Minimum number of single digit primes required whose sum is equal to N in C++

Problem statement

Find the minimum number of single-digit prime numbers required whose sum will be equal to N.

Example

If N = 9 then we require 2 prime numbers i.e. 7 and 2 to make sum 9.

Example

#include <iostream>
using namespace std;
bool isValidIndex(int i, int val) {
   return (i - val) < 0 ? false : true;
}
int getMinPrimes(int n) {
   int arr[n + 1];
   for (int i = 1; i <= n; ++i) {
      arr[i] = 1000000000L;
   }
   arr[0] = arr[2] = arr[3] = arr[5] = arr[7] = 1;
   for (int i = 1; i <= n; ++i) {
      if (isValidIndex(i, 2)) {
         arr[i] = min(arr[i], 1 + arr[i - 2]);
      }
      if (isValidIndex(i, 3)) {
         arr[i] = min(arr[i], 1 + arr[i - 3]);
      }
      if (isValidIndex(i, 5)) {
         arr[i] = min(arr[i], 1 + arr[i - 5]);
      }
      if (isValidIndex(i, 7)) {
         arr[i] = min(arr[i], 1 + arr[i - 7]);
      }
   }
   return arr[n] == 1000000000L ? -1 : arr[n];
}
int main() {
   int n = 9;
   int result = getMinPrimes(n);
   if (result != -1) {
      cout << "Minimum required primes: " << getMinPrimes(n) << endl;
   } else {
      cout << "Not possible to create required sum" << endl;
   }
   return 0;
}

Output

When you compile and execute above program. It generates following output −

Minimum required primes: 2