I was going to suggest that depending on the sparseness of your keys, and whether they are all positive integers, it might be a faster to use exists to test an array. Before I made that assertion, I thought I should write a benchmark.

The results were that the theoretical optimization is pretty useless. The speeds were basically identical.

So, unless someone cares to tell me how badly I screwed up my benchmark, (proper benchmarking is hard, and I don't write them often) I'd have to say "Don't bother with my silly array idea."

Update: BrowserUK was kind enough to demonstrate that my benchmarks were flawed and that the arrays are indeed about 37% faster.

use strict;
use warnings;
use Benchmark;
use List::Util qw(shuffle);

my $count=5000000;

my @existing_indexes    = ( 5,6,8..53,62..106);
my @indexes = (0..200);

my @shuffled_indexes = shuffle @indexes;

my %test_hash;
my @test_array;

@test_array[ @existing_indexes ] = ();
@test_hash{  @existing_indexes } = ();



Benchmark::cmpthese($count, {
    arrayLookup => q{
        my $val = &arrayLookup( \@test_array, \@indexes );
    },
    hashLookup  => q{
        my $val = &hashLookup(  \%test_hash,  \@indexes );
    },
    shuffled_arrayLookup => q{
        my $val = &arrayLookup( \@test_array, \@shuffled_indexes );
    },
    shuffled_hashLookup  => q{
        my $val = &hashLookup(  \%test_hash, \@shuffled_indexes )
    },
} );




sub arrayLookup {
    my $array   = shift;
    my $indexes = shift;

    my @results = map {
        exists $array->[$_];
    } @$indexes;
        
    return \@results;
}

sub hashLookup {
    my $hash = shift;
    my $keys = shift;

    my @results = map {
        exists $hash->{$_};
    } @$keys;
        
    return \@results;
}

[download]

And the results

Run 1:
                         Rate shuffled_arrayLookup hashLookup shuffled
+_hashLookup arrayLookup
shuffled_arrayLookup 409199/s                   --        -1%         
+        -1%         -2%
hashLookup           411286/s                   1%         --         
+        -1%         -2%
shuffled_hashLookup  415076/s                   1%         1%         
+         --         -1%
arrayLookup          417781/s                   2%         2%         
+         1%          --

Run 2:
shuffled_arrayLookup arrayLookup shuffled_hashLookup
hashLookup           412371/s         --                  -0%         
+-1%                 -1%
shuffled_arrayLookup 413976/s         0%                   --         
+-0%                 -1%
arrayLookup          414491/s         1%                   0%         
+ --                 -1%
shuffled_hashLookup  416667/s         1%                   1%         
+ 1%                  --
C:\Documents and Settings\Mark\Desktop\Downloads> exists.pl
                         Rate shuffled_arrayLookup hashLookup shuffled
+_hashLookup arrayLookup
shuffled_arrayLookup 409199/s                   --        -1%         
+        -1%         -2%
hashLookup           411286/s                   1%         --         
+        -1%         -2%
shuffled_hashLookup  415076/s                   1%         1%         
+         --         -1%
arrayLookup          417781/s                   2%         2%         
+         1%          --

Run 3:
                         Rate hashLookup shuffled_arrayLookup arrayLoo
+kup shuffled_hashLookup
hashLookup           412371/s         --                  -0%         
+-1%                 -1%
shuffled_arrayLookup 413976/s         0%                   --         
+-0%                 -1%
arrayLookup          414491/s         1%                   0%         
+ --                 -1%
shuffled_hashLookup  416667/s         1%                   1%         
+ 1%                  --

[download]


TGI says moo

In reply to Re: How do I efficiently predeclare a hash with a set of keys by TGI
in thread How do I efficiently predeclare a hash with a set of keys by lupey

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