RPN_52_v2.pas

program RPN;
uses Contnrs;
var
  s: String; //Input string
  stack: TStack; //Working RPN stack
  a, b, r: LongInt; //Two operands and result
  op: Char;
  buf: String; //Holds a single operand or operator
  l, head, tail: Integer; // Input string parser pointers

procedure my_Push(var stack: TStack; r: LongInt);
var pr: ^LongInt;
begin
  New(pr);
  pr^ := r;
  stack.Push(pr);
end;

function my_Pop(var stack: TStack) : LongInt;
var pv: ^LongInt;
begin
  pv := stack.Pop();
  if pv = nil
     then WriteLn('Stack is already empty!');
  my_Pop := pv^;
  Dispose(pv);
end;

begin
//  WriteLn('Enter an expression in RPN notation: ');
  ReadLn(s);
  s := s + ' ';
  stack := TStack.Create;

  repeat
    head := 1;
    tail := Pos(' ', s);
    if tail = head then begin
      Delete(s, head, tail-head+1);
      l := Length(s);
      continue; // skip leading spaces
    end;
    buf := Copy(s, head, tail-head);

    op := buf[1];
    case op of // Pascal case does not fall through!
      '+':
      begin
        b := my_Pop(stack);
        a := my_Pop(stack);
        r := a + b;
        my_Push(stack, r);
      end;
      '-':
      begin
        b := my_Pop(stack);
        a := my_Pop(stack);
        r := a - b;
        my_Push(stack, r);
      end;
      '*':
      begin
        b := my_Pop(stack);
        a := my_Pop(stack);
        r := a * b;
        my_Push(stack, r);
      end;
      '/':
      begin
        b := my_Pop(stack);
        if b = 0
           then WriteLn('Attempting division by zero!');
        a := my_Pop(stack);
        r := a div b;
        my_Push(stack, r);
      end;
      '0'..'9':
      begin
        Val(buf,r);
        my_Push(stack, r);
      end;
    end; //case
    Delete(s, head, tail-head+1); // delete the space as well
    l := Length(s);
  until (l=0);

  r := my_Pop(stack);
  WriteLn(r);
  stack.Destroy;
//  ReadLn;
end.