Pascal - Subprogram Call by Reference
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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 reference method of passing arguments to a subprogram copies the address of an argument into the formal parameter. Inside the subprogram, the address is used to access the actual argument used in the call. This means that changes made to the parameter affect the passed argument.
In order to pass the arguments by reference, Pascal allows to define variable parameters. This is done by preceding the formal parameters by the keyword var. let us take the example of the procedure swap() that swaps the values in two variables and reflect the change in the calling subprogram.
procedure swap(var x, y: integer); var temp: integer; begin temp := x; x:= y; y := temp; end;
Next, let us call the procedure swap() by passing values by reference as in the following example:
program exCallbyRef; var a, b : integer; (*procedure definition *) procedure swap(var 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 : 200 After swap, value of b : 100
Which shows that now the procedure swap() has changed the values in the calling program.