Using 'sort' to emulate SQL's 'select'...

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

I have a hash I am trying to sort. However, I am trying to sort it by deeper values than has been discussed on the monastery before. (I spent a good hour in the Super Search). So here's some test code:

my %HoH =
( 1 => +{ foo => 'bar',
          baz => 'bletch', },
  2 => +{ foo => 'bar2',
          baz => 'bletch2', },
);

my @fookeys;

# select 'foo' from 'HoH' where 'foo' = 'bar2'
@fookeys = sort { $HoH{$a} -> {foo} eq 'bar2' } keys %HoH;

use Data::Dumper;

# fookeys is [ 1, 2 ]
print Dumper \@fookeys;

# try again
@fookeys = sort { $HoH{$a}{foo} eq 'bar2' } keys %HoH;

# fookeys is [ 1, 2 ]
print Dumper \@fookeys;
[download]

I want every key from HoH where $HoH -> {foo} eq 'bar2'. Seems to me that sort would work. Im actually pretty curious what it thinks im asking it to do. Can anyone shed some light on this?

Thanks,
brother dep.

--
Laziness, Impatience, Hubris, and Generosity.

Comment on Using 'sort' to emulate SQL's 'select'... Select or Download Code

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(ar0n) Re: Using 'sort' to emulate SQL's 'select'... by ar0n (Priest) on Apr 22, 2001 at 19:41 UTC
Why are you using a sort to get the keys? And why the pluses? Pluses are usually used to distinguish between a block of code and a hash, but in this case you don't need the '+' because it'll be considered an anonymous hash anyways... `my %HoH = ( 1 => { foo => 'bar', baz => 'bletch' }, 2 => { foo => 'bar2', baz => 'bletch2' }, ); my @fookeys = grep { $HoH{$_}{foo} eq 'bar2' } keys %HoH; # @fookeys is now 2` [download] [ ar0n ]	[reply] [d/l]
Re: (ar0n) Re: Using 'sort' to emulate SQL's 'select'... by deprecated (Priest) on Apr 22, 2001 at 19:51 UTC
from perldoc perlref: For example, if you wanted a function to make a new hash and return a reference to it, you have these options: sub hashem { { @_ } } # silently wrong sub hashem { +{ @_ } } # ok sub hashem { return { @_ } } # ok On the other hand, if you want the other meaning, you can do this: sub showem { { @_ } } # ambiguous (currently o +k, but may change) sub showem { {; @_ } } # ok sub showem { { return @_ } } # ok The leading +{ and {; always serve to disambiguate the expression to mean either the HASH reference, or the BLOCK. [download] I use HoH's a lot in my code, so I try to make absolutely sure my hashes are as disambiguated as they can be. -- Laziness, Impatience, Hubris, and Generosity.	[reply] [d/l]
Re: Using 'sort' to emulate SQL's 'select'... by lucs (Sexton) on Apr 22, 2001 at 21:35 UTC
Actually, I think you want grep(): @fookeys = grep $HoH{$_}{foo} eq 'bar2', keys %HoH; print "@fookeys\n"; # Only `2' is there. [download] By the way, your usage of sort() is wrong. The comparison function used by sort() must involve both $a and $b, else sort() cannot do its job properly. Also, you should realize that sort() will return a copy of _all_ the elements you ask it to sort; that's why you want grep().	[reply] [d/l]