Re: map vs foreach

[Jonathan Paton]

Dear Michael,

None of the above.  All your code is calling length() twice per loop.  

The best way to write what you want is:

        my $max_length = 0;

        for my $datum (keys %data) {
            my $length = length $datum;

            if ($max_length < $length) {
                $max_length = $length;
            }
        }

But there is a faster way:

        my $max_length = 0;

        while ((my $datum) = each %data) {
            my $length = length $datum;

            if ($max_length < $length) {
                $max_length = $length;
            }
        }

Benchmarked, it all looks like:

          Rate  Fifth Fourth Second  Third  First
Fifth   4539/s     --   -46%   -53%   -57%   -58%
Fourth  8333/s    84%     --   -14%   -21%   -24%
Second  9681/s   113%    16%     --    -8%   -11%
Third  10493/s   131%    26%     8%     --    -4%
First  10929/s   141%    31%    13%     4%     --

And the benchmark code:

use strict;
use warnings;

use Benchmark ':all';

my %data = (
    map { 'x' x rand 100 => undef } 0 .. 1000
);

cmpthese(10000, {
    First  => sub {
        my $max_length = 0;

        for my $datum (keys %data) {
           $max_length = length($datum) if length($datum) > $max_length;
        }
    },
    Second => sub {
        my $max_length = 0;

        map { $max_length = ($max_length > length($_) ) ? $max_length
: length($_) }
            keys %data;
    },
    Third  => sub {
        my $max_length = 0;

        map { $max_length = length($_) if length($_) > $max_length }
            keys %data;
    },
    Fourth => sub {
        my $max_length = 0;

        for my $datum (keys %data) {
            my $length = length $datum;

            if ($max_length < $length) {
                $max_length = $length;
            }
        }
    },
    Fifth  => sub {
        my $max_length = 0;
        my $datum;

        while (($datum) = each %data) {
            my $length = length $datum;

            if ($max_length < $length) {
                $max_length = $length;
            }
        }
    }
});

__END__

Jonathan Paton

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