NAME
Text::Ngram - Basis for n-gram analysis
SYNOPSIS
use Text::Ngram qw(ngram_counts add_to_counts);
my $text = "abcdefghijklmnop";
my $hash_r = ngram_counts($text, 3); # Window size = 3
# $hash_r => { abc => 1, bcd => 1, ... }
add_to_counts($more_text, 3, $hash_r);DESCRIPTION
n-Gram analysis is a field in textual analysis which uses sliding window character sequences in order to aid topic analysis, language determination and so on. The n-gram spectrum of a document can be used to compare and filter documents in multiple languages, prepare word prediction networks, and perform spelling correction.
The neat thing about n-grams, though, is that they're really easy to determine. For n=3, for instance, we compute the n-gram counts like so:
the cat sat on the mat
--- $counts{"the"}++;
--- $counts{"he "}++;
--- $counts{"e c"}++;
...This module provides an efficient XS-based implementation of n-gram spectrum analysis.
There are two functions which can be imported:
ngram_counts
This first function returns a hash reference with the n-gram histogram of the text for the given window size. The default window size is 5.
$href = ngram_counts(\%config, $text, $window_size);The only necessary parameter is $text.
The possible value for \%config are:
- flankbreaks
-
If set to 1 (default), breaks are flanked by spaces; if set to 0, they're not. Breaks are punctuation and other non-alfabetic characters, which, unless you use
punctuation =0> in your configuration, do not make it into the returned hash.Here's an example, supposing you're using the default value for punctuation (1):
my $text = "Hello, world"; my $hash = ngram_counts($text, 5);That produces the following ngrams:
{ 'Hello' => 1, 'ello ' => 1, ' worl' => 1, 'world' => 1, }On the other hand, this:
my $text = "Hello, world"; my $hash = ngram_counts({flankbreaks => 0}, $text, 5);Produces the following ngrams:
{ 'Hello' => 1, ' worl' => 1, 'world' => 1, } - lowercase
-
If set to 0, casing is preserved. If set to 1, all letters are lowercased before counting ngrams. Default is 1.
# Get all ngrams of size 4 preserving case $href_p = ngram_counts( {lowercase => 0}, $text, 4 ); - punctuation
-
If set to 0, punctuation is removed before calculating the ngrams. Set to 1 to preserve it. Default is 0.
# Get all ngrams of size 2 preserving punctuation $href_p = ngram_counts( {punctuation => 1}, $text, 2 ); - spaces
-
If set to 0, no ngrams contaning spaces will be returned
# Get all ngrams of size 3 that do not contain spaces $href = ngram_counts( {spaces => 0}, $text, 3);If you're going to request both types of ngrams, than the best way to avoid calculating the same thing twice is probably this:
$href_with_spaces = ngram_counts($text[, $window]); $href_no_spaces = $href_with_spaces; for (keys %$href_no_spaces) { delete $href->{$_} if / / }
Remember, the default configuration is:
{
spaces => 1,
punctuation => 0,
lowercase => 1,
}add_to_counts
This incrementally adds to the supplied hash; if $window is zero or undefined, then the window size is computed from the hash keys.
add_to_counts($more_text, $window, $href)TO DO
Look further into the tests. Sort them and add more.
SEE ALSO
Cavnar, W. B. (1993). N-gram-based text filtering for TREC-2. In D. Harman (Ed.), Proceedings of TREC-2: Text Retrieval Conference 2. Washington, DC: National Bureau of Standards.
Shannon, C. E. (1951). Predication and entropy of printed English. The Bell System Technical Journal, 30. 50-64.
Ullmann, J. R. (1977). Binary n-gram technique for automatic correction of substitution, deletion, insert and reversal errors in words. Computer Journal, 20. 141-147.
AUTHOR
Maintained by Jose Castro, cog@cpan.org.
Originally created by Simon Cozens, simon@cpan.org.
COPYRIGHT AND LICENSE
Copyright 2004 by Jose Castro
Copyright 2003 by Simon Cozens
This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.