in reply to Hash checking

No need to use a hash if you're just reading numbers from a file. Use an array instead:

#!/usr/bin/perl

use strict;
use warnings;

my $file = 'AXP_FACS.DAT';
open my $FH, '<', $file or die "Error opening file: $!";
my @faclist = (<$FH>); % Read all lines into the array
close $FH;
chomp @faclist;        % Remove newlines from each entry.

my @numbers = 1..5000;
foreach my $integer ( @numbers ) {
    unless ( grep { /\b$integer\b/ } @faclist ) {
        print "Not found: $integer\n";
    }
}

[download]

The '\b' at the start and end of the grep regular expression matches the entire number, not just a single digit.

njcodewarrior

Replies are listed 'Best First'.
Re^2: Hash checking
by GrandFather (Saint) on Apr 25, 2007 at 22:12 UTC
    No need to use a hash ...

    unless you are concerned with execution time. grep performs a linear search through the entire array each time through the loop so the search is O(n2). A hash performs an essentially constant time lookup so the search is O(n).

    DWIM is Perl's answer to Gödel
[reply]

      Thanks for the reply GrandFather. Not that I didn't believe you, but here's the proof:

      #! /usr/bin/perl

use strict;
use warnings;

use File::Spec;
use Data::Dumper;
use Benchmark qw( timethese cmpthese );


my ( undef, undef, $app ) = File::Spec->splitpath( $0 );

open my $DATA, '<', './AXP_FACS.DAT' or die "Error opening file: $!";
my @faclist = (<$DATA>);
chomp @faclist;
close $DATA;

sub grep_by_array {
    my ( $ref ) = @_;
    my @faclist = @$ref;
    my @found;
    foreach my $integer ( 1..5000 ) {
        if ( grep { /\b$integer\b/ } @faclist ) {
            unshift @found, $integer;
        }
    }

    return \@found;

}

# Convert the array to a hash with the numbers as keys
my %list = map { $_ => 1 } @faclist;

sub grep_by_hash {
    my ( $ref ) = @_;
    my %faclist = %$ref;
    my @found;
    foreach my $integer( 1..5000 ) {
        if ( exists $faclist{$integer} ) {
            unshift @found, $integer;
        }
    }

    return \@found;

}

# Benchmark the 2 subs
my $r = timethese( 1000, {
        'array' => sub{ grep_by_array(\@faclist) },
        'hash'  => sub{ grep_by_hash(\%list) },
    }
);

cmpthese( $r );

      RESULTS:

      Benchmark: timing 5000 iterations of array, hash...
     array: 339 wallclock secs (339.02 usr +  0.04 sys = 339.06 CPU) @ 14.75/s (n=5000)
      hash:  8 wallclock secs ( 8.27 usr +  0.00 sys =  8.27 CPU) @ 604.59/s (n=5000)
        Rate array  hash
array 14.7/s    --  -98%
hash   605/s 4000%    --

      That's quite an improvement using a hash!
      You learn something every day...

      njcodewarrior

[reply]

In Section Seekers of Perl Wisdom