Data update

Task/Permutations-Derangements/PascalABC.NET/permutations-derangements.pas

@@ -0,0 +1,23 @@
+function derangements<T>(a: array of T) :=
+    a.Permutations.where(p -> p.where((x, i) -> x = a[i]).Count = 0);
+
+function subFactorial(n: integer): int64;
+begin
+  if n <= 1 then result := 1 - n
+  else result := (n - 1) * (subfactorial(n - 1) + subfactorial(n - 2));
+end;
+
+begin
+  println('Derangements of 1 2 3 4:');
+  foreach var d in derangements(|1, 2, 3, 4|) do
+    d.println;
+
+  println(#10, 'Number of derangements:');
+  println('n   counted   calculated');
+  println('-   -------   ----------');
+  for var n := 1 to 10 do
+    writeln(n:2, derangements(range(1, n).ToArray).count:9, subfactorial(n):10);
+
+  println;
+    println('!20 = ', subfactorial(20));
+end.