LING/C SC/PSYC 438/538Computational Linguistics

Sandiway Fong

Lecture 16: 10/23

Midterm

• graded and returned

• +1 scaling applied

– Final will be on material covered after midterm only

– There will be one or two graded homeworks as well to come

Question 6

[Correction to lecture 15 slides] • Give the deterministic FSA corresponding to:

Answer 6

• [Correction to lecture 15 slides] • Deterministic machine

1

5

2a

3c 4

c

a

b b

c

6a

Recap

• Following Chapter 3 of the textbook

Stage 1: Lexical Intermediate Levels

• Figure 3.17 (top half):tape view of input/output pairs

The Mapping Problem

• Example: (3.11)

• (Context-Sensitive) Spelling Rule: (3.5) e / {x,s,z}^__ s#

rewrites to letter e in left context x^ or s^ or z^ and right context s#

• i.e. insert e after the ^ when you see x^s# or s^s# or z^s#

• in particular, we have x^s# x^es#

Stage 2: Intermediate Surface Levels

• also can be implemented using a FSTimportant!machine is designed to pass input not matching the rule through unmodified (rather than fail)

implements context-sensitive ruleq0 to q2 : left contextq3 to q0 : right context

Stage 2: Intermediate Surface Levels

• Example (3.17)

Stage 2: Intermediate Surface Levels

• Transition table for FST in 3.14

pg.79

Stage 2: Intermediate Surface Levels

• Class Exercise– Let’s build this

FST in Prolog together

– state by state

Stage 2: Intermediate Surface Levels

• A Note on Prolog– it will be useful to employ the cut

predicate for the implementation of “other”

• ! is known as the “cut” predicate– it affects how Prolog backtracks for

another solution– it means “cut” the backtracking off– Prolog will not try any other possible

matching rule on backtracking

FST 3.14 in Prolog

• % To run: ?- q0([f,o,x,^,s,#],L). L = [f,o,x,e,s,#]

• % q0• q0([],[]). % final state case• q0([z|L1],[z|L2]) :- !, q1(L1,L2). % q0 - z -> q1• q0([s|L1],[s|L2]) :- !, q1(L1,L2). % q0 - s -> q1• q0([x|L1],[x|L2]) :- !, q1(L1,L2). % q0 - x -> q1• q0([#|L1],[#|L2]) :- !, q0(L1,L2). % q0 - # -> q0• q0([^|L1],L2) :- !, q0(L1,L2). % q0 - ^:ep -> q0• q0([X|L1],[X|L2]) :- q0(L1,L2). % q0 - other -> q0

FST 3.14 in Prolog

• % q1• q1([],[]). % final state case• q1([z|L1],[z|L2]) :- !, q1(L1,L2). % q1 - z -> q1• q1([s|L1],[s|L2]) :- !, q1(L1,L2). % q1 - s -> q1• q1([x|L1],[x|L2]) :- !, q1(L1,L2). % q1 - x -> q1• q1([^|L1],L2) :- !, q2(L1,L2). % q1 - ^:ep -> q2• q1([#|L1],[#|L2]) :- !, q0(L1,L2). % q1 - # -> q0• q1([X|L1],[X|L2]) :- q0(L1,L2). % q1 - other -> q0

FST 3.14 in Prolog

• % q2• q2([],[]). % final state case• q2(L1,[e|L2]) :- q3(L1,L2). % q2 - ep:e -> q3• q2([s|L1],[s|L2]) :- !, q5(L1,L2). % q2 - s -> q5• q2([z|L1],[z|L2]) :- !, q1(L1,L2). % q2 - z -> q1• q2([x|L1],[x|L2]) :- !, q1(L1,L2). % q2 - x -> q1• q2([#|L1],[#|L2]) :- !, q0(L1,L2). % q2 - # -> q0• q2([X|L1],[X|L2]) :- \+ X = ^, q0(L1,L2). % q2 - other -

> q0

FST 3.14 in Prolog

• % q3• q3([s|L1],[s|L2]) :- q4(L1,L2). % q3 - s -> q4

• % q4• q4([#|L1],[#|L2]) :- q0(L1,L2). % q4 - # -> q0

FST 3.14 in Prolog

• % q5• q5([z|L1],[z|L2]) :- !, q1(L1,L2). % q5 - z -> q1• q5([s|L1],[s|L2]) :- !, q1(L1,L2). % q5 - s -> q1• q5([x|L1],[x|L2]) :- !, q1(L1,L2). % q5 - x -> q1• q5([^|L1],L2) :- !, q2(L1,L2). % q5 - ^:e -> q2• q5([X|L1],[X|L2]) :- \+ X = #, q0(L1,L2). % q5 -

other -> q0