Re: Using $_ in a function if no argument is passed

From: Beau E. Cox (beau_at_beaucox.com)
Date: 04/02/04 

To: beginners@perl.org
Date: Fri, 2 Apr 2004 07:04:16 -1000

On Friday 02 April 2004 06:37 am, JupiterHost.Net wrote:
> Hello List,
>
> It just occurred to me that many Perl functions use $_ if not other
> value is supplied. chomp for instance..., which is very handy...
>
> If one wanted to write a function that used either the given argument or
> $_ how would you do that?
>
> myfunc($myvalue);
> or
> myfunc; #uses the current value of $_
>
> sub myfunc {
>
> my $func_arg = shift || ????; # you wouldn't just do '|| $_;' would you?
>
> ...
>

Hi -

I think what you are trying to do is modify a passed argument - like chomp
does - which really has nothing to do with $_. Normaly a subroutine gets
its arguments from the @_ array and puts them in a private variable,
does its thing, and returns a value. This is commonly called 'pass by
reference' and is a nice, safe way to do things. If you want to operate
on the passed arguments themselves ('pass by value') - which can be more
error prone (at least in some cases), you can do that - as chomp does.

In the sample below, 'to_lower' modifies the incoming argument, and 'to_upper'
does not.

#!/bin/perl

use strict;
use warnings;

my $string = 'hello';
print "before to_upper: $string\n";
to_upper( $string );
print " after to_upper: $string\n";
to_lower( $string );
print " after to_lower: $string\n";

sub to_upper
{
    # actually modified the incoming argument the -
    # 0th element of @_.
    $_[0] = uc $_[0];
}

sub to_lower
{
    # traditional approach
    my $string = shift;
    lc $string;
    return $string;
}

When run, it returns:

before to_upper: hello
 after to_upper: HELLO
 after to_lower: HELLO

Aloha => Beau;

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