Question

I made two different Fibonacci functions, the first one worked perfectly. Then I tried to simplify it in an intuitive way. I thought it would work but for some reason it says ERROR: Out of local stack every time I test it.

Working version:

fibonacci(0,0).
fibonacci(1,1).
fibonacci(N,F) :- N1 is N-1, N2 is N-2, fibonacci(N1,F1), fibonacci(N2,F2), F is F1+F2.

Not working version:

fibonacci(0,0).
fibonacci(1,1).
fibonacci(N,F) :- fibonacci(N-1,F1), fibonacci(N-2,F2), F is F1+F2.

Could someone explain me what is the problem with the second one? Thanks.

Answer 1

Your problem is that in your second one you are recursively calling fibonacci/2 with the term N-1 instead of an integer whose value is N-1.

So, for example if you where calling fibonacci(3, F) it would enter in the third clause and call fibonacci(3-1, F1) instead of fibonacci(2, F1). It would then enter again in the third clause and call fibonacci(3-1-1, F1) and so on.

Note that Prolog uses special operator is to perform arithmetic operations.
The first example is right.