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morphology Flipbook PDF
Morphology • Morphology is concerned with the construction of words and the meaning of their components. • A morpheme is
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Morphology • Morphology is concerned with the construction of words and the meaning of their components. • A morpheme is the smallest meaning-bearing unit of a word. • Words are composed of stems and affixes. • Affixes consist of: prefixes, suffixes, infixes, circumfixes
Examples Root walk noise icon order active elect view
Morphological variants walks, walked, walking noisy, noisily iconic reorder, orderly hyperactive, proactive reelect, reelection preview, previewer, previewers
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Inflectional vs. Derivational • Inflectional: variants that have essentially the same meaning and the same general part-of-speech. Ex: kick, kicks, kicked, kicking
• Derivational: variants that represent a different concept. The part-of-speech may be the same or different. Ex: friend, friendly, friendliness, friendship ! !
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Blindly stripping affixes can produce strange results! preempt news pretend hardly glasses
empt new tend hard glass
TRUE STORIES Mrs Easter atomic ! !
Stemming
Limitations of Stemming
• Some systems use stemming to approximately match terms with variants in documents. • Most stemmers just chop off common prefixes and suffixes. Example: assassination would be stemmed to assassinat which would match: assassination assassinations assassinate assassinated assassinating ! !
Sample Morphology Rules Derived Root Word Word
Affix Repl POS Chars Root
POS Derived
Properties
cats sings walked hairy noisy unfair antiwar
s s ed y y un anti
noun verb verb adj adj adj adj
plural present tense past tense
cat sing walk hair noise fair war
e -
noun verb verb noun noun adj noun
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Errors by the Porter Stemmer organization organ doing doe generalization generic numerical numerous policy police university universe easy easily addition additive negligible negligent execute executive define definite past paste ignore ignorant special specialized arm army head heading
Omissions by the Porter Stemmer european europe analysis analyzes matrices matrix noise noisy sparse sparsity explain explanation resolve resolution triangle triangular urgency urgent cylinder cylindrical
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Morphological Analysis as Search • Morphological analysis can be implemented as exhaustive search of the rule set. • The rules must be applied recursively! • All successful derivations should be returned, so that the NLP system is informed of all possible parts-of-speech for the word. Multiple derivations are common!
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Morphological Analysis Given a word w: IF w is in the dictionary, return its definition! ELSE apply all of the morphology rules to find all possible root forms of w
An Example (1) SUFFIX ed - verb -> adjective (2) SUFFIX ed - verb -> verb TENSE past (3) PREFIX re - verb -> verb
For each rule, strip off the affix and add the replacement chars to produce a candidate root. If the candidate root is in the dictionary with the appropriate POS, then success! • If success, return the derived word with the POS and properties assigned by the rule. • If failure, then recursively apply the rule set to the candidate root to see if it! can be derived. !
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Morphology with FSAs
Simple FSA for Morphology stood, grew, ate, eaten
• Morphological analysis is often performed with finitestate transducers (FST). • An FST is a finite-state automaton that maps between two sets of symbols. • An FST can be viewed as a generator or as a recognizer between pairs of strings.
irreg-past-verb-form
q0!
reg-verb-stem walk, listen
q1!
past (-ed) past participle (-ed)
walk, listen
reg-verb-stem irreg-verb-stem
3rd singular (-s)
q2!
present participle (-ing)
stand, grow, eat
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q3!
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Simple FSA for Adjectives clear, happy, true
q0!
un!"
q1!
adj
q2!
adjAL equal, formal, natural
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q3!
-er -est
q3!