问题
What is wrong with my power function?
pow(_,0,1).
pow(X,Y,Z) :-
pow(X,Y-1,X*Z).
?- pow(2,3,Z).
ERROR: Out of global stack
回答1:
Your Y does not get decremented, you can not use predicates like functions. You also have to unify Z with the result of the multiplication.
pow(_,0,1).
pow(X,Y,Z) :- Y1 is Y - 1,
pow(X,Y1,Z1), Z is Z1*X.
There is also a builtin power function which will be much faster:
pow2(X,Y,Z) :- Z is X**Y.
Also note that pow is not a last call and can not be optimized to use only one stack frame. You should reformulate it to:
pow3(X,Y,Z) :- powend(X,Y,1,Z),!.
powend(_,0,A,Z) :- Z is A.
powend(X,Y,A,Z) :- Y1 is Y - 1, A1 is A*X, powend(X,Y1,A1,Z).
回答2:
Predicates
fac(Integer,Integer,Integer).
Clauses
fac(X,N,X):- N=1,!.
fac(X,N,M):- N1=N-1,fac(X,N1,M1), M= X*M1.
Goal
fac(5,3,X).
回答3:
DOMAINS
num=INTEGER
PREDICATES
nondeterm power(num,num,num)
CLAUSES
power(X,0,1).
power(X,P,F):-X>0,P1=P-1,power(X,P1,F1),F=X*F1.
GOAL
power(2,5,X).
来源：https://stackoverflow.com/questions/1448790/power-function-in-prolog