Notes on Introduction to Prolog Lists, Recursions, Cuts | CS 181, Study notes of Computer Science

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Introduction to PrologLists, Recursions, Cuts CS181: Programming Languages

Vladimir Vacic, Christos Koufogiannakis, University of California at Riverside

Topics:
^ Lists
^ Recursion
^ Cuts

Vladimir Vacic, Christos Koufogiannakis, University of California at Riverside

Lists
^ There are two ways to represent lists

.(a,.(b,.(c,[])))[a,b,c]
^ Lists can contain other lists and variables

[ ] [the, men, [like, to, fish]][a,V1,b,[X,Y]]

Vladimir Vacic, Christos Koufogiannakis, University of California at Riverside

Lists
^ The list with^ head X

and

tail Y^ is written^ [X|Y]

[[b,c]] a [a, [b,c]]

[[b], c] a [a,[b], c]

[c] [a,b] [[a,b], c]

(None) (None) [ ]

[b,c] a [a, b, c]

Tail Head List

Vladimir Vacic, Christos Koufogiannakis, University of California at Riverside

List Matching
^ Example:^ p([1, 2, 3]).p([john, likes, [football, baseball]]).?- p[X|Y].X=

Y=[2,3]

X=john

Y=[likes, [football, baseball]]

Vladimir Vacic, Christos Koufogiannakis, University of California at Riverside

Lists and Recursion
^ More examples:member(X, [X|L]).member(X, [Y|L]) :- member(X,L).last(X, [X]).last(X, [Y|L]) :- last(X, L).

Vladimir Vacic, Christos Koufogiannakis, University of California at Riverside

Recursion
^ Also you must be careful to avoid

circular

definitions

parent(X,Y) :- child(Y,X).child(A,B) :- parent(B,A).

Vladimir Vacic, Christos Koufogiannakis, University of California at Riverside

Recursions
^ Another example of a recursion (however,note that Prolog is not an efficient way to donumerical computations):factorial(0, 1).factorial(N, F) :- N>0,

N1 is N-1,factorial(N1, F1),F is N * F1.

Vladimir Vacic, Christos Koufogiannakis, University of California at Riverside

Cuts
^ Suppose that this clause has been invoked with agoal matching p(X) and the subgoals b(X) and c(X)have been satisfied. On encountering the cut:1)The cut will succeed and PROLOG will try to satisfysubgoals d(X) and e(X).2)If d(X) and e(X) succeed then p(X) succeeds.3)If d(X) and e(X) do not succeed and backtrackingreturns to the cut, then the backtracking process willimmediately terminate and p(X) fails.

Vladimir Vacic, Christos Koufogiannakis, University of California at Riverside

Cuts
^ Example:^ max(A,B,B) :- A < B.max(A,B,A).?- max(3,4,M).M = 4

M = 3

Vladimir Vacic, Christos Koufogiannakis, University of California at Riverside

Cuts
^ Avoiding useless searches:^ member(X,[X|L]) :- !.member(X,[Y|Z]) :- member([X,Z]).?- X is 5,

member(X, [5, 9, 24, 17, 5, 2]),X < 4.

Vladimir Vacic, Christos Koufogiannakis, University of California at Riverside

Cuts
^ Cuts are commonly used in conjunction withthe generate-and-test programming paradigmfind_just_one_solution(X) :-

candidate_solution(X),test_solution(X),!.

Vladimir Vacic, Christos Koufogiannakis, University of California at Riverside

References
^ Clocksin, W.F., and Mellish C.S.^ Programming in Prolog

. 4th edition. New

York: Springer-Verlag. 1994.
Aaby, A.

Prolog Tutorial

. Walla Walla

College. 1997. On line.http://cs.wwc.edu/~cs_dept/KU/PR/Prolog.html