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.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 | /* 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 <[email protected]>, 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 <[email protected]>, 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; } |
