Practical 2

Practical 2, part 4 - Creating Turtles and Patches

In this part of the practical, we will start to create the fundamental components of our model: the turtles and the patches.

Click on the 'Code' tab. You should see the setup procedure. We will now improve it so that it does something more useful.

Change the setup function so that it reads:


                    to setup

                         print "Setting Up Model"

                         clear-all

                         setup-patches

                         setup-turtles

                    end

The clear-all command tells NetLogo to reset everything ready for us to create a new model. The other two lines (setup-patches and setup-turtles) call two new procedures that will contain commands to set up the turtles and patches. We could put these commands directly into the setup procedure, but by putting them into their own separate procedures it makes the model code easier to understand.

Here is the code for the two new procedures. Copy it directly after the end of the setup function.


                    to setup-patches

                         ask patches [ set pcolor green ]

                    end

                    

                    to setup-turtles

                          create-turtles initial-number-of-turtles

                          ask turtles [

                           set shape "sheep"

                           setxy (random 20) (random 20)

                           set color blue

                          ]

                    end

The code above should be fairly self-explanatory. The first procedure (setup-patches) does one thing: it asks all the patches to change their colour to green. This means that each patch will represent a square of grass, ready to be eaten by the turtles.

The second procedure (setup-turtles) creates the turtles. There are some new commands in there that you wont be familiar with:

create-turtles makes a number of new turtles. Instead of saying exactly the number of turtles that we want (e.g. 10), we use the value from the slider (initial-number-of-turtles).
Next, ask turtles is used to run some commands on the turtles that have just been created. Notice that the commands we want to send to the turtles are enclosed in square brackets ([ and ]).
set shape "sheep" uses the set command to change the shape of the turtle. This is the same as changing other turtles variables such as their color. Have a look here if you would like to see a list of all the shapes that are available.
setxy (random 20) (random 20) is a quick way of setting the turtles' x and y coordinates. The random 20 means choose a random number between 0 and 20. This is exactly the same as using two commands, one after the other: set xcor (random 20) set ycor (random 20)
Finally, set color blue makes our sheep blue.

After setting up the mode: turtles and some grass.

Now, try going back to the 'Interface' tab. If you have made any mistakes with the model code, NetLogo will tell you about them. Otherwise you will see the main interface again.

Press the setup button. You should see a few turtles created around the grid.

Press the setup button again. What happens?

That's almost everything. To finish the basic model, move onto the final section to get the sheep moving around the environment.

Activities

Write a procedure called 'make-white' that tells all the turtles to turn white. (For this, you will need to use the ask command as we did in the last practical).
Write a procedure called 'white-then-move' that first makes all the turtles go white, and then moves them forward 1 step. (Hint: the command fd 1 tells a turtle to go one step forward).