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# Copyright (c) 2002 Vivake Gupta (vivakeATomniscia.org). All rights reserved. # # This program is free software; you can redistribute it and/or # modify it under the terms of the GNU General Public License as # published by the Free Software Foundation; either version 2 of the # License, or (at your option) any later version. # # This program is distributed in the hope that it will be useful, # but WITHOUT ANY WARRANTY; without even the implied warranty of # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the # GNU General Public License for more details. # # You should have received a copy of the GNU General Public License # along with this program; if not, write to the Free Software # Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 # USA # # This software is maintained by Vivake (vivakeATomniscia.org) and is available at: # http://www.omniscia.org/~vivake/python/PorterStemmer.py # # Additional modifications were made to incorporate this module into # NLTK. All such modifications are marked with "--NLTK--". The NLTK # version of this module is maintained by NLTK developers, # and is available via http://www.nltk.org/ # # GNU Linking Exception: # Using this module statically or dynamically with other modules is # making a combined work based on this module. Thus, the terms and # conditions of the GNU General Public License cover the whole combination. # As a special exception, the copyright holders of this module give # you permission to combine this module with independent modules to # produce an executable program, regardless of the license terms of these # independent modules, and to copy and distribute the resulting # program under terms of your choice, provided that you also meet, # for each linked independent module, the terms and conditions of # the license of that module. An independent module is a module which # is not derived from or based on this module. If you modify this module, # you may extend this exception to your version of the module, but you # are not obliged to do so. If you do not wish to do so, delete this # exception statement from your version.
Porter Stemmer
This is the Porter stemming algorithm, ported to Python from the version coded up in ANSI C by the author. It follows the algorithm presented in
Porter, M. "An algorithm for suffix stripping." Program 14.3 (1980): 130-137.
only differing from it at the points marked --DEPARTURE-- and --NEW-- below.
For a more faithful version of the Porter algorithm, see
http://www.tartarus.org/~martin/PorterStemmer/
Later additions:
June 2000
The 'l' of the 'logi' -> 'log' rule is put with the stem, so that short stems like 'geo' 'theo' etc work like 'archaeo' 'philo' etc.
This follows a suggestion of Barry Wilkins, research student at Birmingham.
February 2000
the cvc test for not dropping final -e now looks after vc at the beginning of a word, so are, eve, ice, ore, use keep final -e. In this test c is any consonant, including w, x and y. This extension was suggested by Chris Emerson.
-fully -> -ful treated like -fulness -> -ful, and -tionally -> -tion treated like -tional -> -tion
both in Step 2. These were suggested by Hiranmay Ghosh, of New Delhi.
Invariants proceed, succeed, exceed. Also suggested by Hiranmay Ghosh.
Additional modifications were made to incorperate this module into nltk. All such modifications are marked with \"--NLTK--\". The nltk version of this module is maintained by the NLTK developers, and is available from <http://nltk.sourceforge.net> """
## --NLTK-- ## Declare this module's documentation format.
## --NLTK-- ## Import the nltk.stemmer module, which defines the stemmer interface
## --NLTK-- ## Add a module docstring """ A word stemmer based on the Porter stemming algorithm.
Porter, M. \"An algorithm for suffix stripping.\" Program 14.3 (1980): 130-137.
A few minor modifications have been made to Porter's basic algorithm. See the source code of this module for more information.
The Porter Stemmer requires that all tokens have string types. """
# The main part of the stemming algorithm starts here. # b is a buffer holding a word to be stemmed. The letters are in b[k0], # b[k0+1] ... ending at b[k]. In fact k0 = 0 in this demo program. k is # readjusted downwards as the stemming progresses. Zero termination is # not in fact used in the algorithm. # Note that only lower case sequences are stemmed. Forcing to lower case # should be done before stem(...) is called.
## --NEW-- ## This is a table of irregular forms. It is quite short, but still ## reflects the errors actually drawn to Martin Porter's attention over ## a 20 year period! ## ## Extend it as necessary. ## ## The form of the table is: ## { ## "p1" : ["s11","s12","s13", ... ], ## "p2" : ["s21","s22","s23", ... ], ## ... ## "pn" : ["sn1","sn2","sn3", ... ] ## } ## ## String sij is mapped to paradigm form pi, and the main stemming ## process is then bypassed.
"sky" : ["sky", "skies"], "die" : ["dying"], "lie" : ["lying"], "tie" : ["tying"], "news" : ["news"], "inning" : ["innings", "inning"], "outing" : ["outings", "outing"], "canning" : ["cannings", "canning"], "howe" : ["howe"],
# --NEW-- "proceed" : ["proceed"], "exceed" : ["exceed"], "succeed" : ["succeed"], # Hiranmay Ghosh }
"""cons(i) is TRUE <=> b[i] is a consonant.""" else:
"""m() measures the number of consonant sequences between k0 and j. if c is a consonant sequence and v a vowel sequence, and <..> indicates arbitrary presence,
<c><v> gives 0 <c>vc<v> gives 1 <c>vcvc<v> gives 2 <c>vcvcvc<v> gives 3 .... """
"""vowelinstem() is TRUE <=> k0,...j contains a vowel"""
"""doublec(j) is TRUE <=> j,(j-1) contain a double consonant."""
"""cvc(i) is TRUE <=>
a) ( --NEW--) i == 1, and p[0] p[1] is vowel consonant, or
b) p[i - 2], p[i - 1], p[i] has the form consonant - vowel - consonant and also if the second c is not w, x or y. this is used when trying to restore an e at the end of a short word. e.g.
cav(e), lov(e), hop(e), crim(e), but snow, box, tray. """
"""ends(s) is TRUE <=> k0,...k ends with the string s."""
"""setto(s) sets (j+1),...k to the characters in the string s, readjusting k."""
"""r(s) is used further down."""
"""step1ab() gets rid of plurals and -ed or -ing. e.g.
caresses -> caress ponies -> poni sties -> sti tie -> tie (--NEW--: see below) caress -> caress cats -> cat
feed -> feed agreed -> agree disabled -> disable
matting -> mat mating -> mate meeting -> meet milling -> mill messing -> mess
meetings -> meet """ # this line extends the original algorithm, so that # 'flies'->'fli' but 'dies'->'die' etc else:
else: # this line extends the original algorithm, so that # 'spied'->'spi' but 'died'->'die' etc
self.k = self.k + 1 self.setto("e")
"""step1c() turns terminal y to i when there is another vowel in the stem. --NEW--: This has been modified from the original Porter algorithm so that y->i is only done when y is preceded by a consonant, but not if the stem is only a single consonant, i.e.
(*c and not c) Y -> I
So 'happy' -> 'happi', but 'enjoy' -> 'enjoy' etc
This is a much better rule. Formerly 'enjoy'->'enjoi' and 'enjoyment'-> 'enjoy'. Step 1c is perhaps done too soon; but with this modification that no longer really matters.
Also, the removal of the vowelinstem(z) condition means that 'spy', 'fly', 'try' ... stem to 'spi', 'fli', 'tri' and conflate with 'spied', 'tried', 'flies' ... """
"""step2() maps double suffices to single ones. so -ization ( = -ize plus -ation) maps to -ize etc. note that the string before the suffix must give m() > 0. """ # To match the published algorithm, replace this phrase with # if self.ends("abli"): self.r("able") if self.m() > 0: # --NEW-- self.setto("al") self.step2() if self.ends("logi"): self.j = self.j + 1 # --NEW-- (Barry Wilkins) self.r("og") # To match the published algorithm, delete this phrase
"""step3() dels with -ic-, -full, -ness etc. similar strategy to step2.""" if self.ends("ical"): self.r("ic") elif self.ends("ful"): self.r("")
"""step4() takes off -ant, -ence etc., in context <c>vcvc<v>.""" if self.ends("al"): pass else: return else: return # takes care of -ous elif self.ends("iti"): pass else: return else: return if self.ends("ive"): pass else: return else: return else:
"""step5() removes a final -e if m() > 1, and changes -ll to -l if m() > 1. """ self.k = self.k -1
"""In stem(p,i,j), p is a char pointer, and the string to be stemmed is from p[i] to p[j] inclusive. Typically i is zero and j is the offset to the last character of a string, (p[j+1] == '\0'). The stemmer adjusts the characters p[i] ... p[j] and returns the new end-point of the string, k. Stemming never increases word length, so i <= k <= j. To turn the stemmer into a module, declare 'stem' as extern, and delete the remainder of this file. """ ## --NLTK-- ## Don't print results as we go (commented out the next line) #print p[i:j+1] j = len(p) - 1
# copy the parameters into statics
return self.pool[self.b[self.k0:self.k+1]]
return self.b # --DEPARTURE--
# With this line, strings of length 1 or 2 don't go through the # stemming process, although no mention is made of this in the # published algorithm. Remove the line to match the published # algorithm.
else:
## --NLTK-- ## Don't use this procedure; we want to work with individual ## tokens, instead. (commented out the following procedure) #def stem(self, text): # parts = re.split("(\W+)", text) # numWords = (len(parts) + 1)/2 # # ret = "" # for i in xrange(numWords): # word = parts[2 * i] # separator = "" # if ((2 * i) + 1) < len(parts): # separator = parts[(2 * i) + 1] # # stem = self.stem_word(string.lower(word), 0, len(word) - 1) # ret = ret + self.adjust_case(word, stem) # ret = ret + separator # return ret
## --NLTK-- ## Define a stem() method that implements the StemmerI interface.
## --NLTK-- ## Add a string representation function return '<PorterStemmer>'
## --NLTK-- ## This test procedure isn't applicable. #if __name__ == '__main__': # p = PorterStemmer() # if len(sys.argv) > 1: # for f in sys.argv[1:]: # infile = open(f, 'r') # while 1: # w = infile.readline() # if w == '': # break # w = w[:-1] # print p.stem(w)
##--NLTK-- ## Added a demo() function
""" A demonstration of the porter stemmer on a sample from the Penn Treebank corpus. """
from nltk.corpus import treebank from nltk import stem
stemmer = stem.PorterStemmer()
orig = [] stemmed = [] for item in treebank.files()[:3]: for (word, tag) in treebank.tagged_words(item): orig.append(word) stemmed.append(stemmer.stem(word))
# Convert the results to a string, and word-wrap them. results = ' '.join(stemmed) results = re.sub(r"(.{,70})\s", r'\1\n', results+' ').rstrip()
# Convert the original to a string, and word wrap it. original = ' '.join(orig) original = re.sub(r"(.{,70})\s", r'\1\n', original+' ').rstrip()
# Print the results. print('-Original-'.center(70).replace(' ', '*').replace('-', ' ')) print(original) print('-Results-'.center(70).replace(' ', '*').replace('-', ' ')) print(results) print('*'*70)
##--NLTK--
import doctest doctest.testmod(optionflags=doctest.NORMALIZE_WHITESPACE)
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