Dr. Yasser Nada Fall 2010/2011 Lecture 5 Logic Programming PROLOG Dr. Yasser Nada Fall 2010/2011 Lecture 5 Logic Programming

Reversibility of Prolog Programs Consider this program: square(1,1). square(2,4). square(3,9). square(4,16). square(5,25). square(6,36). ?- square(2,X). X=4 ? ? square(X,25). X=5 ? ? PROLOG Logic Programming

Reversibility of Prolog Programs Unlike many other programming languages: No need to write new procedure for finding each argument. Program square/2 is reversible. PROLOG Logic Programming

PROLOG Evaluation in Prolog Difference between: Y is 1+2. and Y = 1+2. In Y is 1+2, the term 1+2 is evaluated to 3 and assigned to Y. In Y=1+2, the term 1+2 is unevaluated and assigned to Y. PROLOG Logic Programming

List Construction Example successor(X,Y) :- Y is X+1. is/2 is a two argument predicate. It is in the form: is(Y,X+1). is/2 is not reversible. ?- successor(3,Y). Y=4 ? ?- successor(X,4). *** Error: uninstantiated variable in arithmetic expression ?- successor(3,5). no ?- PROLOG Logic Programming

PROLOG Calling Patterns Mode successor(+,?). Mode is(?,+). +: input argument. -: output argument. ?: input or output (+ or -). Mode successor(+,?). Mode is(?,+). PROLOG Logic Programming

PROLOG List Processing Test for existence. Test All elements. Return a result – having processed one element. Return a result – having processed all elements. PROLOG Logic Programming

PROLOG Test for Existence Determine one object with a desired property among a list of objects. list_existence_test(Info, [H|T]) :- element_has_property(Info, H). list_existence_test(Info,[H|T]) :- list_existence_test(Info, T). PROLOG Logic Programming

Test for Existence Example Test if a list is a multi-list. nested_list([H|T]) :- iflist(H). nested_list([_H|T]) :- nested_list(T). iflist([]). iflist([H|T]). ?- nested_list([a, [b], c, [d], []]). yes. ?- nested_list([a,b,c]). ?- no PROLOG Logic Programming

Test for Existence Example ?- nested_list([a, [b], c, [d], []), write(y), fail. yyyno ?- If we want to display one y, then we need to use a cut (explained later). Mode nested_list(+). PROLOG Logic Programming

Test for Existence Example member(E, [E|T]). member(E,[H|T]) :- member(E,T). Equivalent to: member(E,[H|T]) :- E=H. element_has_property is E=H. Program fits in this pattern when used with mode member(+,+). PROLOG Logic Programming

Test for Existence Example an_integer([H|T]) :- integer(H). an_integer([H|T]) :- an_integer(T). ?- an_integer([a, fred, 5, yes]). Yes ?- an_integer([a,b,c]). No ?- an_integer([]). ?- PROLOG Logic Programming

PROLOG Test All Elements The elements of a list all satisfy some properties. test_all_have_property(Info,[]). test_all_have_property(Info,[Head|Tail]):- element_has_property(Info,Head), test_all_have_property(Info,Tail). PROLOG Logic Programming

Test All Elements Example testing that a list of elements consists of digits only digits([0,1,2,3,4,5,6,7,8,9]). all_digits([]). all_digits([Head|Tail]):- digits(D), member(Head,D), all_digits(Tail). member(E,[E|T]). membet(E,[H|T]) :- member(E,T). PROLOG Logic Programming

Test All Elements Example Ex 6.1 (2b): no_consonant([]). no_consonant([H|T]) :- member(H, [a,e,o,i,u]), no_consonant(T). ?- no_consonant([a,b,e]). no ?- no_consonant([a,e,i]). yes ?- PROLOG Logic Programming

Return a Result —Having Processed One Element Returning an element that satisfies a property. PROLOG return_after_event(Info,[H|T],Result):- property(Info,H), result(Info,H,T,Result). return_after_event(Info,[Head|Tail],Ans):- return_after_event(Info,Tail,Ans). Logic Programming

Return a Result —Having Processed One Element everything_after_a([H|T],R) :- H = a, R=T. everything_after_a(T,R). ?- everything_after_a([d,a,s,a,f],R). R=[s,a,f] ? ; R=[f] ? ; no ?- This can be written as: everything_after_a([a|T],T). PROLOG Logic Programming

Return a Result —Having Processed All Elements Transform each element from a list into a new element in a new list. process_all(Info,[],[]). process_all(Info,[H1|T1],[H2|T2]):- process_one(Info,H1,H2), process_all(Info,T1,T2). PROLOG Logic Programming

Return a Result —Having Processed All Elements triple([],[]). triple([H1|T1],[H2|T2]):- H2 is 3*H1, triple(T1,T2). ?- triple([1,2,3,4],R). R=[3,6,9,12] ? ?- PROLOG Logic Programming

Return a Result —Having Processed All Elements Another way to write predicate triple is: triple([],Y,Y). triple([H1|T1],L,R):- H2 is 3*H1, triple(T1,[H2|L],R). ?- triple([1,2,3,4],[],R). R=[3,6,9,12] ? ?- PROLOG Logic Programming

Homework Do exercise 6.1 (1b), (2a), 3,4 page 63-64 in the book. PROLOG Logic Programming

Questions PROLOG Logic Programming 22