Print all multiplicative primes <= N in C++

In this problem, we are given an integer n and we have to print all multiplicative primes less than or equal to n.

Multiplicative primes are prime numbers that have a product of their digits also prime numbers. Like 2, 3, 5, 7, 13, 17.

23 is prime but not a multiplicative prime because of 2*3 = 6.

Let’s take an example to understand the problem −

Input: n = 9
Output: 2 3 5 7

To solve this problem, we will find all prime numbers less than n. And check if the number is multiplicative prime. And print all multiplicative prime less than n.

Example

The program illustrates the solution to the problem

 Live Demo

#include <bits/stdc++.h>
using namespace std;
int digitMultiply(int n) {
   int prod = 1;
   while (n) {
      prod = prod * (n % 10);
      n = n / 10;
   }
   return prod;
}
void generateMprime(int n) {
   bool prime[n + 1];
   memset(prime, true, sizeof(prime));
   prime[0] = prime[1] = false;
   for (int p = 2; p * p <= n; p++) {
      if (prime[p]) {
         for (int i = p * 2; i <= n; i += p)
            prime[i] = false;
      }
   }
   for (int i = 2; i <= n; i++) {
      if (prime[i] && prime[digitMultiply(i)])
         cout<<i<<"\t";
   }
}
int main() {
   int n = 10;
   cout<<"All multiplicative Prime Numbers =< "<<n<<" are :\n";
   generateMprime(n);
}

Output

All multiplicative Prime Numbers =< 10 are −
2 3 5 7