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Slicing multilevel hashes

by bobf (Monsignor)
on Apr 07, 2005 at 17:14 UTC ( #445793=perlquestion: print w/replies, xml ) Need Help??
bobf has asked for the wisdom of the Perl Monks concerning the following question:

I'm trying to (ab)use hash slices. I understand the typical (@hash{@keys}) use of a slice, but now I'm trying to do something a bit more complex and take a slice of a hash of hashes. Using a slice in a %HoH when the list is provided as the second 'key' is no problem, but can slices be used to cut across the first set of keys as well? I didn't find anything when I searched for this in the docs or on PM.

Perhaps a bit of code will help define the problem:

use strict; use warnings; my @keys = qw( a c ); my %hash = ( a => 1, b => 2, c => 3, d => 4 ); # typical use - no problem # @hash{a,c} is like $hash{a}, $hash{c} print join( ' ', @hash{@keys} ), "\n"; # 1, 3 my %hoh = ( a => { a => 'a1', b => 'a2', c => 'a3', d => 'a4' }, b => { a => 'b1', b => 'b2', c => 'b3', d => 'b4' }, c => { a => 'c1', b => 'c2', c => 'c3', d => 'c4' }, d => { a => 'd1', b => 'd2', c => 'd3', d => 'd4' } ); # typical use expanded to a %HoH - no problem # @{ $hoh{b} }{a, c} is like $hoh{b}{a}, $hoh{b}{c} print join( ' ', @{ $hoh{b} }{@keys} ), "\n"; # b1, b3 # now I want to slice across the first key instead of the second # want something like $hoh{a}{d}, $hoh{c}{d} print join( ' ', @hoh{@keys}{d} ), "\n"; # syntax error # I tried dereferencing the list of hashrefs, but only # the last ref in the list was used ($hoh{c}) print join( ' ', @{ @hoh{@keys} }{d} ), "\n"; # c4 # do I have to resort to this? print join( ' ', map{ $_->{d} } @hoh{@keys} ), "\n"; # a4 c4

As illustrated in the code above, I used map to get it to work but if there is a way to use a slice I'd like to learn how to do it.

Aside: If this is possible, though, it immediately implies one could slice across both directions at once:

# yikes!! print join( ' ', @hoh{@keys}{@keys} ), "\n"; # a1 a3 c1 c3 ??


Update: clarified the main question

Replies are listed 'Best First'.
Re: Slicing multilevel hashes
by Roy Johnson (Monsignor) on Apr 07, 2005 at 17:33 UTC
    A slice -- even a hash slice -- is not a hash. It's a list (that includes only the values components, not the keys). That's why the sigil is @ and not %. You cannot hash-dereference a list. As dragonchild astutely suggested, you can use map to get the effect you want.
    map @{$_}{@keys}, @hoh{@keys};

    Caution: Contents may have been coded under pressure.

      Thanks for the speedy replies. I did use map to get a working solution (map{ $_->{d} } @hoh{@keys}) but I thought there might be another way to do it (thanks for the example). In the back of my mind I didn't think the @hoh{@keys}{d} idea was possible, but if it was then if anyone would know how to do it, it would be someone at Perl Monks!

Re: Slicing multilevel hashes
by dragonchild (Archbishop) on Apr 07, 2005 at 17:19 UTC
    You're trying to do a slice of a slice - not possible. Try slicing within a map across the outer slice.
Re: Slicing multilevel hashes
by bsb (Priest) on Apr 14, 2005 at 04:42 UTC

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