USP45 has asked for the wisdom of the Perl Monks concerning the following question:

- Is it cool to populate a hash of arrays with qq or qw? 
e.g.

my %animals = (
  pets => [ qw(cat dog) ],
  non  => [ qq(rhino giraffe) ],
);

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- If one of those is correct what goes in the blank if I want to iterate through one of the arrays? 
    foreach $i (???) {
        push ( @some_animals, $animals{pets}[$i] );
    }

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Replies are listed 'Best First'.
Re: Hash of Arrays (populating and iterating)
by pc88mxer (Vicar) on Mar 06, 2008 at 01:37 UTC
    It's not only cool, it's way cool to use qq and qw to populate HoA's.

    For your iteration question, try: 

    foreach $i (0..$#{$animals{pets}}) {
  ...
}

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      Or this way -- why bother with indexes at all? 
      
foreach $animal ( @{ $animals{pets} } ) {
        push ( @some_animals, $animal );
    }

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      Nobody says perl looks like line-noise any more
      kids today don't know what line-noise IS ...
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        That's true. I tried it that way at first and got unexpected results (the array getting treated as 1 item when I tried to sort and make the elements unique) but it's probably because I was using qq incorrectly to populate the hash in the first place. I should have used qw and the elements would have been separate elements.
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      Awesome, that's the ticket. THANKS!
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      Hi pc88mxer,

      This might be a silly question pardon me for that, but i will go ahead and ask this to you,what does $# do in the below code ?

      foreach $i (0..$#{$animals{pets}}) {
  ...
}

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[d/l]

        From perldata:

        The length of an array is a scalar value. You may find the length of array @days by evaluating $#days, as in csh. However, this isn't the length of the array; it's the subscript of the last element, which is a different value since there is ordinarily a 0th element.

        The syntax $#{ ... } extends this idea: it interprets whatever expression is between the braces as an array reference; dereferences it to get an array; and returns the subscript of the last element. For example:

        #! perl
use strict;
use warnings;

my %animals =
(
    pets => [ qw( cat dog hamster ) ],
    non  => [ qw( rhino   giraffe ) ],
);

for my $i (0 .. $#{ $animals{pets} })
{
    print "$i: ", $animals{pets}->[$i], "\n";
}

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        Output:

         1:43 >perl 857_SoPW.pl
0: cat
1: dog
2: hamster

 1:43 >

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        But note that, as Cody Pendant says above, it’s generally better (where possible) to dispense with array indexes altogether.

        Hope that helps,

        Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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