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C Program to calculate Permutation and Combination

Last updated on September 23, 2020


The following is a C program to compute Permutation and Combination:

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/**************************************************
 Program to calculate Permutation and Combination
 * 
 * Enter n: 10
 * Enter r: 4
 * Permutation = 5040
 * Combination = 210
 ***************************************************/

#include<stdio.h> // include stdio.h library
long permutation(int n, int r);
long combination(int n, int r);
long factorial(int num);

int main(void)
{    
    int n, r;

    printf("Enter n: ");
    scanf("%d", &n);

    printf("Enter r: ");
    scanf("%d", &r);

    printf("Permutation = %ld\n", permutation(n, r));    
    printf("Combination = %ld", combination(n, r));

    return 0; // return 0 to operating system
}

long permutation(int n, int r)
{
    return factorial(n) / factorial(n-r);
}

long combination(int n, int r)
{
    return permutation(n, r) / factorial(r);
}

long factorial(int num)
{
    long long fact = 1;

    while(num > 0)
    {
        fact *= num;
        num--;
    }

    return fact;
}

Try it now

Expected Output: 1st run:

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Enter n: 5
Enter r: 3
Permutation = 60
Combination = 10

2nd run:

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Enter n: 5
Enter r: 0
Permutation = 1
Combination = 1

How it works #

Nothing magical here we are simply using the formula to calculate permutation and combination.

The permutations of n different objects taken r at a time is given by:

\begin{gather*}
nPr = \frac{n!}{(n-r)!}
\end{gather*}

The combinations of n different objects taken r at a time is given by:

\begin{gather*}
nCr = \frac{n!}{r!(n-r)!}
\end{gather*}


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