# C++ Permutations of n things Taken r at a Time with k Things Together

C++Server Side ProgrammingProgramming

Given n, r, k, now we have to find how we can select r things from n so that specific k things always occur together, for example.

Input : n = 8, r = 5, k = 2

Output : 960

Input : n = 6, r = 2, k = 2

Output : 2

We need a little knowledge for this problem because this problem is asking us to find the permutation of n and r such that k things come together.

## Approach to Find the Solution

We need to formulate our formula for this question, and that will give us our answer.

## Example

#include <bits/stdc++.h>
using namespace std;
int fact(int n){ // function to calculate factorial of a number
if(n <= 1)
return 1;
return n * fact(n-1);
}
int npr(int n, int r){ // finding permutation
int pnr = fact(n) / fact(n - r);
return pnr;
}
int countPermutations(int n, int r, int k){ // the formula that we came up with
return fact(k) * (r - k + 1) * npr(n - k, r - k);
}
int main(){
int n = 8;
int r = 5;
int k = 2;
cout << countPermutations(n, r, k);
return 0;
}

## Output

960

## Explanation of the Above Code

In the above approach, we try to devise our formula for calculating our answer as for this problem, the formula that we devised is (k!) * (r - k + 1) * P(n-k, r-k). ( P(x, y) is the number of permutations of selection y from x) hence we put up our formula, and we calculate the answer.

## Conclusion

In this tutorial, we solve a problem to find Permutations of n things taken r at a time with k things together. We also learned the C++ program for this problem and the complete approach ( Normal) by which we solved this problem.

We can write the same program in other languages such as C, java, python, and other languages. We hope you find this tutorial helpful.

Published on 25-Nov-2021 10:26:35