part := proc(arr, left, right, pivot)
	local val,safe,i:
	val := arr[pivot]:
	arr[pivot], arr[right] := arr[right], arr[pivot]:
	safe := left:
	for i from left to right do
		if arr[i] < val then
			arr[safe], arr[i] := arr[i], arr[safe]:
			safe := safe + 1:
		end if:
	end do:
	arr[right], arr[safe] := arr[safe], arr[right]:
	return safe:
end proc:

quickselect := proc(arr,k)
	local pivot,left,right:
	left,right := 1,numelems(arr):
	while(true)do
		if left = right then return arr[left]: end if:
		pivot := trunc((left+right)/2);
		pivot := part(arr, left, right, pivot):
		if k = pivot then
			return arr[k]:
		elif k < pivot then
			right := pivot-1:
		else
			left := pivot+1:
		end if:
	end do:
end proc:
roll := rand(1..20):
demo := Array([seq(roll(), i=1..20)]);
map(x->printf("%d ", x), demo):
print(quickselect(demo,7)):
print(quickselect(demo,14)):