to find-clusters
  loop [
    ;; pick a random patch that isn't in a cluster yet
    let seed one-of patches with [cluster = nobody
    and pcolor = 8
    ]
    ;; if we can't find one, then we're done!
if seed = nobody
[
  stop ]
;; otherwise, make the patch the "leader" of a new cluster
;; by assigning itself to its own cluster, then call
;; grow-cluster to find the rest of the cluster
ask seed
[ set cluster self
  grow-cluster ]]
  display
end

to grow-cluster  ;; patch procedure
  ask neighbors4 with [(cluster = nobody
    and pcolor = 8
    ) and
    (pcolor = [pcolor] of myself)]
  [ set cluster [cluster] of myself
    grow-cluster ]
   end

to calc-frequency
  let freq map [[i] -> count patches with [cluster = i]] remove-
   duplicates [cluster] of patches

  set-current-plot "Frequency distribution of urban patch size"
  histogram freq
end