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

2. 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

3. 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

4. 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

5. 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

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

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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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] ? ?-
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Logic Programming

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

22. Questions
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Logic Programming 22