Text::Ngram(3pm) User Contributed Perl Documentation Text::Ngram(3pm)
NAME
Text::Ngram - Ngram analysis of text
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);
As of version 0.14, the %config may instead be passed in as named arguments:
$href = ngram_counts($text, $window_size, %config);
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-alphabetic 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 (default), punctuation is removed before calculating the ngrams. Set to 1 to preserve it.
# Get all ngrams of size 2 preserving punctuation
$href_p = ngram_counts( {punctuation => 1}, $text, 2 );
spaces
If set to 0 (default is 1), no ngrams containing 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 / / }
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
o 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 Alberto Simoes, "ambs@cpan.org".
Previously maintained by Jose Castro, "cog@cpan.org". Originally created by Simon Cozens, "simon@cpan.org".
COPYRIGHT AND LICENSE
Copyright 2006 by Alberto Simoes
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.
perl v5.14.2 2012-01-25 Text::Ngram(3pm)