NetLogoExamples - Emergent Wiki

Computer languages are best described by example. This page gives examples to illustrate concepts described in UsingNetLogo.

1. Defining variables

globals [time randomize-initial-state?]
patches-own [state resource-level]

Remember, agents have built-in variables: xcor, ycor, color, heading for turtles; pxcor, pycor, pcolor for patches.

2. Defining procedures

to update
     command-1
     command-2
     command-3
end

3. Commands

You must specify whom you're commanding:

   ask patches [
      task-1
      task-2
   ]

You can't ask anything of a turtle in the middle of an ask-patches block.

4. Things you can do

Change values of variables using set:

   set state (random 5)   ; assigns random integer 0 to 4.
   set time (time + 1)    ; increments variable time
   set pcolor blue        ; turns patch blue

Move turtles:

   right random 360       ; changes turtle's heading a random number of degrees
   forward 2              ; moves turtle in direction of heading, two distance units

Create turtles, sprout turtles

5. How do you specify whom you're asking?

All patches or turtles: ask patches [ ]
One patch: ask patch 0 0 [ ]
One patch northeast of the one you're already asking: ask patch-at 1 -1 [ ]
Moore neighbors or von Neumann neighbors (patches only): ask neighbors [ ] or ask neighbors4 [ ]
Agents that have some property:

   ask turtles with [TRUE/FALSE TEST] [TASK]
   ask patches with [TRUE/FALSE TEST] [TASK]
   ask breeds with [TRUE/FALSE TEST] [TASK]
   ask neighbors4 with [resource-level > 3] [set resource resource + (random 2) + 1]

6. Loops

   while [TRUE/FALSE TEST] [
      task-1
      task-2
   ]

So, when do you use loops?

When you want the model to keep running until something happens.
When you want something to happen a set number of times. Use an index variable:

   set i 0
   while [i < 5][
      task-1
      task-2
      set i (i + 1)
   ]

7. Conditionals

Make a procedure dependent on something.
How?

   if (TRUE/FALSE TEST) [
      task-1
      task-2
   ]

   ifelse (SOME TRUE/FALSE TEST) [
      THINGS TO DO IF TRUE
   ]
   [
      THINGS TO DO IF FALSE
   ]

8. What are all these TRUE/FALSE TESTs?

Comparisons

=, !=, <, <=, etc.
T/F variables: mouse-down?, randomize-initial-states?

Trailing question-mark is a useful reminder.
Booleans (like all variables) initialize to zero, not false.

What can be compared with what?

Variables
reporters

count turtles
count patches with [pcolor = 4]

Mathematical expressions

count neighbors with [state = 2] mod 2

9. Interface stuff

Use buttons to trigger procedures, sliders to set variables, switches to set booleans.
Use monitors to track the value of global variables. The value can be a string.
Buttons can be regular or "forever." Forever buttons repeat their code until reclicked.
If you define a variable in the interface, don't define it in the code.
To edit, resize, or move an interface element, drag the cursor in, across border. Then move or resize, or click Edit.
The lattice resized in two ways or in combination:

Change number of cells.
Change size of cells.
I usually set cell number first, then resize using the cursor.