who

who

This is a built-in turtle variable. It holds the turtle’s “who number” or ID number, an integer greater than or equal to zero. You cannot set this variable; a turtle’s who number never changes.

Who numbers start at 0. A dead turtle’s number will not be reassigned to a new turtle until you use the clear-turtles or clear-all commands, at which time who numbering starts over again at 0.

Example:

show [who] of turtles with [color = red]
;; prints a list of the who numbers of all red turtles
;; in the Command Center, in random order
crt 100
  [ ifelse who < 50
      [ set color red ]
      [ set color blue ] ]
;; turtles 0 through 49 are red, turtles 50
;; through 99 are blue

You can use the turtle reporter to retrieve a turtle with a given who number. See also turtle.

Note that who numbers aren’t breed-specific. No two turtles can have the same who number, even if they are different breeds:

clear-turtles
create-frogs 1
create-mice 1
ask turtles [ print who ]
;; prints (in some random order):
;; (frog 0): 0
;; (mouse 1): 1

Even though we only have one mouse, it is mouse 1 not mouse 0, because the who number 0 was already taken by the frog.