in reply to Re: null keys
in thread null keys

Are we talking empty string or undef here? They act differently, depending.

Normally they do, but we're talking about hash keys here. A hash key can only be a string, so any number, reference or undefined value is stringified. undef happens to stringify to the emtpy string, so $foo{+undef} and $foo{''} are the same thing. 

2;0 juerd@ouranos:~$ perl -le'$foo{+undef} = "Hello, world!"; print $f
+oo{""}'
Hello, world!

[download]

- Yes, I reinvent wheels.
- Spam: Visit eurotraQ.

Replies are listed 'Best First'.
Re: Re: Re: null keys
by marvell (Pilgrim) on Jun 12, 2002 at 16:27 UTC
    Which is shown nicely by: 
    
#!/usr/bin/perl

use Data::Dumper;

$hash{+undef} = "pleasure";
$hash{''} = "pain";

print Dumper(\%hash);

    [download]

    Outputs: 

    $VAR1 = {
          '' => 'pain',
        };

    [download]

    --
    Steve Marvell

[reply]
[d/l]
[select]
Re: Re: Re: null keys
by marvell (Pilgrim) on Jun 12, 2002 at 16:26 UTC
    Update: I tried to cancel the submission and ended up in another double submission fiasco. I realied my error, hence the latter post.

    --
    Steve Marvell

[reply]

      sub bar () { 'quux' }

$foo{undef} = 'bareword undef is treated as string';
$foo{+undef} = 'function undef is treated as function because +
                is not valid in a bareword string';
$foo{bar} = 'key bar is used';
$foo{+bar} = 'key quux is used';
$foo{bar()} = 'key quux is used again (the old value is overwritten)';

      [download]

      - Yes, I reinvent wheels.
- Spam: Visit eurotraQ.

[reply]
[d/l]

      If the stuff inside the curlies looks like a word it's automaticaly quoted.

      Thus $hash{undef} and $hash{'undef'} are equivalent.Try : 

      $ perl -MO=Deparse -e "print $hash{undef}"

      [download]

      If you want to use an undef as a hash key you have to use either $hash{+undef} or something like my $NULL = undef; $hash{$NULL}

      Update: Erm ... hash keys are strings. Always. So $hash{+undef} us equivalent to $hash{''}. Oh well.

        Jenda

[reply]
[d/l]

In Section Seekers of Perl Wisdom