Print all prime numbers less than or equal to N in C++

In this problem, we are given a number N and we have to print all prime numbers less than or equal to N.

Let’s take an example to understand the topic better −

Input: 10
Output: 2 3 5 7

A prime number is a number that can be divided by only one and the number itself. Example: 2, 3.

A simple approach is to iterate from 2 to N and divide the number by it. If the number is not divisible, then it’s a prime number. Print the number. Do this till the number is equal to N. This approach is not that efficient.

A more effective approach will be checking for prime number by iterating from 2 to √N.

Example

 Live Demo

#include <bits/stdc++.h>
using namespace std;
bool isPrimeNumber(int n){
   if (n <= 1)
      return false;
   if (n <= 3)
      return true;
   if (n % 2 == 0 || n % 3 == 0)
      return false;
   for (int i = 5; i * i <= n; i = i + 6)
      if (n % i == 0 || n % (i + 2) == 0)
         return false;
   return true;
}
void printPrime(int n){
   for (int i = 2; i <= n; i++) {
      if (isPrimeNumber(i))
         cout<<i<<" ";
      }
   }
int main(){
   int n = 41;
   cout<<"Prime numbers less than or equal to "<<n<<" are \n";
   printPrime(n);
}

Output

Prime numbers less than or equal to 41 are

2 3 5 7 11 13 17 19 23 29 31 37 41