it eats grass You ll also see its energy going down whenever it moves 80010 25070 300 Rh cont ow 300 Eh sa 4 350b 800 250 004 5500 Soon Wo W booo 3000 450 4500 5000 70010 2000 300 SFR 40050 BD TVs50 More procedures Now our turtles are eating let s make them reproduce and die too Let s also make the grass grow back We ll add all three of these of these behaviors now by making three separate procedures one for each behavior e Go to the Procedures tab e Rewrite the go procedure as follows to go move turtles eat grass reproduce check death regrow grass end e Add the procedures for reproduce check death and regrow grass as shown below to reproduce ask turtles if energy gt 50 set energy energy 50 hatch 1 set energy 50 end to check death ask turtles if energy lt 0 die end file Applications NetLogo 204 0 2 docs tutorial3 Atml Page 10 of 16 NetLogo 4 0 2 User Manual Tutorial 3 Procedures 4 21 09 11 28 PM to regrow grass ask patches if random 100 lt 3 set pcolor green end Each of these procedures uses the i f command Each turtle when it runs reproduce checks the value of the turtle s energy variable If it is greater than 50 then the turtle runs the commands inside the
12. NetLogo 204 0 2 docs tutorial2 html Page 2 of 7 NetLogo 4 0 2 User Manual Tutorial 2 Commands 4 21 09 11 28 PM Command Center W Clear observer gt ask patches set pcolor yellow observer gt ask turtles set color brown O v observer patches links e Choose turtles from the popup menu e Typeset color pink and press return e Press the tab key until you see patches gt in the bottom left corner e Typeset pcolor white and press return What does the View look like now Do you notice any differences between these two commands and the observer commands from earlier The observer oversees the world and therefore can give a command to the patches or turtles using ask Like in the first example observer gt ask patches set pcolor yellow the observer has to ask the patches to set their pcolor to yellow But when a command is directly given to a group of agents like in the second example patches gt set pcolor white you only have to give the command itself e Press setup What happened Why did the View revert back to the old version with the black background and white road Upon pressing the setup button the model will reconfigure itself back to the settings outlined in the Procedures tab The Command Center is not often used to permanently change the model It is most often used as a tool to customize current models and allows for you to manipulate the NetLogo world to further answer those What if que
13. NetLogo 4 0 2 User Manual Tutorial 1 Models 4 21 09 11 28 PM If you read the Sample Model Party section you got a brief introduction to what it s like to interact with a NetLogo model This section will go into more depth about the features that are available while you re exploring the models in the Models Library Throughout all of the tutorials we ll be asking you to make predictions about what the effects of making changes to the models will be Keep in mind that the effects are often surprising We think these surprises are exciting and provide excellent opportunities for learning Some people have found it helpful to print out the tutorials in order to work through them When the tutorials are printed out there s more room on your computer screen for the NetLogo model you re looking at Sample Model Wolf Sheep Predation We ll open one of the Sample Models and explore it in detail Let s try a biology model Wolf Sheep Predation a predator prey population model e Open the Models Library from the File menu NetLogo Maite Edit Tools Zoon 8 New 36N tl Open dO ir Models Library 3M ra Save aS Save AS Save As Applet Print af P e Choose Wolf Sheep Predation from the Biology section and press Open The Interface tab will fill up with lots of buttons switches sliders and monitors These interface elements allow you to interact with the model Buttons are blue they set up start and stop the m
14. adsheet and database programs such as Excel You can also export a plot by control clicking Mac or right clicking Windows it and choosing Export from the popup menu Monitors Monitors are another method of displaying information in a model Here are the monitors in Wolf Sheep Predation time ticks sheep wolves grass 4 The monitor labeled time ticks tells us how much time has passed in the model The other monitors show us the population of sheep and wolves and the amount of grass Remember the amount of grass is divided by four to keep the plot from getting too tall The numbers displayed in the monitors update continuously as the model runs whereas the plots show you data from the whole course of the model run Note that NetLogo has also another kind of monitor called agent monitors These will be introduced in Tutorial 2 Controlling the View If you look at the interface tab you ll see a strip of controls along the top edge in the toolbar These controls change various aspects of the view Let s experiment with the effect of these controls e Press setup and then go to start the model running file Applications NetLogo 204 0 2 docs tutoriall html Page 4 of 8 NetLogo 4 0 2 User Manual Tutorial 1 Models 4 21 09 11 28 PM e As the model runs move the speed slider to the left What happens This slider is helpful if a model is running too fast for you to see what s going on in detail e Move t
15. alled setup and a forever button called go Many models also have a once button called go once or step once which is like go except that it advances the model by one tick time step instead of over and over Using a once button like this lets you watch the progress of the model more closely Stopping a forever button is the normal way to stop a model It s safe to pause a model by stopping a forever button then make it go on by pressing the button again You can also stop a model with the Halt item on the Tools menu but you should only do this if the model is stuck for some reason Using Halt may interrupt the model in the middle of an action and as the result the model could get confused e If you like experiment with the setup and go buttons in the Wolf Sheep Predation model Do you ever get different results if you run the model several times with the same settings Controlling speed Speed Slider The speed slider allows you to control the speed of a model that is the speed at which turtles move patches change color and so on yyy normal speed When you move the slider to the left the model slows down so there are longer pauses between each tick time step That makes it easier to see what is happening You might even slow the model down so far as to see exactly what a single turtle is doing When you move the speed slider to the right of the center the model will speed up NetLogo will start skippin
16. d would enhance the look of the model while the others are more behavioral We will be focusing more on the simpler or cosmetic changes throughout most of this tutorial Tutorial 3 will go into greater detail about behavioral changes which require changing the Procedures tab To make these simple changes we will be using the Command Center The Command Center The Command Center is located in the Interface Tab and allows you to enter commands or directions to the model Commands are instructions you can give to NetLogo s agents turtles patches links and the observer Refer to the Interface Guide for details explaining the different parts of the Command Center In Traffic Basic e Press the setup button e Locate the Command Center e Click the mouse in the white box at the bottom of the Command Center e Type the text shown here Command Center WH Clear observer gt ask patches set pcolor yellow J v e Press the return key What happened to the View You may have noticed the background of the View has turned all yellow and the street has disappeared file Applications NetLogo 204 0 2 docs tutorial2 html Page 1 of 7 NetLogo 4 0 2 User Manual Tutorial 2 Commands 4 21 09 11 28 PM Why didn t the cars turn yellow too Looking back at the command that was written we asked only the patches to change their color In this model the cars are represented by a different kind of agent called turtles Therefo
17. e Monitor for turtle 0 type set color pink What happens in the View Did anything change in the Turtle Monitor A second way to change one turtle s color is to go directly to the color variable in the Turtle Monitor and change the value e Select the text to the right of color in the Turtle Monitor e Type ina new color such as green 2 What happened The third way to change an individual turtle s or patch s color is to use the observer Since the observer oversees the NetLogo world it can give commands that affect individual turtles as well as groups of turtles e In the Command Center select observer from the popup menu or use the tab key e Typeask turtle 0 set color blue and press return What happens Just as there are Turtle Monitors there are also Patch Monitors Patch monitors work very similarly to Turtle Monitors Can you make a patch monitor and use it to change the color of a single patch If you try to have the observer ask patch 0 set pcolor blue you ll get an error message Command Center W Clear ERROR Expected a number here rather than a list or block patches gt ask patch set pcolor blue v To ask an individual turtle to do something we use its who number But patches don t have who numbers therefore we need to refer to them some other way Remember patches are arranged on a coordinate system Two numbers are needed to plot a point on a graph an x axis value and a
18. e two words Let s look at what you typed in and see what each line of your procedure does e to setup begins defining a procedure named setup e clear all resets the world to an initial empty state All the patches turn black and any turtles you might have created disappear Basically it wipes the slate clean for a new model run e create turtles 100 creates 100 turtles They start out standing at the origin that is the center of patch 0 0 e ask turtles tells each turtle to run independently the commands inside the brackets Every command in NetLogo is run by some agent ask is a command too Here the observer is running the ask command itself in turn causing the turtles to run commands e setxy random xcor random ycor isa command using reporters A reporter as opposed to a command reports a result First each turtle runs the reporter random xcor which will report a random number from the allowable range of turtle coordinates along the X axis Then each turtle runs the reporter random ycor same for the Y axis Finally each turtle runs the setxy command with those two numbers as inputs That makes the turtle move to the point with those coordinates e end completes the definition of the setup procedure When you re done typing switch to the Interface tab and press the setup button you made before You will see the turtles scattered around the world file Applications NetLogo 204 0 2 docs tutorial3 Atml 4 21 0
19. enu You might try typing turt les gt pen down into the Command Center and then pressing the go button Also inside the move turtles procedure you can try changing right random 360toright random 45 Play around It s easy and the results are immediate and visible one of NetLogo s many strengths When you feel you ve done enough experimenting for now you re ready to continue improving the model you are building 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 e Go back to the setup procedure We can rewrite it as follows to setup clear all setup patches setup turtles end e The new definition of setup refers to two new procedures To define setup patches add this to setup patches ask patches set pcolor green end The setup patches procedure sets the color of every patch to green to start with A turtle s color variable is color a patch s is pcolor The only part remaining in our new setup that is still undefined is setup turtles e Add this procedure too file Applications NetLogo 204 0 2 docs tutorial3 hAtml Page 4 of 16 NetLogo 4 0 2 User Manual Tutorial 3 Procedures 4 21 09 11 28 PM to setup turtles create turtles 100 ask turtles setxy random xcor random ycor end Did you notice that the new setup turtles
20. ering information from a model using plots and monitors In this section the focus will start to shift from observing models to manipulating models You will start to see the inner workings of the models and be able to change how they look Sample Model Traffic Basic Go to the Models Library File menu Open up Traffic Basic found in the Social Science section Run the model for a couple minutes to get a feel for it Consult the Information tab for any questions you may have about this model In this model you will notice one red car in a stream of blue cars The stream of cars are all moving in the same direction Every so often they pile up and stop moving This is modeling how traffic jams can form without any cause such as an accident a broken bridge or an overturned truck No centralized cause is needed for a traffic jam to form You may alter the settings and observe a few runs to get a full understanding of the model As you are using the Traffic Basic model have you noticed any additions you would like to make to the model Looking at the Traffic Basic model you may notice the environment is fairly simple a black background with a white street and number of blue cars and one red car Changes that could be made to the model include changing the color and shape of the cars adding a house or street light creating a stop light or even creating another lane of traffic Some of these suggested changes are cosmetic an
21. ess the Create button in the Plot dialog again to create a second new pen Enter the name of this pen as grass and press OK in the Enter Pen Name dialog see image below Select the color for this pen and change it to green Select OK in the Plot dialog box file Applications NetLogo 204 0 2 docs tutorial3 html Page 12 of 16 NetLogo 4 0 2 User Manual Tutorial 3 Procedures 4 21 09 11 28 PM A ACF Plot Name Totals X axis label time Xmin O0 X max 10 Y axis label totals Y min O0 Y max 10 v Autoplot Plot Pens Choose Pen to Edit turtles Rename Create Color l black 0 O C Enter Pen Name l egend 7 ___Custom Color Enter a name for the pen a 1 grass OK Note that when you create the plot you can also set the minimum and maximum values on the X and Y axes 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 e Setup and run the model again You can now watch the plot being drawn as the model is running Your plot should have the general shape of the one below though your plot might not look exactly the same Remember that we left Autoplot on This allows the plot to readjust itself when it runs out of room If you forget which pen is which click on the Pens label on the top right corner of the plot You might t
22. ever they are slightly modified to demonstrate NetLogo s perspective features Curricular Models These are models designed to be used in schools in the context of curricula developed by the CCL at Northwestern University Some of these are models are also listed under Sample Models others are unique to this section See the info tabs of the models for more information on the curricula they go with Code Examples These are simple demonstrations of particular features of NetLogo They ll be useful to you later when you re extending existing models or building new ones For example if you wanted to put a histogram within your model you d look at Histogram Example to find out how HubNet Computer Activities This section contains participatory simulations for use in the classroom For more information about HubNet see the HubNet Guide What s Next If you want to learn how to explore models at a deeper level Tutorial 2 Commands will introduce you to the NetLogo modeling language In Tutorial 3 Procedures you can learn how to alter and extend existing models and build your own models file Applications NetLogo 204 0 2 docs tutoriall html Page 8 of 8 NetLogo 4 0 2 User Manual Tutorial 2 Commands 4 21 09 11 28 PM In Tutorial 1 you had the opportunity to view some of the NetLogo models and you have successfully navigated your way through Opening and running models pressing buttons changing slider and switch values and gath
23. expanding this model if you d like experimenting with different variables and behaviors for the agents Alternatively you may want to revisit the first model in the tutorial Wolf Sheep Predation This is the model you used in Tutorial 1 In the Wolf Sheep Predation model you saw sheep move around consume resources that are replenished occasionally grass reproduce under certain conditions and die if they ran out of resources But that model had another type of creature moving around wolves The addition of wolves requires some additional procedures and some new primitives Wolves and sheep are two different breeds of turtle To see how to use breeds study Wolf Sheep Predation Alternatively you can look at other 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 interesting just to watch patches and turtles forming patterns to try to create a game to play or whatever Hopefully you have learned some things both in terms of the NetLogo language and about how to go about building a model The entire set of procedures 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 Notice that this listing is full of comments which begin with semicolons Comments let you mix an explanation
24. first set of brackets In this case it decreases the turtle s energy by 50 then hatches a new turtle with an energy of 50 The hatch command is a NetLogo primitive which looks like this hatch number commands This turtle creates number new turtles each identical to its parent and asks the new turtle s that have been hatched to run commands You can use the commands to give the new turtles different colors headings or whatever In our case we run one command We set the energy for the newly hatched turtle to be 50 When each turtle runs check death it will check to see if its energy is less or equal to 0 If this is true then the turtle is told to die die is a NetLogo primitive When each patch runs regrow grass it will check to see if a random integer from O to 99 is less than 3 If so the patch color is set to green This will happen 3 of the time on average for each patch since there are three numbers 0 1 and 2 out of 100 possible that are less than 3 e Switch to the Interface tab now and press the setup and go buttons You should see some interesting behavior in your model now Some turtles die off some new turtles are created hatched and some grass grows back This is exactly what we set out to do If you continue to watch your monitors in your model you will see that the count turtles and green patches monitors both fluctuate Is this pattern of fluctuation predictable Is there a relationship between the variables
25. g frames that is it won t update the view at the end of every tick only some ticks Showing the state of the world takes time so showing it less often means the model progresses faster Note that if you push the speed slider well to the right the view update may update so frequently that the model appears to have slowed down It hasn t as you can see by watching the tick counter Only the frequency of updates has lessened Adjusting Settings Sliders and Switches The settings within a model give you an opportunity to work out different scenarios or hypotheses Altering the settings and then running the model to see how it reacts to those changes can give you a deeper understanding of the phenomena being modeled Switches and sliders give you access to a model s settings Here are the switches and sliders in Wolf Sheep Predation file Applications NetLogo 204 0 2 docs tutoriall html Page 2 of 8 NetLogo 4 0 2 User Manual Tutorial 1 Models 4 21 09 11 28 PM setup go g Grass settings Sheep settings Wolf settings Let s experiment with their effect on the behavior of the model e Open Wolf Sheep Predation if it s not open already e Press setup and go and let the model run for about a 100 time ticks Note there is a readout of the number of ticks right above the plot e Stop the model by pressing the go button What happened to the sheep over time Let s take a look and see what
26. he speed slider to the middle e Try moving the speed slider to the right e Now try checking and unchecking the view updates checkbox What happens Fast forwarding the model and turning off view updates are useful if you re impatient and want a model to run faster Fast forwarding moving the speed slider to the right drops view updates so the model can run fast since updating the view takes time that could be used for updating the view When view updates are off completely The model continues to run in the background and plots and monitors still update but if you want to see what s happening you need to turn view updates back on by rechecking the box Most models run much faster when view updates are off The size of the view is determined by five separate settings Min and Max X Min and Max Y and Patch Size Let s take a look at what happens when we change the size of the view in the Wolf Sheep Predation model There are more world and view settings than there s room for in the toolbar The Settings button lets you get to the rest of the settings e Press the Settings button in the toolbar A dialog box will open containing all the settings for the view file Applications NetLogo 204 0 2 docs tutoriall html Page 5 of 8 NetLogo 4 0 2 User Manual Tutorial 1 Models 4 21 09 11 28 PM e OC Model Settings World 25 25 Location of origin Center HH minimum x coordinate for patches max pxcor
27. how energy Cancel ok Switch e Now go back to the go procedure using the Procedures tab with the Toolbar e Rewrite the eat grass procedure as follows to eat grass ask turtles if pcolor green set pcolor black set energy energy 10 ifelse show energy set label energy set label The eat grass procedure introduces the 1 e1se command Look at the code carefully Each turtle when it runs these new commands checks the value of show energy determined by the switch If the switch is on comparison is true and the turtle will run the commands inside the first set of brackets In this case it assigns the value for the energy to the label of the turtle If the comparison is false the switch is off then the turtle runs the commands inside the second set of brackets In this case it removes the text labels by setting the label of the turtle to be nothing In NetLogo a piece of text is called a string A string is a sequence of letters and other characters written between double quotes Here we have two double quotes right next to each other with nothing in between them That s an empty string If a turtle s label is an empty string no text is attached to the turtle e Test this in the Interface tab by running the model using the setup and go buttons switching the show energy switch back and forth file Applications NetLogo 204 0 2 docs tutorial3 Atml Page 9 of 16 Net
28. ke to keep go as simple as possible so that it is easy to understand Eventually it will include many other things you want to have happen as the model runs such as calculating something or plotting the results Each of these things to do will have its own procedure and each procedure will have its own unique name The go button you made in the Interface tab is a forever button meaning that it will continually run its commands until you shut it off by clicking on it again After you have pressed setup once to create the turtles press the go button Watch what happens Turn it off and you ll see that all the turtles stop in their tracks Note that if a turtle moves off the edge of the world it wraps that is it appears on the other side This is the default behavior It can be changed see the Topology section of the Programming Guide for more information Experimenting with commands We suggest you start experimenting with other turtle commands Type commands into the Command Center like turtles gt set color red or add commands to setup go ormove turtles Note that when you enter commands in the Command Center you must choose turtles gt patches gt orobserver gt in the popup menu on the left depending on which agents are going to run the commands It s just like using ask turtles orask patches but saves typing You can also use the tab key to switch agent types which you might find more convenient than using the m
29. min pxcor and min pycor change too That s because by default the origin 0 0 is in the center of the world What happened to the shape of the view e Press the setup button Now you can see the new patches you have created e Edit the view by pressing the Settings button again e Change the patch size to 20 and press OK What happened to the size of the view Did its shape change Editing the view also lets you change other settings including the font size of labels and whether the view uses shapes Feel free to experiment with these and other settings as well Once you are done exploring the Wolf Sheep Predation model you may want to take some time just to explore some of the other models available in the Models Library The Models Library The library contains five sections Sample Models Perspective Demos Curricular Models Code Examples and HubNet Computer Activities Sample Models The Sample Models section is organized by subject area and currently contains more than 210 models We are continuously working on adding new models to it so come visit this section at a later date to view the new additions to the library Some of the folders in Sample Models have folders inside them labeled unverified These models are complete and functional but are still in the process of being reviewed for content accuracy and quality of code Perspective Demos These models are all models that are also in Sample Models how
30. nonetheless it is generally good practice to call tick after your agents have done their actions but before you plot Now that your model uses ticks you ll probably want to use the menu at the top of the Interface tab to change from continuous updates to tick based updates That means that NetLogo will only update that is redraw the view that is the display area where you see your agents between ticks never in the middle of a tick This makes your model run faster and ensures a consistent appearance since the updates will happen at consistent times See the Programming Guide for a fuller discussion of view updates Some more details First instead of always using 100 turtles you can have a variable number of turtles e Make a slider variable called number using the monitor icon on the Toolbar and click on an open spot in the Interface Try changing the minimum and maximum values in the slider e Then inside of setup turtles instead of create turtles 100 you can type to setup turtles create turtles number ask turtles setxy random xcor random ycor end Test this change and compare how having more or fewer turtles initially affect the plots over time Second wouldn t it be nice to adjust the energy the turtles gain and lose as they eat grass and reproduce e Make a slider called energy from grass e Make another slider called birth energy e Then inside of eat grass make this change to eat grass ask tur
31. odel Sliders and switches are green they alter model settings Monitors and plots are beige they display data If you d like to make the window larger so that everything is easier to see you can use the zoom menu at the top of the window When you first open the model you will notice that the view is empty all black To begin the model you will first need to set it up e Press the setup button What do you see appear in the view e Press the go button to start the simulation As the model is running what is happening to the wolf and sheep populations e Press the go button to stop the model Controlling the Model Buttons When a button is pressed the model responds with an action A button can be a once button or a forever button You can tell the difference between these two types of buttons by a symbol on the face of the button Forever buttons have two arrows in the bottom right corners like this file Applications NetLogo 204 0 2 docs tutoriall html Page 1 of 8 NetLogo 4 0 2 User Manual Tutorial 1 Models 4 21 09 11 28 PM Once buttons don t have the arrows like this setup Once buttons do one action and then stop When the action is finished the button pops back up Forever buttons do an action over and over again When you want the action to stop press the button again It will finish the current action then pop back up Most models including Wolf Sheep Predation have a once button c
32. olors are represented in the computer as numbers The same is true for the agents For example every turtle has an ID number we call its who number Let s take a closer look at the turtle monitor What is this turtle s who number What color is this turtle file Applications NetLogo 204 0 2 docs tutorial2 html Page 5 of 7 NetLogo 4 0 2 User Manual Tutorial 2 Commands 4 21 09 11 28 PM What shape is this turtle This turtle monitor is showing a turtle who that has a who number of 0 a color of 15 red see above chart and the shape of a car There are two other ways to open a turtle monitor besides right clicking or control clicking depending on your operating system One way is to choose Turtle Monitor from the Tools menu then type the who number of the turtle you want to inspect into the who field and press return The other way is to type inspect turtle 0 or other who number into the Command Center You close a turtle monitor by clicking the close box in the upper left hand corner Macintosh or upper right hand corner other operating systems Now that we know more about Agent Monitors we have three ways to change an individual turtle s color One way is to use the box called an Agent Commander found at the bottom of an Agent Monitor You type commands here just like in the Command Center but the commands you type here are only done by this particular turtle e In the Agent Commander of the Turtl
33. oses Now you have a setup button Pressing the button runs a procedure called setup A procedure is a sequence of NetLogo commands that we assign a new name We haven t defined that procedure yet we will soon Because the button refers to a procedure that doesn t exist yet the button turns red f interface Information Procedures j 5 an Button w r E vie apes continuous oy Edit Delete Add normal speed Natacha m4 gt ticks 0 a E E m setup s E If you want to see the actual error message click the button Now we ll create the setup procedure so the error message will go away e Switch to the Procedures tab e Type the following to setup clear all create turtles 100 ask turtles setxy random xcor random ycor end When you re done the Procedures tab looks like this file Applications NetLogo 204 0 2 docs tutorial3 Atml Page 1 of 16 NetLogo 4 0 2 User Manual Tutorial 3 Procedures e080 NetLogo Untitled Interface Information Procedures QPiv Procedures w Find Check to setup clear all create turtles 100 ask turtles setxy random xcor random ycor J end Note that the lines are indented different amounts Most people find it helpful to indent their code like this but it is not mandatory It makes the code easier to read and change Your procedure began with the word to and ended with the word end Every new procedure you create will begin and end with thes
34. ove turtles end But what is move turtles Is it a primitive in other words built in to NetLogo like clear al11 is No it s another procedure that you re about to add So far you have introduced two procedures that you added yourself setup and go e Add the move turtles procedure after the goprocedure to go move turtles end to move turtles ask turtles right random 360 forward 1 end file Applications NetLogo 204 0 2 docs tutorial3 hAtml Page 3 of 16 NetLogo 4 0 2 User Manual Tutorial 3 Procedures 4 21 09 11 28 PM Pd Note there are no spaces around the dash in move turtles 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 a single name Here is what each command in the move turtles procedure does e ask turtles says that each turtle should run the commands in the brackets e right random 360 is another command that uses a reporter First each turtle picks a random whole number between 0 and 359 random doesn t include the number you give it as a possible result Then the turtle turns right this number of degrees e forward 1 makes the turtle move forward one step Why couldn t we have just written all of these commands in go instead of in a separate procedure We could have but during the course of building your project it s likely that you ll add many other parts We d li
35. plained along the way Agents and procedures In Tutorial 2 you learned how to use the command center 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 You ve already used types of agents you can give commands to in NetLogo patches turtles links and the observer Patches are stationary and arranged in a grid Turtles move over that grid Links connect two turtles The observer oversees everything that s going on and does whatever the turtles patches and links can t do for themselves All four types of agents can run NetLogo commands All three can also run procedures A procedure combines a series of NetLogo commands into a single new command that you define You will now learn to write procedures that make turtles move eat reproduce and die You will also learn how to make monitors sliders and plots The model we ll build is a simple ecosystem model not unlike parts of Wolf Sheep Predation from Tutorial 1 Making the setup button To start a new model select New from the File menu Then begin by creating a setup button Click the Button icon in the toolbar at the top of the Interface tab Click wherever you want the button to appear in the empty white area of the Interface tab A dialog box for editing the button opens Type setup in the box labeled Commands Press the OK button when you re done the dialog box cl
36. r use the tab key file Applications NetLogo 204 0 2 docs tutorial2 html Page 4 of 7 NetLogo 4 0 2 User Manual Tutorial 2 Commands 4 21 09 11 28 PM e Typeset pcolor red 2 The spacing around the is important By subtracting from red you make it darker e Typeset pcolor red 2 By adding to red you make it lighter You can use this technique on any of the colors listed in the chart Agent Monitors and Agent Commanders In the previous activity we used the set command to change the colors of all the cars But if you recall the original model contained one red car amongst a group of blue cars Let s look at how to change only one car s color e Press setup to get the red car to reappear e f you are on a Macintosh hold down the Control key and click on the red car On other operating systems click on the red car with the right mouse button e lf there is another turtle close to the red turtle you ll see more than one turtle listed at the bottom of the menu Move your mouse over the turtle selections notice when your mouse highlights a turtle menu item that turtle is highlighted in the view Select inspect turtle from the sub menu for the red turtle A turtle monitor for that car will appear ooo rk Taking a closer look at this turtle monitor we can see all of the variables that belong to the red car A variable is a place that holds a value that can be changed Remember when it was mentioned that all c
37. re the cars did not received these instructions and thus did not change What happened in the Command Center You may have noticed that the command you just typed is now displayed in the white box in the middle of the Command Center as shown below Com mand Center eee baun observer gt ask patches set pcolor yellow observer gt e Type in the white box at the bottom of the Command Center the text shown below Command Center A Clear observer gt ask patches set pcolor yellow observer gt ask turtles set color brown J v Was the result what you expected Your View should have a yellow background with a line of brown cars in the middle The NetLogo world is a two dimensional world that is made up of turtles patches and an observer The patches create the ground in which the turtles can move around on and the observer is a being that oversee everything that is going on in the world For a detailed description and specifics about this world refer to the NetLogo Programming Guide In the Command Center we have the ability to give the observer a command the turtles a command or the patches a command We choose between these options by using the popup menu located in the bottom left corner of the Command Center You can also use the tab key on your keyboard to cycle through the different options e In the Command Center click on the observer gt in the bottom left corner file Applications
38. re brown Think of the patches as being like square tiles in a room with a tile floor By default exactly in the middle of the room is a tile labeled file Applications NetLogo 204 0 2 docs tutoriall html Page 6 of 8 NetLogo 4 0 2 User Manual Tutorial 1 Models 4 21 09 11 28 PM 0 0 meaning that if the room was divided in half one way and then the other way these two dividing lines would intersect on this tile We now have a coordinate system that will help us locate objects within the room How many tiles away is the 0 0 tile from the right side of the room How many tiles away is the 0 0 tile from the left side of the room In NetLogo the number of tiles from right to left is called world width And the number of tiles from top to bottom is world height These numbers are defined by top bottom left and right boundaries max pycor world height 0 0 min pycor In these diagrams max pxcor is 3 min pxcor is 3 max pycor is 2 and min pycor is 2 When you change the patch size the number of patches tiles doesn t change the patches only get larger or smaller on the screen Let s look at the effect of changing the minimum and maximum coordinates in the world OT file Applications NetLogo 204 0 2 docs tutoriall html Page 7 of 8 NetLogo 4 0 2 User Manual Tutorial 1 Models 4 21 09 11 28 PM e Using the Settings dialog that is still open change max pxcor to 30 and max pycor value to 10 Notice that
39. ry running the model several times to see what aspects of the plot are the same and which are different Tick counter To make comparisons between plots from one model run and another it is often useful to do the comparison for the same length of model run Learning how to stop or start an action at a specific time can help make this happen by stopping the model at the same point each model run Keeping track of how many times the go procedure is run is a useful way to cue these actions To keep track of this we will use NetLogo s built in tick counter e Change the go procedure to go if ticks gt 500 stop move turtles eat grass reproduce check death regrow grass tick do plots file Applications NetLogo 204 0 2 docs tutorial3 Atml Page 13 of 16 NetLogo 4 0 2 User Manual Tutorial 3 Procedures 4 21 09 11 28 PM end e Now setup and run the model The graph and model won t keep running forever They should stop automatically when the tick counter in the Interface tab s toolbar reaches 500 The tick command advances the tick counter by 1 ticks is a reporter which reports the current value of the tick counter clear all takes care of resetting the tick counter to 0 when a new run starts Note that we put tick before do plots That s so if we write plotting code that uses the value of the tick counter it will see the new value not the old value In this tutorial we won t actually write any code like that but
40. s procedure to eat grass ask turtles if pcolor green set pcolor black set energy energy 10 We are using the i command for the first time Look at the code carefully Each turtle when it runs these commands compares the value of the patch color it is on op color to the value for green A turtle has direct access to the variables of the patch it is standing on If the patch color is green the comparison reports t rue and only then will the turtle runs the commands inside the brackets otherwise it skips them The commands make the turtle change the patch color to black and increase its own energy by 10 The patch turns black to signify that the grass at that spot has been eaten and the turtle is given more energy from having just eaten the grass Next let s make the movement of turtles use up some of the turtle s energy e Rewrite move turtles as follows to move turtles ask turtles right random 360 forward 1 set energy energy 1 As each turtle wanders it will lose one unit of energy at each step e Switch to the Interface tab now and press the setup button and the go button You ll see the patches turn black as turtles travel over them file Applications NetLogo 204 0 2 docs tutorial3 Atml Page 6 of 16 NetLogo 4 0 2 User Manual Tutorial 3 Procedures 4 21 09 11 28 PM Monitors Next you will create two monitors in the Interface tab with the Toolbar You make them just like buttons and
41. stions that pop up as you are investigating the models The Procedures tab is explained in the next tutorial and in the Programming Guide Now that we have familiarized ourselves with the Command Center let s look at some more details about how colors work in NetLogo Working With Colors You may have noticed in the previous section that we used two different words for changing color color and pcolor What is the difference between color and pcolor e Choose turtles from the popup menu in the Command Center or use the tab key e Typeset color blue and press return What happened to the cars Think about what you did to make the cars turn blue and try to make the patches turn red If you try to ask the patches to set color red an error message occurs file Applications NetLogo 204 0 2 docs tutorial2 html Page 3 of 7 NetLogo 4 0 2 User Manual Tutorial 2 Commands 4 21 09 11 28 PM Command Center 2 Clear turtles gt set color yellow ERROR You can t use COLOR in a patch context because COLOR is turtle onlLy patches gt set color red iii e Type set pcolor red instead and press return We call color and pcolor variables Some commands and variables are specific to turtles and some are specific to patches For example the color variable is a turtle variable while the pcolor variable is a patch variable Go ahead and practice altering the colors of the turtles and patches using the set command
42. tion s sliders e Read the contents of the Information tab located above the toolbar to learn what each of this models sliders represents The Information tab offers guidance and insight into the model Within this tab you will find an explanation of the model suggestions on things to try and other information You may want to read the Information tab before running a model or you might want to just start experimenting then look at the Information tab later What would happen to the sheep population if there was more initial sheep and less initial wolves at the beginning of the simulation e Turn the grass switch off e Set the initial number sheep slider to 100 e Set the initial number wolves slider to 20 e Press setup and then go file Applications NetLogo 204 0 2 docs tutoriall html Page 3 of 8 NetLogo 4 0 2 User Manual Tutorial 1 Models 4 21 09 11 28 PM e Let the model run for about 100 time ticks Try running the model several times with these settings What happened to the sheep population Did this outcome surprise you What other sliders or switches can be adjusted to help out the sheep population e Set initial number sheep to 80 and initial number wolves to 50 This is close to how they were when you first opened the model e Set sheep reproduce to 10 0 e Press setup and then go e Let the model run for about 100 time ticks What happened to the wolves in this run
43. tles if pcolor green set pcolor black set energy energy energy from grass ifelse show energy set label energy set label end e And inside of reproduce make this change to reproduce ask turtles if energy gt birth energy set energy energy birth energy hatch 1 set energy birth energy file Applications NetLogo 204 0 2 docs tutorial3 Atml Page 14 of 16 NetLogo 4 0 2 User Manual Tutorial 3 Procedures 4 21 09 11 28 PM Finally what other slider could you add to vary how often grass grows back Are there rules you can add to the movement of the turtles or to the newly hatched turtles that happen only at certain times Try writing them What s next So now you have a simple model of an ecosystem Patches grow grass turtles wander eat the grass reproduce and die You have created an interface containing buttons sliders switches monitors and plots You ve even written a series of procedures to give the turtles something to do 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 Programming Guide All of the primitives are listed and described in the the NetLogo Dictionary Also you can continue experimenting with and
44. value the label the label if energy gt birth energy set energy energy birth energy set energy birth energy hatch 1 end to check death ask turtles if energy lt 0 die end to regrow grass ask patches if random 100 lt 3 end to do plots set current plot 3 out of 100 times set pcolor green Totals take away birth energy to give birth give this birth energy to the offspring ee AE A removes the turtle if it has no energy left the patch color is set to green which plot we want to use next ee AE A set current plot pen turtles which pen we want to use next plot count turtles what will be plotted by the current pen set current plot pen grass which pen we want to use next plot count patches with end file Applications NetLogo 204 0 2 docs tutorial3 html pcolor green what will be plotted by the current pen ee A A Page 16 of 16
45. w many of them are green That s what with does it makes a smaller agentset of just those agents for whom the condition in the brackets is true The condition is pcolor green so that gives us just the green patches file Applications NetLogo 204 0 2 docs tutorial3 html Page 8 of 16 NetLogo 4 0 2 User Manual Tutorial 3 Procedures 4 21 09 11 28 PM Now we have two monitors that will report how many turtles and green patches we have to help us track what s going on in our model As the model runs the numbers in the monitors will automatically change e Use the setup and go buttons and watch the numbers in the monitors change Switches and labels The turtles aren t just turning the patches black they re also gaining and losing energy As the model runs try using a turtle monitor to watch one turtle s energy go up and down It would be nicer if we could see every turtle s energy all the time We will now do exactly that and add a switch so we can turn the extra visual information on and off e Tocreate a switch click on the switch icon on the Toolbar in the Interface tab and click on an open spot in the Interface A dialog box will appear e In the Global variable section of the dialog box type show energy Don t forget to include the question mark in the name See image below m4 gt gt ticks 0 setup go m count turtles green patches 50 1225 O fy f f i Global variable s
46. would happen to the sheep if we change one of the settings e Turn the grass switch on e Press setup and go and let the model run for a similar amount of time as before What did this switch do to the model Was the outcome the same as your previous run Just like buttons switches have information attached to them Their information is set up in an on off format Switches turn on off a separate set of directions These directions are usually not necessary for the model to run but might add another dimension to the model Turning the grass switch on affected the outcome of the model Prior to this run the growth of the grass stayed constant This is not a realistic look at the predator prey relationship so by setting and turning on a grass growth rate we were able to model all three factors sheep wolf and grass populations Another type of setting is called a slider Sliders are a different type of setting then a switch A switch has two values on or off A slider has a range of numeric values that can be adjusted For example the initial number sheep slider has a minimum value of 0 and a maximum value of 250 The model could run with O sheep or it could run with 250 sheep or anywhere in between Try this out and see what happens As you move the marker from the minimum to the maximum value the number on the right side of the slider changes this is the number the slider is currently set to Let s investigate Wolf Sheep Preda
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