Re: Using $_ in a function if no argument is passed
From: Beau E. Cox (beau_at_beaucox.com)
Date: 04/02/04
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To: beginners@perl.org Date: Fri, 2 Apr 2004 07:18:25 -1000
On Friday 02 April 2004 07:04 am, Beau E. Cox wrote:
> 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
>
That's all well and good, but after re-reading you query,
here is what you really want:
#!/bin/perl
use strict;
use warnings;
my $string = 'hello';
print "before to_upper w/arg: $string\n";
to_upper( $string );
print " after to_upper w/arg: $string\n";
$_ = 'hiya';
print "before to_upper no/arg: $_\n";
&to_upper;
print " after to_upper no/arg: $_\n";
sub to_upper
{
# point to passed arg or caller's $_
my $arg = $_[0] ? \$_[0] : \$_;
# modify in place
$$arg = uc $$arg;
}
Aloha => Beau;
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