!YS-v0

v =: +[9 8 7 6 5 0 1 2 3 4]

defn main():
  say:
    map \(quickselect v _):
      v.#:range

defn quickselect(a k):
  loop arr a:V, current-k k:
    pivot-idx =: arr.#:rand-int
    pivot =: arr.$pivot-idx
    remaining =:
      concat subvec(arr 0 pivot-idx):
             subvec(arr pivot-idx.++)
    left =: remaining.filter(\(_ < pivot))
    right =: remaining.filter(\(_ >= pivot))
    left-count =: left.#
    cond:
      current-k == left-count: pivot
      current-k < left-count:
        recur: left:V current-k
      else:
        recur right:V: (current-k - left-count).--