turtles-own
[
  nearby-classmates         ;; agentset of turtles within some specified radius
  closest-classmate         ;; the nearest turtle
  stopped?                  ;; if the turtle hasn't moved
  rule                      ;; the turtle's rule (the rules are named by strings)
  origin                    ;; the turtle's original position
  close-classmates          ;; turtles within the too-close range
  far-classmates            ;; turtles within the too-far range
  total-distance-moved      ;; total distance moved by the turtle
  previous-patch            ;; patch the turtle was previously on
]

;;
;; SETUP AND HELPERS
;;

to setup
  clear-all
  set-default-shape turtles "circle"
  make-turtles
  reset-ticks
end

to make-turtles
  if [count patches in-radius initial-radius] of patch 0 0 < num-turtles-to-create
  [ user-message (word "There aren't enough patches in initial-radius to create the number "
                       "of turtles you've asked for.  Please make initial-radius larger or decrease "
                       "the number of turtles.")
    stop ]
  ask patch 0 0 [
      create-initial-turtles num-random-min   "random-min"   violet
      create-initial-turtles num-random       "random"       brown
      create-initial-turtles num-random-away  "random-away"  pink
      create-initial-turtles num-open-min     "open-min"     blue
      create-initial-turtles num-open-min-max "open-min-max" orange ]
  set-common-variables
end

;; reports the total number of turtles the user has requested
to-report num-turtles-to-create
  report num-random-min +
         num-random +
         num-random-away +
         num-open-min +
         num-open-min-max
end

;; ask n random patches without a turtle on them that are within initial-radius of the
;; asker to sprout a turtle with the rule set to turtle-rule and color to turtle-color
to create-initial-turtles [n turtle-rule turtle-color]
  ask n-of n (patches in-radius initial-radius with [not any? turtles-here])
  [ sprout 1
    [ set rule turtle-rule
      set color turtle-color ] ]
end

;; initializes turtle variables
;; also moves the turtles a bit so that they are a bit more randomly scattered
to set-common-variables
  ask turtles
  [ set xcor xcor - 0.5 + random-float 1
    set ycor ycor - 0.5 + random-float 1
    set origin patch-here
    set stopped? false
    set total-distance-moved 0 ]
end


;;
;; GO AND TURTLE STRATEGIES
;;

to go
  if all? turtles [stopped?] [ stop ]
  ask turtles [
    set previous-patch patch-here
    if rule = "random-min"   [ move-random-min ]
    if rule = "random"       [ move-random ]
    if rule = "random-away"  [ move-random-away ]
    if rule = "open-min"     [ move-open-min ]
    if rule = "open-min-max" [ move-open-min-max ]
    set total-distance-moved (total-distance-moved + (distance previous-patch))
  ]
  tick
end

;; set a random heading and move at
;; each time step, with no stopping condition
to move-random ;; turtle procedure
  rt random 360
  avoid-walls
  fd step-size
end

;; turtle procedure: set a random heading and move until
;; all other turtles are at least "too-close" away
to move-random-min
  set nearby-classmates other turtles in-radius too-close
  ifelse any? nearby-classmates  ; if there aren't turtles nearby, stop, you are scattered
  [ rt random 360
    avoid-walls
    fd step-size
    set stopped? false ]
  [ set stopped? true ]
end

;; set heading in the direction opposite
;; of the closest turtle, without stopping
to move-random-away ;; turtle procedure
  set closest-classmate min-one-of other turtles [distance myself]
  face closest-classmate
  rt 180
  avoid-walls
  fd step-size
  set stopped? false
end

;; set heading towards the largest open space,
;; stopping when all other turtles are at least "too-close" away
to move-open-min ;; turtle procedure
  set nearby-classmates other turtles in-radius too-close
  ifelse any? nearby-classmates
  [ facexy (mean [xcor] of nearby-classmates)
           (mean [ycor] of nearby-classmates)
    rt 180
    avoid-walls
    fd step-size
    set stopped? false ]
  [ set stopped? true ]
end

;; when turtles are too close, move to an open space,
;; when turtles get too far, move in towards a more populated area, and otherwise, stop
to move-open-min-max ;; turtle procedure
  set close-classmates other turtles in-radius too-close
  set far-classmates other turtles in-radius too-far
  ifelse any? close-classmates  ; move to an open space
  [ facexy (mean [xcor] of close-classmates)
    (mean [ycor] of close-classmates)
    rt 180
    avoid-walls
    fd step-size
    set stopped? false ]
  [ ifelse not any? far-classmates  ; move to a more populated space in the 'room'
    [ facexy mean [xcor] of other turtles
      mean [ycor] of other turtles
      avoid-walls
      fd step-size
      set stopped? false ]
    [ set stopped? true ] ]
end

;; if a turtle gets to a wall, it turns around
to avoid-walls ;; turtle procedure
  if not can-move? 1
  [ rt 180 ]
end


; Copyright 2004 Uri Wilensky.
; See Info tab for full copyright and license.
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PENS
"default" 1.0 1 -13791810 true "" ""

@#$#@#$#@
## WHAT IS IT?

This model simulates students' ideas about scattering, which takes place just before exercising in gym. The students in a class start out all bunched up, and the teacher asks them to spread out or scatter.  This simulation shows the spread of the group when the individual students follow simple rules to decide whether to move and where.  The scatterers move according to rules that were gleaned from several interviews with sixth-grade students.

The students were asked: "At the beginning of a Physical Education class, the students are standing close together. The teacher tells the students to scatter so they may perform calisthenics. What happens? Can you describe and explain?"  The students described the scattering process verbally, using coins to simulate the process and drawing a series of pictures to depict the succeeding steps.

## HOW IT WORKS

This model implements several scattering rules, which the user can mix and match:

RANDOM-MIN: Move in a random direction until you are far enough away from all of your neighbors. Random-min turtles (the violet ones) set their heading and move in a random direction if and only if there are turtles that are too close.

OPEN-MIN: Move into the largest open nearby space until you are far enough away from all of your neighbors. Open-min turtles (blue) also stop when other turtles are too close. If there are turtles that are too close, they find the heading that will take them to the largest open space.

OPEN-MIN-MAX: Move into the largest open nearby space until you are far enough away, but not too far. If you're too close, move away. If you're too far, move closer. The open-min-max turtles (orange) move to the largest open space if other turtles are too close. If the other turtles are too far away (that is, there aren't turtles within a certain space), the turtle in question moves to a more populated area.

Two of the scattering rules do not have stopping conditions, so they continue scattering indefinitely:

RANDOM-AWAY: Move in a random direction away from the person that is closest too you. Random-away turtles (pink) look at the turtle closest to them and move in the opposite direction.

RANDOM: Move about randomly, disregarding all other scatterers. Random turtles (brown) just run around in a random direction, never looking at the turtles around them.

The scattering students move at equal speeds, if they move at all. Two students cannot occupy the same location. Whether or not to move and where to move depends on each of the rules that are described. The color of the scattering students reflects the rule that they are following.

## HOW TO USE IT

Use the sliders to the right of the view to specify the number of turtles to create with each rule.

With the first three rules, the TOO-CLOSE slider sets the distance, in patches, that the turtles must be away from other turtles before stopping. The NUM-RANDOM-MIN slider indicates the number of turtles to create that move in a random direction, stopping when they are at least the distance specified by TOO-CLOSE from other turtles. Use the NUM-OPEN-MIN slider to specify how many turtles to create that move to the largest open space if there are turtles within TOO-CLOSE.

NUM-OPEN-MIN-MAX specifies the number of turtles to create that must have turtles within a certain range in order to stop. TOO-CLOSE indicates how far away other turtles must be for a turtle to stop, and TOO-FAR indicates how close the turtle in question must be to at least another turtle for it to stop.

The NUM-RANDOM-AWAY slider indicates the number of turtles to create that move away from the closest turtle, without ever stopping. NUM-RANDOM indicates the number of turtles that move in random directions without stopping.

To indicate the radius within which you want the turtles to be initially scattered, use the INITIAL-RADIUS slider. INITIAL-RADIUS must be large enough to fit the number of turtles indicated by the above sliders. See NetLogo Features, below.

To vary the distance that a turtle moves each time step, change the STEP-SIZE slider.

Press SETUP to create the number of turtles indicated, in the INITIAL-RADIUS specified. Press GO to start the model and the plots.

The plot AVERAGE DISTANCE FROM ORIGIN shows the average distance that turtles of a certain rule have moved from their original position over time. It is color-coded to match the colors of the turtles. To observe the average distance of turtles of a certain rule, watch the line of the same color.

The MOBILITY OF SCATTERERS histogram displays the average total distance moved by turtles within a certain range away from the origin. Each of the bars plots the average total distance of turtles in five concentric circles radiating from the center patch. Note that this differs from that of the AVERAGE DISTANCE FROM ORIGIN plot, which measures the distance from the turtle's present spot to its original spot, in that it adds up each step taken by the turtle.

The TICKS monitor displays the number of clock cycles that GO has run.

## THINGS TO NOTICE

While the group is spreading, notice the individual turtles' routes.  You can use the "watch turtle" functionality (by clicking on a turtle) or pen-downing a single turtle to observe its path.  Try this with a number of turtles. How do the individual and group patterns relate?

Like two people walking towards each other down a hallway trying to avoid colliding, the rules of the turtles deciding their heading based on other turtles can conflict. For instance, several neighboring turtles moving to the largest open space may choose to move to the same open spot and, in doing so, move closer to the other turtles. This can form clusters of turtles, even while the group is spreading and even in the periphery of the group.

Certain rules get to a settled, or scattered, state more quickly than others do. Which rules are better for efficient scattering?

Observe the shape of the "Average distance from Origin" plot: Why is it shaped the way it is?  What other phenomena behave in a similar way?

When observing the MOBILITY OF SCATTERERS plot, notice how scatterers farther on the outside will sometimes move a greater total distance or a smaller total distance, depending on the rule.

There are often threshold values where the scatterers will never settle down, and the model will not stop. For instance, if TOO-CLOSE is too great and there are too many turtles, they will never have enough room to completely scatter.

Some of the rules produce a more ordered end formation than others. Why is this, and which ones exhibit this behavior?  Do any regions show higher densities than others?

When certain rules are mixed together, segregation occurs. That is, turtles of a certain rule tend to be surrounded by more turtles of the same rule than not. Which combinations of rules produce this segregation more than others?

## THINGS TO TRY

When experimenting with the move-into-range turtles, vary the TOO-CLOSE and TOO-FAR sliders concurrently with the STEP-SIZE slider. If the step size is too big and the TOO-CLOSE and TOO-FAR values creating too narrow of a range, will the scatterers ever settle?

Try mixing rules to explore how turtles of different rules interact with each other.

Watch what happens when a few random-scatter turtles run around a set of turtles with stopping conditions, like move-to-open-space turtles.

Vary the STEP-SIZE slider, and see what effect this has on the model. Does a bigger step size make for more or less efficient scattering?

Try varying INITIAL-RADIUS to make the starting formation tighter or more spread out.

Pen-down one turtle, and watch its path as it moves about the world. How does it compare with the group?

Which rule seems to simulate real situations of scattering people?

## EXTENDING THE MODEL

Try to think of other rules that scatterers follow. For example, figure out rules to get the turtles to fill up the entire space. Alternatively, turtles could move towards a friend or away from an enemy. Also, try to think of stopping conditions other than being a certain distance from other turtles.

Turtles could have a variable `stubbornness`, indicating how likely a turtle is to move or not. A `dawdle` or `friendliness` variable could indicate a likeliness to hang around other turtles.

Try giving the scatterers the ability to back track. For instance, if a scatterer moving to an open space finds that their movement takes them to a more populated area, they could move back to where they were before.

Allow turtle to have varying speeds or step sizes, simulating the differing stride lengths of individual scatterers

Devise other ways to model the scattering in the aggregate sense; for example, measure the density in a certain area.

What if there was a location (e.g. a fire) that people were trying to get away from while scattering?  Devise a way to model such a situation.  You can include physical structures, such as walls and doors.

## NETLOGO FEATURES

The model creates turtles by asking patches to `sprout` a turtle initialized with a certain rule. Because a patch can only `sprout` one turtle, only a certain number of turtles can fit in a certain radius. The model verifies that the user hasn't asked for more turtles than can fit in the initial-radius specified to avoid an error.

## CREDITS AND REFERENCES

The study is described in Levy, S.T., & Wilensky, U. (2004). Making sense of complexity: Patterns in forming causal connections between individual agent behaviors and aggregate group behaviors. In U. Wilensky (Chair) and S. Papert (Discussant) Networking and complexifying the science classroom: Students simulating and making sense of complex systems using the HubNet networked architecture. The annual meeting of the American Educational Research Association, San Diego, CA, April 12 - 16, 2004.

Thanks to Stephanie Bezold for her work on this model.

## HOW TO CITE

If you mention this model or the NetLogo software in a publication, we ask that you include the citations below.

For the model itself:

* Wilensky, U. (2004).  NetLogo Scatter model.  http://ccl.northwestern.edu/netlogo/models/Scatter.  Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.

Please cite the NetLogo software as:

* Wilensky, U. (1999). NetLogo. http://ccl.northwestern.edu/netlogo/. Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.

## COPYRIGHT AND LICENSE

Copyright 2004 Uri Wilensky.

![CC BY-NC-SA 3.0](http://ccl.northwestern.edu/images/creativecommons/byncsa.png)

This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License.  To view a copy of this license, visit https://creativecommons.org/licenses/by-nc-sa/3.0/ or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA.

Commercial licenses are also available. To inquire about commercial licenses, please contact Uri Wilensky at uri@northwestern.edu.

This model was created as part of the projects: PARTICIPATORY SIMULATIONS: NETWORK-BASED DESIGN FOR SYSTEMS LEARNING IN CLASSROOMS and/or INTEGRATED SIMULATION AND MODELING ENVIRONMENT. The project gratefully acknowledges the support of the National Science Foundation (REPP & ROLE programs) -- grant numbers REC #9814682 and REC-0126227.

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Polygon -7500403 true true 195 90 240 150 225 180 165 105
Polygon -7500403 true true 105 90 60 150 75 180 135 105

plant
false
0
Rectangle -7500403 true true 135 90 165 300
Polygon -7500403 true true 135 255 90 210 45 195 75 255 135 285
Polygon -7500403 true true 165 255 210 210 255 195 225 255 165 285
Polygon -7500403 true true 135 180 90 135 45 120 75 180 135 210
Polygon -7500403 true true 165 180 165 210 225 180 255 120 210 135
Polygon -7500403 true true 135 105 90 60 45 45 75 105 135 135
Polygon -7500403 true true 165 105 165 135 225 105 255 45 210 60
Polygon -7500403 true true 135 90 120 45 150 15 180 45 165 90

square
false
0
Rectangle -7500403 true true 30 30 270 270

square 2
false
0
Rectangle -7500403 true true 30 30 270 270
Rectangle -16777216 true false 60 60 240 240

star
false
0
Polygon -7500403 true true 151 1 185 108 298 108 207 175 242 282 151 216 59 282 94 175 3 108 116 108

target
false
0
Circle -7500403 true true 0 0 300
Circle -16777216 true false 30 30 240
Circle -7500403 true true 60 60 180
Circle -16777216 true false 90 90 120
Circle -7500403 true true 120 120 60

tree
false
0
Circle -7500403 true true 118 3 94
Rectangle -6459832 true false 120 195 180 300
Circle -7500403 true true 65 21 108
Circle -7500403 true true 116 41 127
Circle -7500403 true true 45 90 120
Circle -7500403 true true 104 74 152

triangle
false
0
Polygon -7500403 true true 150 30 15 255 285 255

triangle 2
false
0
Polygon -7500403 true true 150 30 15 255 285 255
Polygon -16777216 true false 151 99 225 223 75 224

truck
false
0
Rectangle -7500403 true true 4 45 195 187
Polygon -7500403 true true 296 193 296 150 259 134 244 104 208 104 207 194
Rectangle -1 true false 195 60 195 105
Polygon -16777216 true false 238 112 252 141 219 141 218 112
Circle -16777216 true false 234 174 42
Rectangle -7500403 true true 181 185 214 194
Circle -16777216 true false 144 174 42
Circle -16777216 true false 24 174 42
Circle -7500403 false true 24 174 42
Circle -7500403 false true 144 174 42
Circle -7500403 false true 234 174 42

turtle
true
0
Polygon -10899396 true false 215 204 240 233 246 254 228 266 215 252 193 210
Polygon -10899396 true false 195 90 225 75 245 75 260 89 269 108 261 124 240 105 225 105 210 105
Polygon -10899396 true false 105 90 75 75 55 75 40 89 31 108 39 124 60 105 75 105 90 105
Polygon -10899396 true false 132 85 134 64 107 51 108 17 150 2 192 18 192 52 169 65 172 87
Polygon -10899396 true false 85 204 60 233 54 254 72 266 85 252 107 210
Polygon -7500403 true true 119 75 179 75 209 101 224 135 220 225 175 261 128 261 81 224 74 135 88 99

wheel
false
0
Circle -7500403 true true 3 3 294
Circle -16777216 true false 30 30 240
Line -7500403 true 150 285 150 15
Line -7500403 true 15 150 285 150
Circle -7500403 true true 120 120 60
Line -7500403 true 216 40 79 269
Line -7500403 true 40 84 269 221
Line -7500403 true 40 216 269 79
Line -7500403 true 84 40 221 269

x
false
0
Polygon -7500403 true true 270 75 225 30 30 225 75 270
Polygon -7500403 true true 30 75 75 30 270 225 225 270
@#$#@#$#@
NetLogo 6.2.2
@#$#@#$#@
set too-close 1.5
set too-far 2.0
set num-open-min-max 50
setup
repeat 75 [ go ]
@#$#@#$#@
@#$#@#$#@
@#$#@#$#@
@#$#@#$#@
default
0.0
-0.2 0 0.0 1.0
0.0 1 1.0 0.0
0.2 0 0.0 1.0
link direction
true
0
Line -7500403 true 150 150 90 180
Line -7500403 true 150 150 210 180
@#$#@#$#@
0
@#$#@#$#@
