Understanding call by value-result and call by reference differnce

Question

So I have this piece of Pascal code:

program P;

var a: array [1..2] of Integer;
var i :Integer;
var k :Integer;

procedure update(x,y,z: Integer);
    begin
        x := x+1;
        y := x*2;
        x := y;    
        k := x;
     end


begin
     a[1] := 5; a[2] := 10;
     k := 3;
     for i:=1 to 2 do
       begin
          update(a[i],a[2],k);
          print(a);
          print(k)
       end
end.

(assume that 'print' prints elements of array separated by spaces, and then prints a new line and also for an integer it just prints it)

And i'm trying to understand how different the output would be if the function call was by value-result or by reference.

obviously, if it was just by-value, it's easy to tell that the procedure wouldn't make any difference to the actual parameter, i.e the output (in by value) should be: 5 10 3 5 10 3.

I think that if it was be value-result it would have been, at least the first iteration: 12 12 12. at the case of by reference I got confused. What would it be?

sorry, my bad, although it doesn't really make a difference as the subject of the question has nothing to do with the language it's written in. — wannabe programmer, Jun 19 '14 at 18:57
What happens when you try it? If you do so, you'll clearly see the difference. (Also, the tag is entirely relevant when you post code. It's usually a good idea to pay attention to advice you receive here, especially when it comes from people with a considerable amount of experience here. You'll get answers much more quickly if you tag it in accordance with the language of the code you've posted; people who are familiar with that language will see it, and it's relevant for future readers of this site when searching for information.) — Ken White, Jun 19 '14 at 22:12
To illustrate the difference, write your `print` function to output the array contents, remove the loop, and run the code in the sequence `print`, `update`, `print`, and compare the outputs. — Ken White, Jun 19 '14 at 22:14

score 1 · Accepted Answer · answered Nov 23 '14 at 16:57

You haven't declared any variable parameters. The variable K will be changed, but that doesn't count in this context and is generally considered bad practice.

PROGRAM ParamTest;

VAR
   A, B : Integer;

PROCEDURE TestProc(X : Integer; VAR Y : Integer; CONST Z : Integer);

BEGIN
   X := X + Z;
   Y := Y + Z;
END;

BEGIN
   A := 10;
   B := 10;
   TestProc(A, B, 5);
   WriteLn(A, ' ', B);
END.

X : Call by value
Y : Call by reference
Z : Call by constant

The output of this program should be 10 15.

With declaring Zas constant parameter you promise to the compiler that you don't change that value. The following procedure variant should give an error while compiling.

PROCEDURE TestProc(X : Integer; VAR Y : Integer; CONST Z : Integer);

BEGIN
   X := X + Z;
   Y := Y + Z;
   Z := Z + 1;
END;

Understanding call by value-result and call by reference differnce

1 Answers1