Try running this: 

#!/usr/bin/perl

use strict;

my $a = 2;
f($a);
print "$a\n";

sub f {
    my $x = shift;
    $x = 5;
}

[download]
You'll notice it prints '2', while according to you it should print '5'.

This is precisely why a reference should be passed to the function. If you don't, you actually pass a value (2) and initialize the local variable $x to it. Later in the sub the value of $x is change to 5. Now why should that change the value of $a in the main program? It shouldn't since the subroutine only knows about the value of $a which happens to be 2, not about $a itself so the value of $a will remain 2 as it was before the function call.

Changing the code above to: 

#!/usr/bin/perl

use strict;

my $a = 2;
f(\$a);
print "$a\n";

sub f {
    my $x = shift;
    $$x = 5;
}

[download]
will actually print '5' rather than '2'. Now you pass the address of the variable $a to the sub and it gets stored in $x. The value at the address in $x is set to 5 by dereferencing $x with $$x. Since $x is the address of $a, it's value is indeed changed to 5.

Hope this helps, -gjb-

In reply to Re: Re: Parameter passing by gjb
in thread Parameter passing by Anonymous Monk

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