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