Binomial coefficient recursive function in C - brings wrong result, why?

Question

beginner here trying to understand the source of the bug.

I have written this recursive function for finding the binomial coefficient between two numbers which is apparently correct in concept. However, for these two numbers, n =4 and k=2, I should be getting 6 as a result whereas I actually get 16. Any idea why is that happening?

#include<stdio.h>

int binomial(int n, int k)
{
  if ((k = 0) || (k == n))
    return 1;
  if (k>n)
    return 0;

  return binomial(n - 1, k - 1) + binomial(n - 1, k);
}

int main()    
{
  int a, b, res;
  a = 4;
  b = 2;
  res = binomial(a, b);
  printf("The result is %d", res);
  return 0;
}

please consider accepting answers if they that helped you (it also gives you 2 rep points :) ) — Mitch Wheat, Apr 19 '18 at 12:12

score 5 · Answer 1 · answered Apr 19 '18 at 11:48

5

This line looks wrong as it assigns 0 to k:

if ((k=0) || (k==n))

You probably mean:

if ((k==0) || (k==n))

answered Apr 19 '18 at 11:48

Klas Lindbäck

33,105
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score 5 · Answer 2 · answered Apr 19 '18 at 11:48

5

This line

 if ((k = 0) || (k == n))

should be

  if ((k == 0) || (k == n))  
         ^^

You were assigning zero to k.

As @Michael Walz points out, it's good practice to compile with -Wall to turn on all compilation warnings.

answered Apr 19 '18 at 11:48

Mitch Wheat

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Ohhhh, I see! Will pay more attention to syntax next time. Thanks! – franx Apr 19 '18 at 12:08
I am not really familiar with -wall though – franx Apr 19 '18 at 12:09
add it to your compile command assuming you are using gcc: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html – Mitch Wheat Apr 19 '18 at 12:10

Binomial coefficient recursive function in C - brings wrong result, why?

2 Answers2