Finite-state automata 3 Morphology Day 14 LING Computational Linguistics Harry Howard Tulane University

25-Sept-2009LING , Prof. Howard, Tulane University2 Course organization   NLTK is installed on the computers in this room!  How would you like to use the Provost's $150?

SLP §2.2 Finite-state automata Recognition as search

25-Sept-2009LING , Prof. Howard, Tulane University4 Non-deterministic recognition: Search  In a non-deterministic FSA, there is at least one path through the machine for a string that is in the language defined by the machine.  There is no path through the machine that leads to an accept state for a string not in the language.  But not all paths directed through the machine for an accept string lead to an accept state.

25-Sept-2009LING , Prof. Howard, Tulane University5 Non-deterministic recognition  So success in non-deterministic recognition occurs when a path is found through the machine that ends in an accept.  Failure occurs when all of the possible paths for a given string lead to failure.

25-Sept-2009LING , Prof. Howard, Tulane University6 Back to the example ba a a !$ q0q0 q1q1 q2q2 q2q2 q3q3 q4q4

25-Sept-2009LING , Prof. Howard, Tulane University7 Example q0q0 baaa! q1q1 baaa! q2q2 baaa! q2q2 baaa! q2q2 baaa! X q3q3 baaa! q4q4 baaa!

25-Sept-2009LING , Prof. Howard, Tulane University8 Summary  States in the search space are pairings of tape positions and states in the machine.  By keeping track of as yet unexplored states, a recognizer can systematically explore all the paths through the machine given an input.

25-Sept-2009LING , Prof. Howard, Tulane University9 Keeping track  But how do you keep track?  Depth-first/last in first out (LIFO)/stack  Unexplored states are added to the front of the agenda, and they are explored by going to the most recent.  Breadth-first/first in first out (FIFO)/queue  Unexplored states are added to the back of the agenda, and they are explored by going to the most recent.

25-Sept-2009LING , Prof. Howard, Tulane University10 Depth-first/LIFO/stack q2q2 q 18 q 12 q 41 q 27 q2q2 q 12 q 27 q 50 q 31 stack

25-Sept-2009LING , Prof. Howard, Tulane University11 Breadth-first/FIFO/queue q2q2 q 18 q 12 q 41 q 27 q2q2 q 12 q 27 q 50 q 31 queue

SLP §2.2 Finite-state automata Comparison

25-Sept-2009LING , Prof. Howard, Tulane University13 Equivalence  Non-deterministic machines can be converted to deterministic ones with a fairly simple construction.  That means that they have the same power:  non-deterministic machines are not more powerful than deterministic ones in terms of the languages they can accept.

25-Sept-2009LING , Prof. Howard, Tulane University14 Why bother?  Non-determinism doesn’t get us more formal power and it causes headaches, so why bother?  More natural (understandable) solutions.

SLP §3 Words and transducers Intro

25-Sept-2009LING , Prof. Howard, Tulane University16 Concepts and terminology  study of spelling  study of word composition  to build a structured representation of a word or sentence  input to this process  a process that applies without limitations  Can all forms be stored in advance?  orthography  morphology  parsing  surface or input form  productive

25-Sept-2009LING , Prof. Howard, Tulane University17 Concepts and terminology  the minimal meaning-bearing unit in a language  the main unit  additional units  a unit that:  precedes the main one  follows the main one  surrounds the main one  is inserted within the main one  a language in which the main unit can have many additional units  morpheme  stem  affix  prefix  suffix  circumfix  infix  agglutinative

25-Sept-2009LING , Prof. Howard, Tulane University18 Concepts and terminology  Combining an affix to a stem does not change the part of speech of the stem.  Combining an affix to a stem DOES change the part of speech of the stem.  Combining multiple stems.  Combining a stem with a phonologically reduced stem.  inflection  derivation  compounding  cliticization

SLP §3 Words and transducers §3.1 Survey of (mostly) English morphology

25-Sept-2009LING , Prof. Howard, Tulane University20 Inflectional morphology stem-s-ingpreteritepast part. walkwalkswalkingwalked trytriestryingtried mapmapsmappingmapped eateatseatingateeaten catchcatchescatchingcaught beisbeingwasbeen

Next time P4 SLP §3.2ff