Pascal - Subprogram Call by Value

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Category: Chapter 3

Published: Sunday, 14 April 2013 16:20

Written by Sternas Stefanos

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The call by value method of passing arguments to a subprogram copies the actual value of an argument into the formal parameter of the subprogram. In this case, changes made to the parameter inside the function have no effect on the argument.

By default, Pascal uses call by value method to pass arguments. In general, this means that code within a subprogram cannot alter the arguments used to call the subprogram. Consider the procedure swap() definition as follows.

procedure swap(x, y: integer); 
var
   temp: integer;
begin
   temp := x;
   x:= y;
   y := temp;
end;
 

Now let us call the procedure swap() by passing actual values as in the following example:

program exCallbyValue;
var
   a, b : integer;
(*procedure definition *)
procedure swap(x, y: integer); 
var
   temp: integer;
begin
   temp := x;
   x:= y;
   y := temp;
end;
begin
   a := 100;
   b := 200;
   writeln('Before swap, value of a : ', a );
   writeln('Before swap, value of b : ', b );
(* calling the procedure swap  by value   *)
   swap(a, b);
   writeln('After swap, value of a : ', a );
   writeln('After swap, value of b : ', b );
end.
 

When the above code is compiled and executed, it produces following result:

Before swap, value of a :100
Before swap, value of b :200
After swap, value of a :100
After swap, value of b :200

The program shows that there is no change in the values though they had been changed inside the subprogram.