How to implement N choose R?

Question

I just wrote this code :

#include <iostream>

using namespace std;

unsigned long long int choose(unsigned long long int k, unsigned long long int n)
{
    if (k==n or k==0)
    {
        return 1;
    }
    else
    {
        return (choose(n-1,k-1)+choose(n-1,k));
    }
}

int main(){
    cout << choose(3, 6);
}

but I got Run-Time Error, I think my problem is in my variables, I did debugging but I couldn't find the answer, why I got run time error?

possible duplicate of [Binomial coefficient](http://stackoverflow.com/questions/4256188/binomial-coefficient) — Anthony, Jun 04 '14 at 12:45
@anthony-arnold if u have my answer help me,else don't bother me — user3700182, Jun 04 '14 at 12:56

score 2 · Answer 1 · answered Jun 04 '14 at 12:47

2

Since your variables are unsigned, substracting 1 from them when they're already 0 will make them roll over to the max value.

When n reaches 0 and you then call choose(n-1,k), this'll happen and that's the source of the issue (stackoverflow).

answered Jun 04 '14 at 12:47

Luchian Grigore

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How Can i fix it?i think i have to insert some IF – user3700182 Jun 04 '14 at 12:49
@user3700182 `or n == 0`? – Luchian Grigore Jun 04 '14 at 12:49
do yo mean :: if(k==n or k==0 or n==0) ?? – user3700182 Jun 04 '14 at 12:55
@user3700182 do you think that makes sense? – Luchian Grigore Jun 04 '14 at 13:05
@user3700182 some other piece of advice - unless this is your coding convention, use the more idiomatic `||` instead of `or`. – Luchian Grigore Jun 04 '14 at 13:24

How to implement N choose R?

1 Answers1