June 2018 Update

Task/Quickselect-algorithm/Maple/quickselect-algorithm.maple

@@ -0,0 +1,36 @@
+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)):