Random Walk Periodic Boundary Condition

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;; variable declarations ;;
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globals
[
  sum-distance
  root-mean-square
]

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;; setup procedures ;;
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;; set up the patches and creates turtles

to setup
  clear-all
  set-patch-size (200 / boundary)
  resize-world (- boundary) boundary 0 (boundary * 2)
  ask patches [ set pcolor gray + 3 ]
  setup-turtles
  reset-ticks
end 

;; determines the number of turtles and their color

to setup-turtles
  ;; the origin is at the bottom of the view so there is no need
  ;; to relocate the turtles upon creation
  crt number-of-particles
  [
    set color black
    ;; set pen-size 3
    set heading 0
    pd
  ]
end 

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;; runtime procedures ;;
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to go
  if any? turtles with [ patch-at 0 1 = nobody ]
  [ 
    ask turtles [ set ycor 0 ]
    clear-drawing
  ]
  
  ;; have the turtles randomly pick either right or left and
  ;; move in that direction
  ask turtles
  [
    ifelse ((random-float 1) < left-probability )
    [
      ;; go left
      set heading 90
      forward 1
      set heading 0
      forward 1
    ]
    [
      ;; go right
      set heading 270
      forward 1
      set heading 0
      forward 1
    ]
    
    ;; set the color of the turtles to give a rough idea of how many turtles
    ;; are at each location -- the lighter the color, the greater the number
    ;; of turtles
    set color scale-color blue (count turtles-here) (number-of-particles / 5) 0
  ]

  ;; calculate root mean square of distance of all particles
  set sum-distance 0
  foreach sort turtles
  [
    ask ? [ set sum-distance (sum-distance + xcor * xcor) ]
  ]
  set root-mean-square (sqrt (sum-distance / (count turtles)))
  tick
end