prolog中的幂函数 [英] power function in prolog

问题描述

我的幂函数出了什么问题?

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

推荐答案

你的 Y 没有递减，你不能使用函数之类的谓词.您还必须将 Z 与乘法的结果统一.

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.

还有一个内置的幂函数会更快:

There is also a builtin power function which will be much faster:

pow2(X,Y,Z) :- Z is X**Y.

还要注意 pow 不是最后一次调用，不能优化为只使用一个堆栈帧.您应该将其重新表述为:

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).

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