directly modify hash from subroutine

marksman has asked for the wisdom of the Perl Monks concerning the following question:

Why doesn't this work as I expect?

my %h = (a,b,c,d);
&func(\%h);

while ((my $key, my $val) = each %h)
{
  print "key is $key val is $val\n";
}

sub func
{
   $_->{'e'} = 'f';
}
[download]

The output is

key is c val is d
key is a val is b
[download]

but I also want it to print out
key is e val is f

How can I get it to produce what I want?

Comment on directly modify hash from subroutine Select or Download Code

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Re: directly modify hash from subroutine by stevieb (Canon) on Sep 04, 2009 at 23:47 UTC
`#!/usr/bin/perl use strict; use warnings; my %h = qw( a b c d ); func( \%h ); while ( my ( $key, $val ) = each %h ) { print "key is $key val is $val\n"; } sub func { my $incoming_hash_ref = shift; $incoming_hash_ref->{'e'} = 'f'; }` [download] prints: `key is e val is f key is c val is d key is a val is b` [download] IOW, the $_ isn't doing what you think it is doing (AFAICT). Steve	[reply] [d/l] [select]
Re: directly modify hash from subroutine by marksman (Novice) on Sep 05, 2009 at 00:04 UTC
Ok, yeah if I replace $_ with $_[0] then it works. I guess I didn't understand the way function parameters were passed. So the arguments are always in @_, not $_, is that correct?	[reply]
Re^2: directly modify hash from subroutine by chromatic (Archbishop) on Sep 05, 2009 at 00:26 UTC
That is (almost always) correct. There's a special case related to subroutine prototypes in Perl 5.10, but you can safely and successfully ignore that for a very, very long time.	[reply]
Re^2: directly modify hash from subroutine by biohisham (Priest) on Sep 05, 2009 at 19:56 UTC
Great, well you know that @_ would hold the arguments passed to a subroutine and you correlated that with array element indexing. Function (subroutines) parameters are passed either directly or by reference, but you have to remember: using & to call a subroutine would disable prototype checking on the arguments if you have specified a subroutine that accepts certain types of arguments. modifying a reference inside a subroutine would modify the original data as is the case with references modification in general. Excellence is an Endeavor of Persistence. Chance Favors a Prepared Mind.	[reply] [d/l]