Testing array of hash values

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

Monks ~

Two questions here:

Why doesn't this work like I expect ("false expectations" is my best answer, but it isn't helping me),
Is there a better way to do what I'm attempting?

I want to format and display a heading iff that heading's children contain data.

#!/usr/bin/perl -T

use strict;
use warnings;

# prints nothing, as expected
print do_head( "Empty" );

# prints "<H4>Appearance</H4>"; wish it didn't
my @p{ 'eyes', 'hair', 'etc' } = ();
print do_head( "Appearance", @p{ 'eyes', 'hair', 'etc' });


#------------------------------------------------
sub do_head {
    my $heading = shift;
    my @data = @_;

    return "<H4>$heading</H4>" if @data;
}
[download]

I expected the do_head sub to put an empty list into @data; said list would then evaluate as false in a scalar context. This doesn't work, of course, which is why I'm here. I'm sure this is based on my faulty understanding of arrays and hashes. I've re-read the Camel on this subject and not attained enlightenment. Help?

Thanks
--
man with no legs, inc.

Comment on Testing array of hash values Download Code

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Re: Testing array of hash values by athomason (Curate) on Dec 11, 2001 at 07:52 UTC
Like dws, I'm surprised this would run; it doesn't for me under 5.6.1. Your declaration of `%p` almost has the syntax for a hash slice, except that you can't do that in a `my` declaration. Something like `my %p; @p{ 'eyes', 'hair', 'etc' } = ();` would be legitimate, if unusual. As for why do_head doesn't do what you think, I believe you're misunderstanding the behavior of `undef` in an array. `undef` in an array or hash doesn't just disappear, it's still something. This is notably different from its behavior in a list (which is not an array!). For instance, see the behavior of this snippet: `@a = ( ); @c = ( undef ); die "a" if @a; die "b" if ( undef, undef ); die "c" if @c` Particularly, it dies with 'c', not 'a' or 'b'. In a boolean context--like an if statement--an array takes the value of its length, not its members. In my example, this is 1, which is a true value. Likewise, your three `undef`s give @data a true value. So, your list of `undef`s is in fact true when assigned to an array. You were probably expecting the behavior of a list, which in a boolean context evaluates to its last element (e.g., `die "d" if ( 1, undef ); die "e" if ( undef, 1 );` dies with 'e'). The solution, if you wish to keep the behavior of printing when the elements are defined, can be found in dws' post.	[reply] [d/l] [select]
Re: Testing array of hash values by dws (Chancellor) on Dec 11, 2001 at 07:26 UTC
It looks like this is intended to be a CGI, and yet it doesn't emit the correct response header, so I'll assume this is just a code fragment. What, exactly, do you expect `@p{'eyes', 'hair', 'etc'} =();` to do? It rather looks like a syntax error to me. If you're trying to pre-load a hash with keys, you might try `my %p; $p{$_} = undef for qw(eyes hair etc);` [download] But that begs the question of what your `do_print` is trying to do, since `@p{ 'eyes', 'hair', 'etc' }` results in a three element array, with each element being `undef`. Assuming that you're trying to return the heading if and only if any one of the elements is present, you could do `sub do_head { my($heading, @data) = @_; return "<h4>$heading</h4>" if grep { defined($_) } @data; return ""; }` [download]	[reply] [d/l] [select]
Another posible initializer by Fletch (Bishop) on Dec 11, 2001 at 07:32 UTC
He may have been trying for: `my %p; @p{ qw( eyes hair etc ) } = ( undef ) x 3;` [download] Of course that's crufty in that you have to hard code in the number of elements.	[reply] [d/l]
Re: Another posible initializer by dvergin (Monsignor) on Dec 11, 2001 at 07:47 UTC
Actually, `my %p; @p{'eyes', 'hair', 'etc'} = ();` [download] is a common idiom for using a hash slice to load a hash with keys and undef values.	[reply] [d/l]
Re: Re: Another posible initializer by Fletch (Bishop) on Dec 11, 2001 at 08:18 UTC
Re: Re: Re: Another posible initializer by dvergin (Monsignor) on Dec 11, 2001 at 11:55 UTC
Re: Re: Testing array of hash values by legLess (Hermit) on Dec 11, 2001 at 10:33 UTC
Yes, it's a fragment, but it runs as it should from the command line. The line `@p{'eyes', 'hair', 'etc'} =();` [download] is just to show that those keys are empty. It was indeed the `undef` that was getting me. Your code works. Thanks. -- man with no legs, inc.	[reply] [d/l]


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