Question

I am trying to generate Fibonacci series and provided the code for the same below. When I run this code for smaller values, it outputs correct results. However, when I try and calculate series for a number say ’50’, it out puts correct results upto the 47th number and result for 48,49 and 50th term are incorrect. I tried using unsigned long int as well but it did not correct the results. Can some please suggest what am I doing wrong here.
Thanks.

#include<stdio.h>
unsigned long long int fibr(unsigned long long int);

int main(){
    unsigned long long  int n;
    printf("Enter a number\n");
    scanf("%llu",&n);
    //res=fibr(n);
      while(n>=0){
          printf("%llu\n",fibr(n));
          n--;
           }
     }
unsigned long long int fibr(unsigned long long int n){
     if((n==0)||(n==1))
         return n;
     else return fibr(n-1)+fibr(n-2);
     }

‘After the suggestions , I incorporated unsigned long long int. Have modified the code above but now it gives a seg fault. Any hints please. I am not allowed to use any other library except the standards one available. ‘

Answer 1

Here’s the answer to your second question:

I think you had this problem from the beginning:

while(n>=0){

is an infinite loop since n is an unsigned integer. n will go negative due to the decrement. But since it’s unsigned, it will wrap around and cause a stack-overflow in your recursion.

Also, your algorithm is probably slowest way to do this. It runs in exponential time. So it will take a very long time to run it when n is big.

A better way is just this:

int n = 48;
return (unsigned long long)(pow(1.6180339887498948,n) * 0.44721359549995794 + 0.5);

This method will run in constant time. 🙂