Re: undef as a key in a hash.

I had to try this myself, because I couldn't believe it:

$a{undef} = 1;
print defined for keys %a;

# Output
1

# This doesn't work either:
%a = (undef, 1);
[download]

I wonder what he was thinking...

Ted Young

Comment on Re: undef as a key in a hash. Download Code

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Re^2: undef as a key in a hash. by Joost (Canon) on Sep 25, 2007 at 18:45 UTC
Those two don't do the same thing: the first line uses the key "undef" since barewords in the `$hash{bareword}` construct are automatically quoted. The second one uses the string "" as the key, as explained below. In any case, you're right that anything recieved from keys() can't be undefined. "What should it profit a man, if he should win a flame war, yet lose his cool?"	[reply] [d/l]
Re^2: undef as a key in a hash. (constant.pm "bug") by lodin (Hermit) on Sep 26, 2007 at 12:05 UTC
I wonder what he was thinking... The hash-style usage of constant wasn't added until perl 5.7.1, and the `defined $name` check was around before that. Basically all of 5.7.0's code was kept intact and put in the `for` loop. The `defined $name` check was just accidentally (I presume) left inside the `for` loop. Here's how it should look. `my $multiple = ref $_[0]; if ( $multiple ) { ... } else { unless (defined $_[0]) { require Carp; Carp::croak("Can't use undef as constant name"); } $constants{+shift} = undef; } foreach my $name ( keys %constants ) { ... }` [download] Not that this matters much, because `use constant undef, 'x';` [download] will fail anyway. You just get a slightly less clear error message. lodin	[reply] [d/l] [select]