http://www.perlmonks.org?node_id=695177


in reply to Idiom: hashes as sets

One tweak to save memory: use undef instead of 1 for the value and use exists to check for an item in the set. It makes things harder to read though.

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Re^2: Idiom: hashes as sets
by Narveson (Chaplain) on Jul 02, 2008 at 19:22 UTC

    I agree, and note what this means for inserting a list:

    my %set;
@set{qw(I agree)} = ();
print sort keys %set;      # Iagree

    [download]

    Just didn't want you to say

    @set{@list} = (undef) x scalar @list;

    even though that would work.

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      I don't understand the reply. Could you, Narveson, please rephrase?
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Re^2: Idiom: hashes as sets
by Anonymous Monk on Jul 03, 2008 at 14:22 UTC
    use !undef and get the best of both worlds :)(tye learn me this)
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      Whoa, 1 and undef are on par , optimization? 
      #!/usr/bin/perl -- 

use Devel::Size qw(size total_size);
use strict;
use warnings;

{
my %h;
    
    for ( 1 .. 20_000 ){
        $h{1}{$_}=1;
        $h{'$_'}{$_}=$_;
        $h{'undef'}{$_}=undef;
        $h{'!undef'}{$_}= !undef;
        $h{'!!undef'}{$_}= !!undef;
        $h{'!!!undef'}{$_}= !!!undef;
        $h{'sv_yes'}{$_}= !0;
        $h{'sv_no'}{$_}= !1;
    }

    printf "%10s %10s %10s\n", 'value', 'size', 'total_size'; 
    printf "%10s %10d %10d\n",$_, size($h{$_}),total_size($h{$_}) for 
+sort keys %h;
}

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      ActivePerl Build 813, perl, v5.8.7 built for MSWin32-x86-multi-thread 
           value       size total_size
  !!!undef     660026    1420026
   !!undef     660026    1400026
    !undef     660026    1420026
        $_     660026    1328920
         1     660026     980026
     sv_no     660026    1400026
    sv_yes     660026    1420026
     undef     660026     900026

      [download]
      strawberry-perl-5.10.0.1-1.exe , v5.10.0 built for MSWin32-x86-multi-thread 
           value       size total_size
  !!!undef     660014    1540014
   !!undef     660014    1540014
    !undef     660014    1540014
        $_     660014    1616018
         1     660014     980014
     sv_no     660014    1540014
    sv_yes     660014    1540014
     undef     660014     980014

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