I am trying to write a prolog program for fibbonaci series.This code is going in infinite loop .Can someone tell what's the error?

Question

fib(0,0).   
fib(1,1).  
fib(A,Result):-    
    fib(A-1,R),  
    fib(A-2,P),  
    Result is R+P.

Your program does not stop. Consider performing a Cut. – Hanif Bin Ariffin Mar 06 '19 at 17:38 — Hanif Bin Ariffin, Mar 06 '19 at 17:38

score 2 · Accepted Answer · answered Mar 07 '19 at 07:57

To understand why, first narrow down the reason for non-termination. Here is the smallest part of your program that still loops:

fib(0,0) :- false.
fib(1,1) :- false.
fib(A,Result):-    
    fib(A-1,R), false,
    fib(A-2,P),  
    Result is R+P.

No matter what the arguments are, this program will loop. And thus your original program will loop too.

To fix this you need to change something in the visible part: You need to ensure that A > 1. In this manner the program now terminates.

Further, as noted correctly by @andsanmar, A-1 alone is not a number, but just the term -(A,1) so it can never be 0 nor 1. So either write

fib(N,0) :- N =:= 0.
fib(N,1) :- N =:= 1.
...

Or add (is)/2 as suggested by @andsanmar.

For more on how to narrow down reasons for non-termination see failure-slice.

score 0 · Answer 2 · answered Mar 06 '19 at 10:10

0

When you do fib(A-1,R), being A=4, for example, what's going to be sent is 4-1 instead of 3, you need to do Aminus1 is A -1, fib(Aminus1,R). The same with the A-2

answered Mar 06 '19 at 10:10

andsanmar

1
3

Please note that fib(0,1) still loops – false Mar 07 '19 at 07:43

I am trying to write a prolog program for fibbonaci series.This code is going in infinite loop .Can someone tell what's the error?

2 Answers2