Kth prime number greater than N in C++

In this tutorial, we are going to write a program that finds the k-th prime number which is greater than the given number n.

Initialise the number n.
Find all the prime numbers till 1e6 and store it in a boolean array.
Write a loop that iterates from n + 1 to 1e6.
- If the current number is prime, then decrement k.
- If k is equal to zero, then return i.
Return -1.

Example

Let's see the code.

Live Demo

#include <bits/stdc++.h>
using namespace std;
const int MAX_SIZE = 1e6;
bool prime[MAX_SIZE + 1];
void findAllPrimes() {
   memset(prime, true, sizeof(prime));
   for (int p = 2; p * p <= MAX_SIZE; p++) {
      if (prime[p]) {
         for (int i = p * p; i <= MAX_SIZE; i += p) {
            prime[i] = false;
         }
      }
   }
}
int findKthPrimeGreaterThanN(int n, int k) {
   for (int i = n + 1; i < MAX_SIZE; i++) {
      if (prime[i]) {
         k--;
      }
      if (k == 0) {
         return i;
      }
   }
   return -1;
}
int main() {
   findAllPrimes();
   int n = 5, k = 23;
   cout << findKthPrimeGreaterThanN(n, k) << endl;
   return 0;
}

Output

If you run the above code, then you will get the following result.

Conclusion

If you have any queries in the tutorial, mention them in the comment section.

Hafeezul Kareem

Published on 09-Apr-2021 13:06:07

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