GIS Geog5061M and Geocomputation Unit 5 Practical
StarLogo
StarLogo is a program that can be used to build and experiment with complex systems. You will be using this for the practical project assessment should you choose to do this one. This practical will help you to become familiar with the basic StarLogo environment and its controls. Once you have undertaken this practical and experimented with some of the past students projects, youll be ready to start building your own application. This practical have been adapted from materials written by the StarLogo Development Team, MIT Media Lab, Alan Epstein and Rebekah Wahba. If you havent already downloaded the StarLogo package, go to: http://education.mit.edu/starlogo/ click on the Download button and follow the instructions. In this practical we will create a colony of artificial termites.
1 The StarLogo Environment
Start the StarLogo program. You should then see two different floating windows. The first contains what is called the Control Center. There are four panels in the Control Center, but you will only be able to see two at a time: one for the Turtles (agents that you will control) and one for the Observer. Within each Control Center there is a Command Center and a Procedures window as shown in Figures 1 and 2 below. To move between the Turtles and the Observer Command Centers, click on the tab. Here are some basics to do with the Turtle and Observer Command Centers:
When you want to tell a turtle to do something you can type the command in the Turtle Command Center or in the Procedures window. Commands typed in the Command Center are executed immediately. Commands typed in the Procedures window must be called by either a button or another command in the Command Center window. Commands typed in the Turtle window will be executed by all the turtles. If you want the observer to do something (like create new turtles or change the colour of all of the patches), you need to click on the Observer button and type those commands into the Observer window.
The second floating window is the StarLogo screen (Figure 3). As you issue commands in the Command Center, you will see the actions happen in the StarLogo window. You should see a single red turtle in the middle of the black area.
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Figure 1: The Turtle Command Center
Figure 2: The Observer Command Center
Figure 3: The StarLogo screen
2 Moving, Clearing and Creating Turtles
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When you first open StarLogo, you will be given some turtles to start with. Type fd 10 (which means forward 10 paces) in the Turtle Command Center followed by return or enter. Watch what happens in the StarLogo screen. Since the turtles were facing forward, they moved out in different directions and created a circle. Also notice that you only had to type fd 10 once and all of the turtles moved at the same time. Each command you type is run by all of the turtles. To remove the turtles type clearturtles or ct (shorthand version) in the Observer Command Center. Lets now add 10 turtles. Type create-turtles(10) or crt(10) in the Observer Command Center and press enter or return. All the newly created turtles will be stacked on top of each other so use the fd command to make them move forward.
3 Making a Procedure
A procedure is a collection of commands that execute at the same time. To run a procedure, simply type the name of the procedure. All procedures go in one of the Procedures windows of the Command Center. The Observer can only control the creation of the turtles and the patches (more on this later) but it can also send messages to the turtles asking them to do things using the ask-turtles command. Type the following commands in the Observer Procedure window:
to setup crt 30 ask-turtles [fd 10] end
The word to is used to start every procedure followed by the name of the procedure, in this case setup. The word end must also close every procedure so that the program knows when your commands are finished. Lets now try to run the procedure. First type ct in the Observer Command Center to clear the turtles. Now type setup, and watch your turtles appear. Typing setup caused the setup procedure to run. However, suppose that you don't want to have to type ct every time you start over. You can incorporate this line directly into the setup procedure:
to setup ct crt 30 ask-turtles [fd 10] end
Now the turtles will be automatically cleared each time you type setup. Run the modified procedure now. At this point you should save your work. Choose File Save Project As from either the Command Center or StarLogo window, type a name for the project and click OK.
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4 Buttons
To make it even easier to run the setup procedure, you can create a setup button. Then when you press the button, StarLogo will run the procedure called setup. Click on the icon that shows a blue button with a finger at the top of the main StarLogo window. Then click on the white space below the icons. When a window pops up, enter a name for the button and type setup in the Logo Instruction box as shown in Figure 4. You can enter a tooltip if you wish (i.e. what appears when you hover your mouse over the button). Finally, select the location where you put your setup procedure. Since you entered the commands in the Observer Procedures select Observer.
Figure 4: Creating a Button
You now have a setup button that will run the StarLogo procedure called setup. Go ahead and push the button to test it. Note that you can also move the button around in the white space.
5 Modifying the Colours of your Turtles
You have just created 30 turtles (or termites). Now let's make all of your termites blue. Use the setcolor command, or just setc blue, to set the colour of the turtles. [Note the American spelling of colour in the command should you use the full command!] Type setc blue in the Turtle Command Center. Now try making them red (setc red) and add this line to your setup procedure as shown below:
to setup ct crt 30 ask-turtles [setc red fd 10] end
Now that you have 30 red termites, lets give them something to do. First we will fill part of the termites' environment with woodchips. Later we can give the termites rules about what to do with the woodchips.
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6 Creating Patches
You're now ready to create the woodchips. We will do this using something called patches. While we could use turtles to create woodchips, it is easier to use patches. You can draw in the patches or woodchips by using the StarLogo drawing tools. Click on the icon that looks like a pencil on the left hand side of the StarLogo window and then choose one of the yellow shades. Now click on the black canvas to place the woodchips. You can click on individual squares or use it like a pencil to shade in larger areas.
Figure 5: Making patches with the drawing tools
You can also set the colour of the whole patch-canvas in the Observer Command Center with the ask-patches command using setpc with the name or number of a colour:
ask-patches [setpc blue]
Another way to change the colour of a single patch is to use the Turtle Command stamp. For example, type the following command in the Turtle Command Center:
repeat 100 [fd 10 stamp yellow]
This command tells the turtles to walk 10 steps and then change the colour of the patch they are on to yellow. The turtles will repeat this action 100 times.
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7 Computer Generated Woodchips
The manual drawing of woodchips does not create a random arrangement of woodchips. If we really want to scatter the woodchips we need to change the direction of each turtle, and use the random command. Type this in the Turtle Command Center:
repeat 100 [seth random 360 jump random 200 stamp yellow]
This command tells each turtle to jump in a random direction and stamp the patch that it lands on yellow. Each turtle should repeat this process 100 times. Now we can write a procedure that puts woodchips on the patch canvas. Enter this procedure in the Turtle Procedures window.
to patch-setup repeat 100 [seth random 360 jump random 200 stamp yellow] end
Next we can add this command to the setup procedure in the Observer Procedures window, which now looks like this:
to setup ct crt 30 ask-turtles [setc red fd 10] ask-turtles [patch-setup] end
Now when you press the setup button, you will see red termites and yellow woodchips. If you want to clear the patches, use the command cp. You can therefore use both ct and cp to clear the patches and the turtles. Add that to your setup procedure:
to setup ct cp crt 30 ask-turtles [setc red fd 10] ask-turtles [patch-setup] end
Note: If you know you want to clear both turtles and patches, you can use the command ca, or clear all. Let's add ca to our setup procedure as follows:
to setup crt ca 100 ask-turtles [setc red fd 10] ask-turtles [patch-setup] end
You have now completed the setup procedure for your program, and learned something about making a lot of termites, colouring things, and using procedures.
8 Creating a Multi-Procedure Program
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How can you make this bunch of termites interact with the woodchips? To do this, you need to think about all the tasks the termites need to accomplish. If you break down the tasks, then it's easier to write a procedure for them. You want the termites to search for woodchips and place them into piles. But to place woodchips into piles, termites need to first find a woodchip. But to find woodchips, they need to be able to move around to get to the woodchips. Once they've found a woodchip, they need to pick it up. Then they need to find a place to put it and then to actually put it down. To summarise, the termites need to:
know how to move search for woodchips find a pile for the new chip place the woodchip down in the pile.
You will now learn how to program the termites so that they can carry out these four steps. 8.1 Movement Lets start by getting the termites to move around, since that's the first thing a termite needs to know how to do before it can do more complicated things. To do this, type this procedure in the Turtle Procedures window:
to wiggle fd 1 rt random 50 lt random 50 end
With this procedure, the termite moves forward 1 unit and then wiggles by turning a little to the left and to the right. Try typing wiggle a couple of times in the Turtle Command Center and watch what happens. 8.2 Search for Woodchips The next task the termites need to be able to do is find a woodchip. You can call the procedure search-for-chip. If a termite sees a yellow woodchip, then it removes it from the patch. To do this, type this procedure in the Turtle Procedures window. Leave a line below the wiggle procedure.
to search-for-chip if pc = yellow [stamp black jump 20 stop] wiggle search-for-chip end
This procedure means that every a termite checks whether the patch it is on is yellow, and if so, it turns the patch to black (to signify that it picked up that woodchip). The termite then jumps far away and stops.
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You can test this procedure by typing search-for-chip in the Turtle Command Center. This will result in the termites moving around and turning many of the yellow woodchip patches to black. Of course, the red termites will be covering the black patches so you won't be able to see them yet. So now you have search-for-chip which changes yellow woodchips to black to indicate that they have been picked up and are no longer on the ground. You can also change your search-for-chip procedure to do wiggle before checking for a woodchip, and to keep going as long as it hasn't found one. Find the search-for-chip procedure in the Turtle Procedures window and change it to look like this:
to search-for-chip wiggle if pc = yellow [stamp black stop] search-for-chip end
8.3 Find a New Pile Now that the termite is carrying the woodchip, it needs to find a new pile to place it. This procedure will look a little like search-for-chip. To do this, type this procedure in the Turtle Procedures window. Leave a line below the search-for-chip procedure.
to find-new-pile if pc = yellow [stop] wiggle find-new-pile end
All you had to do for find-new-pile was tell the termite to wiggle, and if it sees a yellow woodchip, to stop. Otherwise, keep looking (i.e. do find-new-pile again). Test the latest procedure by typing find-new-pile in the Turtle Command Center. Your termites will move around looking for an existing woodchip on the ground and then stop. 8.4 Find a Place to Drop the Woodchip You just wrote a procedure for the termite to find a new pile to drop its chip. However, the termite just stops once it has found a spot. Now you need to write a procedure for the termite to put down its woodchip.
You know a patch is empty if its colour is black, i.e. if pc = black. To drop the woodchip, you can make the black patch turn yellow by stamping it yellow. To do this, type this procedure in the Turtle Procedures window. Leave a line below the find-new-pile procedure.
to find-empty-patch wiggle
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if pc = black [stamp yellow stop] find-empty-patch end
Here you tell the termite to wiggle, and if it finds an empty (black) patch, stamp the patch yellow (the same as dropping a yellow woodchip), and stop. If not, keep searching for an empty patch.
9 Continue the Search
After the termites have found an empty patch and dropped the woodchip, you'll want them to keep looking around for more woodchips so that the piles of chips can form quickly. You can write a simple get-away procedure to accomplish this:
to get-away seth random 360 jump 20 if pc = black [stop] get-away end
seth random 360 sets the heading of the termite to be some random compass between 0 and 360 degrees. jump 20 is similar to fd 20, but much faster, more like teleporting. If a termite lands on a black patch, stop. Otherwise, keep jumping to get away. Add get-away after it puts the woodchip down in our find-empty-patch procedure:
to find-empty-patch wiggle if pc = black [stamp yellow get-away stop] find-empty-patch end
Now ...