The Porter stemming algorithm (or ‘Porter stemmer’) is a process for removing the commoner morphological and inflectional endings from words in English. It is used as part of a term normalization process that is usually done when setting up Information Retrieval systems. The rules in the Porter algorithm are separated into five distinct phases numbered from 1 to 5. They are applied to the words in the text starting from phase 1 and moving on to phase 5. Further, they are applied sequentially one after the other as commands in a program.
Originally written in 1979 at Computer Laboratory, Cambridge (England), it was reprinted in 1997 in the book “Readings in Information Retrieval“. Initially it was written in BCPL language. Here is the list of implementations in other programming languages including C, Java and Pearl implementations done by author himself.
This porter stemming algorithm extensively use conditional operators such as C if-else and while loops.
This is the Porter stemming algorithm, coded up in ANSI C by the author himself. You can compile it on Unix with ‘gcc -O3 -o stem stem.c’ after which ‘stem’ takes a list of inputs and sends the stemmed equivalent to stdout.
/* This is the Porter stemming algorithm, coded up in ANSI C by the
author. It may be be regarded as canonical, in that it follows the
algorithm presented in
Porter, 1980, An algorithm for suffix stripping, Program, Vol. 14,
no. 3, pp 130-137,
only differing from it at the points marked --DEPARTURE-- below.
See also http://www.tartarus.org/~martin/PorterStemmer
The algorithm as described in the paper could be exactly replicated
by adjusting the points of DEPARTURE, but this is barely necessary,
because (a) the points of DEPARTURE are definitely improvements, and
(b) no encoding of the Porter stemmer I have seen is anything like
as exact as this version, even with the points of DEPARTURE!
You can compile it on Unix with 'gcc -O3 -o stem stem.c' after which
'stem' takes a list of inputs and sends the stemmed equivalent to
stdout.
The algorithm as encoded here is particularly fast.
Release 1: was many years ago
Release 2: 11 Apr 2013
fixes a bug noted by Matt Patenaude <matt@mattpatenaude.com>,
case 'o': if (ends("\03" "ion") && (b[j] == 's' || b[j] == 't')) break;
==>
case 'o': if (ends("\03" "ion") && j >= k0 && (b[j] == 's' || b[j] == 't')) break;
to avoid accessing b[k0-1] when the word in b is "ion".
Release 3: 25 Mar 2014
fixes a similar bug noted by Klemens Baum <klemensbaum@gmail.com>,
that if step1ab leaves a one letter result (ied -> i, aing -> a etc),
step2 and step4 access the byte before the first letter. So we skip
steps after step1ab unless k > k0.
*/
#include <string.h> /* for memmove */
#define TRUE 1
#define FALSE 0
/* 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.
*/
static char * b; /* buffer for word to be stemmed */static int k,k0,j; /* j is a general offset into the string */
/* cons(i) is TRUE <=> b[i] is a consonant. */
static int cons(int i)
{ switch (b[i])
{ case 'a': case 'e': case 'i': case 'o': case 'u': return FALSE;
case 'y': return (i==k0) ? TRUE : !cons(i-1);
default: return TRUE;
}
}
/* 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
....
*/
static int m()
{ int n = 0;
int i = k0;
while(TRUE)
{ if (i > j) return n;
if (! cons(i)) break; i++;
}
i++;
while(TRUE)
{ while(TRUE)
{ if (i > j) return n;
if (cons(i)) break;
i++;
}
i++;
n++;
while(TRUE)
{ if (i > j) return n;
if (! cons(i)) break;
i++;
}
i++;
}
}
/* vowelinstem() is TRUE <=> k0,...j contains a vowel */
static int vowelinstem()
{ int i; for (i = k0; i <= j; i++) if (! cons(i)) return TRUE;
return FALSE;
}
/* doublec(j) is TRUE <=> j,(j-1) contain a double consonant. */
static int doublec(int j)
{ if (j < k0+1) return FALSE;
if (b[j] != b[j-1]) return FALSE;
return cons(j);
}
/* cvc(i) is TRUE <=> i-2,i-1,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.
*/
static int cvc(int i)
{ if (i < k0+2 || !cons(i) || cons(i-1) || !cons(i-2)) return FALSE;
{ int ch = b[i];
if (ch == 'w' || ch == 'x' || ch == 'y') return FALSE;
}
return TRUE;
}
/* ends(s) is TRUE <=> k0,...k ends with the string s. */
static int ends(char * s)
{ int length = s[0];
if (s[length] != b[k]) return FALSE; /* tiny speed-up */ if (length > k-k0+1) return FALSE;
if (memcmp(b+k-length+1,s+1,length) != 0) return FALSE;
j = k-length;
return TRUE;
}
/* setto(s) sets (j+1),...k to the characters in the string s, readjusting
k. */
static void setto(char * s)
{ int length = s[0];
memmove(b+j+1,s+1,length);
k = j+length;
}
/* r(s) is used further down. */
static void r(char * s) { if (m() > 0) setto(s); }
/* step1ab() gets rid of plurals and -ed or -ing. e.g.
caresses -> caress
ponies -> poni
ties -> ti
caress -> caress
cats -> cat
feed -> feed
agreed -> agree
disabled -> disable
matting -> mat
mating -> mate
meeting -> meet
milling -> mill
messing -> mess
meetings -> meet
*/
static void step1ab()
{ if (b[k] == 's')
{ if (ends("\04" "sses")) k -= 2; else
if (ends("\03" "ies")) setto("\01" "i"); else
if (b[k-1] != 's') k--;
}
if (ends("\03" "eed")) { if (m() > 0) k--; } else
if ((ends("\02" "ed") || ends("\03" "ing")) && vowelinstem())
{ k = j;
if (ends("\02" "at")) setto("\03" "ate"); else
if (ends("\02" "bl")) setto("\03" "ble"); else
if (ends("\02" "iz")) setto("\03" "ize"); else
if (doublec(k))
{ k--;
{ int ch = b[k];
if (ch == 'l' || ch == 's' || ch == 'z') k++;
}
}
else if (m() == 1 && cvc(k)) setto("\01" "e");
}
}
/* step1c() turns terminal y to i when there is another vowel in the stem. */
static void step1c() { if (ends("\01" "y") && vowelinstem()) b[k] = 'i'; }
/* 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. */
static void step2() { switch (b[k-1])
{
case 'a': if (ends("\07" "ational")) { r("\03" "ate"); break; }
if (ends("\06" "tional")) { r("\04" "tion"); break; }
break;
case 'c': if (ends("\04" "enci")) { r("\04" "ence"); break; }
if (ends("\04" "anci")) { r("\04" "ance"); break; }
break;
case 'e': if (ends("\04" "izer")) { r("\03" "ize"); break; }
break;
case 'l': if (ends("\03" "bli")) { r("\03" "ble"); break; } /*-DEPARTURE-*/
/* To match the published algorithm, replace this line with
case 'l': if (ends("\04" "abli")) { r("\04" "able"); break; } */
if (ends("\04" "alli")) { r("\02" "al"); break; }
if (ends("\05" "entli")) { r("\03" "ent"); break; }
if (ends("\03" "eli")) { r("\01" "e"); break; }
if (ends("\05" "ousli")) { r("\03" "ous"); break; }
break;
case 'o': if (ends("\07" "ization")) { r("\03" "ize"); break; }
if (ends("\05" "ation")) { r("\03" "ate"); break; }
if (ends("\04" "ator")) { r("\03" "ate"); break; }
break;
case 's': if (ends("\05" "alism")) { r("\02" "al"); break; }
if (ends("\07" "iveness")) { r("\03" "ive"); break; }
if (ends("\07" "fulness")) { r("\03" "ful"); break; }
if (ends("\07" "ousness")) { r("\03" "ous"); break; }
break;
case 't': if (ends("\05" "aliti")) { r("\02" "al"); break; }
if (ends("\05" "iviti")) { r("\03" "ive"); break; }
if (ends("\06" "biliti")) { r("\03" "ble"); break; }
break;
case 'g': if (ends("\04" "logi")) { r("\03" "log"); break; } /*-DEPARTURE-*/
/* To match the published algorithm, delete this line */
} }
/* step3() deals with -ic-, -full, -ness etc. similar strategy to step2. */
static void step3() { switch (b[k])
{
case 'e': if (ends("\05" "icate")) { r("\02" "ic"); break; }
if (ends("\05" "ative")) { r("\00" ""); break; }
if (ends("\05" "alize")) { r("\02" "al"); break; }
break;
case 'i': if (ends("\05" "iciti")) { r("\02" "ic"); break; }
break;
case 'l': if (ends("\04" "ical")) { r("\02" "ic"); break; }
if (ends("\03" "ful")) { r("\00" ""); break; }
break;
case 's': if (ends("\04" "ness")) { r("\00" ""); break; }
break;
} }
/* step4() takes off -ant, -ence etc., in context <c>vcvc<v>. */
static void step4()
{ switch (b[k-1])
{ case 'a': if (ends("\02" "al")) break; return;
case 'c': if (ends("\04" "ance")) break;
if (ends("\04" "ence")) break; return;
case 'e': if (ends("\02" "er")) break; return;
case 'i': if (ends("\02" "ic")) break; return;
case 'l': if (ends("\04" "able")) break;
if (ends("\04" "ible")) break; return;
case 'n': if (ends("\03" "ant")) break;
if (ends("\05" "ement")) break;
if (ends("\04" "ment")) break;
if (ends("\03" "ent")) break; return;
case 'o': if (ends("\03" "ion") && j >= k0 && (b[j] == 's' || b[j] == 't')) break;
if (ends("\02" "ou")) break; return;
/* takes care of -ous */ case 's': if (ends("\03" "ism")) break; return;
case 't': if (ends("\03" "ate")) break;
if (ends("\03" "iti")) break; return;
case 'u': if (ends("\03" "ous")) break; return;
case 'v': if (ends("\03" "ive")) break; return;
case 'z': if (ends("\03" "ize")) break; return;
default: return;
}
if (m() > 1) k = j;
}
/* step5() removes a final -e if m() > 1, and changes -ll to -l if
m() > 1. */
static void step5()
{ j = k;
if (b[k] == 'e')
{ int a = m();
if (a > 1 || a == 1 && !cvc(k-1)) k--;
}
if (b[k] == 'l' && doublec(k) && m() > 1) k--;
}
/* 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.
*/
int stem(char * p, int i, int j)
{ b = p; k = j; k0 = i; /* copy the parameters into statics */ if (k <= k0+1) return k; /*-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. */
step1ab();
if (k > k0) {
step1c(); step2(); step3(); step4(); step5();
}
return k;
}
/*--------------------stemmer definition ends here------------------------*/
#include <stdio.h>
#include <stdlib.h> /* for malloc, free */#include <ctype.h> /* for isupper, islower, tolower */
static char * s; /* a char * (=string) pointer; passed into b above */
#define INC 50 /* size units in which s is increased */static int i_max = INC; /* maximum offset in s */
void increase_s()
{ i_max += INC;
{ char * new_s = (char *) malloc(i_max+1);
{ int i; for (i = 0; i < i_max; i++) new_s[i] = s[i]; } /* copy across */ free(s); s = new_s;
}
}
#define LETTER(ch) (isupper(ch) || islower(ch))
static void stemfile(FILE * f)
{ while(TRUE)
{ int ch = getc(f);
if (ch == EOF) return;
if (LETTER(ch))
{ int i = 0;
while(TRUE)
{ if (i == i_max) increase_s();
ch = tolower(ch); /* forces lower case */
s[i] = ch; i++;
ch = getc(f);
if (!LETTER(ch)) { ungetc(ch,f); break; }
}
s[stem(s,0,i-1)+1] = 0;
/* the previous line calls the stemmer and uses its result to
zero-terminate the string in s */ printf("%s",s);
}
else putchar(ch);
}
}
int main(int argc, char * argv[])
{ int i;
s = (char *) malloc(i_max+1);
for (i = 1; i < argc; i++)
{ FILE * f = fopen(argv[i],"r");
if (f == 0) { fprintf(stderr,"File %s not found\n",argv[i]); exit(1); }
stemfile(f);
}
free(s);
return 0;
}