quicksort.pas

Program Quicksort;
{
  My first Pascal code.
  Populates and array numbers of length N with random numbers between 1 and M, sorts it in place using quicksort, then prints each element of the list.
  The quicksort algorithm is translate from C as found in "The Most Beautiful Code I Never Wrote" by Jon Bentley as found in the book "Beautiful Code"
}

Uses math;

Const
  M = 1000;
  N = 20;

Var
  numbers : Array [1..N] of Integer;
  i : Integer;
  x: Integer;

procedure swap(i, j : integer);
var
  x : integer;
begin
  x := numbers[i];
  numbers[i] := numbers[j];
  numbers[j] := x;
end;

function randint(i, j : integer) : integer;
begin
  randint := Random(j - i) + i
end;

procedure quicksort(l, u : integer);
var
  i, m : integer;
begin
  if l < u then
  begin
    swap(l, randint(l, u));
    m := l;
    for i := l + 1 to u do
      if numbers[i] < numbers[l] then
      begin
        m := m + 1;
        swap(m, i);
      end;
    swap(l, m);
    quicksort(l, m - 1);
    quicksort(m + 1, u);
  end;
end;

begin
  Randomize;
  for i := 0 to N do begin
    numbers[i] := randint(1, M);
  end;

  quicksort(1, N);

  for x in numbers do
    Writeln(x);
end.