A subroutine example

[eltikia]

Can you give us an example of a subroutine in Perl? I'm studying for a Java exam and I'd like to know more. Here's a quotation from a handbook.

Different languages handle parameters in different ways. Pass-by-value is used by many languages, including Java. [...] The other approach is pass-by-reference. It is used by default in a few languages, such as Perl.

[Stagyrite]

Let's consider what the handbook says, and we need both Perl 1 and Perl 5 interpreters to find out how it works.

No call-by-reference in Perl 1

A pass-by-reference example wouldn't work in Perl 1, and here is a good try.

sub trySwap {
    $temp = $_[0];
    $_[0] = $_[1];
    $_[1] = $temp;
}

$a = 1;
$b = 2;
do trySwap ($a, $b);
print $a;
print $b;

This Perl 1 program prints 12 which is different from what we expect.

Call-by-reference in Perl 5

Call-by-reference is the default Perl 5 mechanism, just like the handbook states.

sub swap {
    $temp = $_[0];
    $_[0] = $_[1];
    $_[1] = $temp;
}

$a = 1;
$b = 2;
swap ($a, $b);
print $a;
print $b;

It prints 21 just like expected.

Something similar in Perl 1

Here's a traditional although sophisticated Hello World program.

#!./perl

sub printSwapped {
    $temp = $_[0];
    $_[0] = $_[1];
    $_[1] = $temp;
    print $_[0];
    print $_[1];
}

$a = "world\n";
$b = "hello, ";
do printSwapped( $a, $b );

Its output is "hello, world" printed in a single line.