Data commit

Task/Quickselect-algorithm/Picat/quickselect-algorithm.picat

@@ -0,0 +1,37 @@
+main =>
+  L = [9,8,7,6,5,0,1,2,3,4],
+  Len = L.len,
+  println([select(L,1,Len,I) : I in 1..Len]),
+  nl.
+
+select(List, Left, Right, K) = Select =>
+  if Left = Right then
+    Select = List[Left]
+  else
+    PivotIndex  = partition(List, Left, Right, random(Left,Right)),
+    if K == PivotIndex then
+      Select = List[K]
+    elseif K < PivotIndex then
+      Select = select(List, Left, PivotIndex-1, K)
+    else
+      Select = select(List, PivotIndex+1, Right, K)
+    end
+  end.
+
+partition(List, Left, Right, PivotIndex) = StoreIndex =>
+  PivotValue = List[PivotIndex],
+  swap(List,PivotIndex,Right),
+  StoreIndex = Left,
+  foreach(I in Left..Right-1)
+    if List[I] @< PivotValue then
+      swap(List,StoreIndex,I),
+      StoreIndex := StoreIndex+1
+    end
+  end,
+  swap(List,Right,StoreIndex).
+
+% swap L[I] <=> L[J]
+swap(L,I,J) =>
+  T = L[I],
+  L[I] := L[J],
+  L[J] := T.