Look Ahead Example

;;  This procedure sets up the patches and turtles

to setup
  ;;  Clear everything.
  clear-all

  ;;  This will create a "checkerboard" of blue patches.  Every third patch will be
  ;;  blue (remember modulo gives you the remainder of a division).
  ask patches with [pxcor mod 3 = 0 and
                    pycor mod 3 = 0]
    [ set pcolor blue ]

  ;;  This will make the outermost patches blue.  This is to prevent the turtles
  ;;  from wrapping around the world.  Notice it uses the number of neighbor patches rather than
  ;;  a location. This is better because it will allow you to change the behavior of the turtles
  ;; by changing the shape of the world (and it is less mistake-prone)
  ask patches with [count neighbors != 8]
    [ set pcolor blue ]

  ;;  This will create turtles on 200 randomly chosen
  ;;  black patches.
  ask n-of 200 (patches with [pcolor = black])
  [
    sprout 1
      [ set color red ]
  ]
  reset-ticks
end 

;;  This procedure makes the turtles move

to go
  ask turtles
  [
    ;;  This important conditional determines if they are about to walk into a blue
    ;;  patch.  It lets us make a decision about what to do BEFORE the turtle walks
    ;;  into a blue patch.  This is a good way to simulate a wall or barrier that turtles
    ;;  cannot move onto.  Notice that we don't use any information on the turtle's
    ;;  heading or position.  Remember, patch-ahead 1 is the patch the turtle would be on
    ;;  if it moved forward 1 in its current heading.
    ifelse [pcolor] of patch-ahead 1 = blue
      [ lt random-float 360 ]   ;; We see a blue patch in front of us. Turn a random amount.
      [ fd 1 ]                  ;; Otherwise, it is safe to move forward.
  ]
  tick
end 


; Public Domain:
; To the extent possible under law, Uri Wilensky has waived all
; copyright and related or neighboring rights to this model.