Stemming Algorithm - AIT CSIM Program

Stemming Algorithm - AIT CSIM Program

Stemming Algorithms 9142608 9142609 From www.mis.nsysu.edu.tw/~syhwang/Courses/IR/StemmingAlgorithms.ppt, modified by Sumanta The Porter Algorithm Word = Stem + Affix(es)

E.g., generalizations = general + ization + s Stemming is the determination of the stem of a given word Porters stemmer is a rule-based algorithm

E.g., ational ate (apply: relational relate) Porters stemmer is heuristic, in that it is a practical method not guaranteed to be optimal The Porter Stemmer: Definitions Definitions CONSONANT: a letter other than A, E, I, O, U, and Y preceded by consonant (in TOY, consonants

are T,Y; in SYZYGY they are S, Z, G) VOWEL: any other letter With this definition all words and parts of words are of form : [C](VC)m[V] C=string of one or more consonants (con+) and [C] indicates arbitrary presence of the contents, i.e., possibly empty string as well. V=string of one or more vowels and [V] indicates arbitrary E.g., Troubles

C VC VC = C(VC)2 m is the measure of the word m = 0: TR, EE, TREE, Y, BY m = 1: TROUBLE, OATS, TREES, IVY m = 2: TROUBLES, PRIVATE, OATEN, ORRERY Spring 2002 NLE

3 Rule Format Rules are of the form (condition) S1 S2 where S1 and S2 are suffixes. Given a set of rules, only the one with the longest matching suffix S1 is applies. Conditions: 1. m --- measure of the stem m = k or m > k, where k is an integer

2.*X --- the stem ends with a given letter X 3.*v*--- the stem contains a vowel 4.*d --- the stem ends in double consonant 5.*o --- the stem ends with a consonant-vowel-consonant sequence, where the final consonant is not w, x or y, (e.g., wil, hop) Rules are divided into sets and in each successive step one set of rules is applied. Porter Steps Each step corresponds to a set of rules. The rules in

a step are examined in sequence , and only one rule from a step can apply { } step1a(word); step1b(stem); if (the second or third rule of step 1b was used)

step1b1(stem); step1c(stem); step2(stem); step3(stem); step4(stem); step5a(stem); step5b(stem); Should be: ti

if the second or third rule of step 1b was used Examples/Problems Step 1a Step 4 computers computer comput Step 1b singing sing

generalizations information instructor Try words of your own Porters Mishaps On-line Porters at http://textanalysisonline.com/nltk-porter-stemmer gives gas (noun) ga

gases (plural) gase gasses (verb, present tense) gass gassing (verb, present continuous) gass gaseous (adjective) gaseou This is not good all these words should ideally reduce to the same stem. Trade-off: More rules (accurate but slow) vs Less rules (efficient but sometimes wrong). Google does give different results for gas and gases, so maybe they use these Porter rules:-)

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