# Grouping Turtles Example

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turtles-own [ ;; a number representing the group this turtle is a member of, or -1 if this ;; turtle is not in a group. my-group ] ;;; ABOUT THE PROBLEM ;;; ;;; In some cases, you have a group of turtles that you wish to divide into ;;; groups of roughly the same size. ;;; ;;; Note that if you have a fixed number of groups known in advance, there are ;;; probably better ways to do this, using breeds, for instance. The ;;; techniques here are only necessary when you need to dynamically modify the ;;; size or number of the groups. ;;; ;;; Conceptually, there are two basic ways to form groups: by desired group ;;; size, or by desired number of groups. In either case, if the target number ;;; evenly divides the total number of turtles, there's no problem. But if it ;;; doesn't, then we have to make some decisions. ;;; ;;; When creating groups of a fixed size, say k, it's most natural to form as ;;; many groups of k turtles as possible, and then consider the remainder of ;;; the turtles "extra", and create one smaller group containing only those ;;; turtles. That way, all but one of the groups will have the desired size. ;;; ;;; When creating a fixed number of groups, it's most natural to assign one ;;; turtle to each group, and continue in this way until all turtles have been ;;; assigned. This, for instance, is the strategy one would most likely adopt ;;; in trying to distribute a large number of ping-pong balls as evenly as ;;; possible among a fixed number of buckets. In this case, all the groups ;;; will have either size k or size k + 1, for some k. ;;; ;;; The following two procedures implement each of these strategies. Which is ;;; appropriate for your model depends on your requirements. Other variations ;;; on these schemes are, of course, possible. ;;; this procedure randomly assigns turtles to groups based on the desired ;;; size of the groups. all the groups will have the desired size except for ;;; at most one group, which contains the remainder of the turtles. more ;;; formally, if there are n turtles, and the desired group size is k, this ;;; procedure will produce j = floor (n / k) groups of k turtles, and if ;;; n mod k > 0, it will produce one group of n mod k turtles. to assign-by-size ;; all turtles are initially ungrouped ask turtles [ set my-group -1 ] let unassigned turtles ;; start with group 0 and loop to build each group let current 0 while [any? unassigned] [ ;; place a randomly chosen set of group-size turtles into the current ;; group. or, if there are less than group-size turtles left, place the ;; rest of the turtles in the current group. ask n-of (min (list group-size (count unassigned))) unassigned [ set my-group current ] ;; consider the next group. set current current + 1 ;; remove grouped turtles from the pool of turtles to assign set unassigned unassigned with [my-group = -1] ] end ;;; this procedure randomly assigns turtles to groups based on the desired ;;; number of groups. all the groups will have as close as possible to the ;;; same number of turtles. more formally, if there are n turtles, and the ;;; desired number of groups is j, then let the initial group size be ;;; k = ceiling (n / j). this procedure will produce n mod j groups of size ;;; k and j - (n mod j) groups of size k - 1. to assign-by-number ;; figure out the larger of the two group sizes let tmp-group-size ceiling (count turtles / number-of-groups) ;; all turtles are initially ungrouped ask turtles [ set my-group -1 ] let unassigned turtles ;; start with group 0 and loop to build each group let current 0 while [any? unassigned] [ ;; place a randomly chosen set of tmp-group-size turtles into the current group ask n-of tmp-group-size unassigned [ set my-group current ] ;; consider the next group. if we're done building the larger groups, ;; reduce the group size by 1 for the rest of the groups. set current current + 1 if current = ((count turtles) mod number-of-groups) [ set tmp-group-size tmp-group-size - 1 ] ;; remove grouped turtles from the pool of turtles to assign set unassigned unassigned with [my-group = -1] ] end ;;; ;;; the following procedures are used to implement the visualization, ;;; and really don't have anything to do with grouping. ;;; unless you're particularly interested, it's safe to ignore the rest of the ;;; code. ;;; ;;; sets up the model to setup clear-all crt number-of-turtles [ ;; we want the color to be related to the who number of the turtles, so ;; that it will be fairly obvious if turtles are grouped in an ordered way. set color scale-color green who (number-of-turtles / -4) (number-of-turtles * 1.2) ;; randomly place them initially setxy random-xcor random-ycor ;; turtles start out ungrouped set my-group -1 ] reset-ticks end ;; causes the turtles to run around, going to their group's "home" if they're ;; in a group. to go ask turtles [ ;; if i'm in a group, move towards "home" for my group if my-group != -1 [ face get-home ] ;; wiggle a little and always move forward, to make sure turtles don't all ;; pile up lt random 5 rt random 5 fd 1 ] tick end ;; figures out the home patch for a group. this looks complicated, but the ;; idea is simple. we just want to lay the groups out in a regular grid, ;; evenly spaced throughout the world. we want the grid to be square, so in ;; some cases not all the positions are filled. to-report get-home ;; turtle procedure ;; calculate the minimum length of each side of our grid let side ceiling (sqrt (max [my-group] of turtles + 1)) report patch ;; compute the x coordinate (round ((world-width / side) * (my-group mod side) + min-pxcor + int (world-width / (side * 2)))) ;; compute the y coordinate (round ((world-height / side) * int (my-group / side) + min-pycor + int (world-height / (side * 2)))) end ; Public Domain: ; To the extent possible under law, Uri Wilensky has waived all ; copyright and related or neighboring rights to this model.

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File | Type | Description | Last updated | |
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Grouping Turtles Example.png | preview | Preview for 'Grouping Turtles Example' | almost 8 years ago, by Uri Wilensky | Download |

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