power function in prolog

Question

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

ebo · Accepted Answer · 2012-11-26T16:03:25.820

18

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).

edited Nov 26 '12 at 16:03

answered Sep 19 '09 at 15:53

ebo

8,985
3
31
37

@ebo: Your definitions pow/3 and pow3/3 do not terminate! Try `pow(1,0,0)`. This should fail. – false Feb 19 '12 at 23:21
1

The Z parameter should not be bound. Also this is an academic example. Not all edge cases are considered to lower the complexity of the implementation. – ebo Feb 20 '12 at 11:13
Kind of strange, but fixed. – ebo Nov 26 '12 at 16:04
All three examples terminate in SWI-Prolog 5.10.4. – ebo Nov 26 '12 at 18:20
@ebo: They don't for `pow3(1,0,0)` – false Feb 26 '14 at 15:53

score 2 · Answer 2 · answered Feb 25 '14 at 18:19

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).

answered Feb 25 '14 at 18:19

Maik

29
1

`fac(5,3,1)` should fail, but it loops. – false Feb 25 '14 at 18:20
in my case `fac(5,3,1)` return `no`, without loops, I use Visual_Prolog_v52_pe. – Maik Feb 26 '14 at 15:41
Impossible. You must have a different program. – false Feb 26 '14 at 15:48

score 1 · Answer 3 · edited Nov 24 '12 at 15:23

1

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).

edited Nov 24 '12 at 15:23

John Conde

217,595
99
455
496

answered Apr 16 '11 at 08:58

Kushan Randima

11
1

power function in prolog

3 Answers3

Linked