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Definite Clause Grammar

Definite Clause Grammar. CS440 Sept 29, 2009. Solution to HW1. Maximum maximum([N],N):-number(N). maximum([X|List], M):- number(X), maximum(List, N), M is max(X,N). Solution to HW1. Depth depth([],1):-!. depth([X|List], M):- depth(X,N1), depth(List, N2), !,

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Definite Clause Grammar

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  1. Definite Clause Grammar CS440 Sept 29, 2009

  2. Solution to HW1 • Maximum maximum([N],N):-number(N). maximum([X|List], M):- number(X), maximum(List, N), M is max(X,N).

  3. Solution to HW1 • Depth depth([],1):-!. depth([X|List], M):- depth(X,N1), depth(List, N2), !, M is max(N1+1,N2). depth(_,0).

  4. Definite Clause Grammar • Convenient way to represent grammatical relationship for parsing. • Well used in natural language processing application. • Prolog is well suited for natural language processing. • Eg: [The, cat, chases, the, mouse] Sentence->nounphrase, verbphrase. verbphrase->verb, nounphrase.

  5. GCD -Difference List diffList(X,L,R) :- append(X,R,L). • How to parse : Sentence->nounphrase, verbphrase. 1. generate and test Sentence(X):- append(N,V,X) nounphrase(N), verbphrase(V). 2. difference list – get the part each predicate wants and pass the remainder to next. sentences(S,R):- nounphrase(P, S, R1), R1 = S-P verbphrase(V, R1, R). R = R1 – V

  6. DCG Syntax • --> operator indicates a DCG rule, replacing the normal neck ( :- ) used for Prolog clauses. For example, the sentence grammar rule can be written using a DCG rule: sentence --> subject, verb, object. • Each goal is assumed to refer to the head of a DCG rule, and the preprocessor adds two extra arguments for the difference list. • Curly braces {} are used to isolate normal Prolog goals from the DCG preprocessor. For example: subject --> modifier, noun, {write('found subject')}. • Square brackets [], list notation, are used to indicate terminal symbols of the grammar. For example: noun --> [cat]

  7. GCD -Difference List • Thus, • each goal refers to one predicates head and two argument for different list. • Eg: term(Z) -> number(X), “*”, number(Y), {Z is X*Y} • Will be • term(Z, S, R):- • number(X,S,R1), number(Y,R1,R), Z is X*Y

  8. Single Digit Calculus • Eg: 3*4+5*8+6*3 • expr(Z) --> term(X), "+", expr(Y), {Z is X + Y}. expr(Z) --> term(Z). term(Z) --> number(X), "*", term(Y), {Z is X * Y}. term(Z) --> number(Z). number(X) --> [C], {"0"=<C, C=<"9", X is C - "0"}.

  9. Single Digit Calculus • expr(A, B, C) :- term(D, B, E), E=[43|F], expr(G, F, H), A is D+G, C=H. • expr(A, B, C) :- term(A, B, C).

  10. Single Digit Calculus • term(A, B, C) :- number(D, B, E), E=[42|F], term(G, F, H), A is D*G, C=H. • term(A, B, C) :- number(A, B, C).

  11. Single Digit Calculus • number(A, B, C) :- B=[D|E], [48]=<D, D=<[57], A is D-[48], C=E.

  12. Predicates Call • expr(Z,”1*2*4+5”,[]) . Need one extra argument for remainder after parsing. • expr(Z,”1*3*4+50”,R). R?

  13. THANKS

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