Print all prime numbers less than or equal to N

Given a number N, the task is to print all prime numbers less than or equal to N.

Examples:

Input: 7
Output: 2, 3, 5, 7

Input: 13
Output: 2, 3, 5, 7, 11, 13 

Naive Approach: Iterate from 2 to N, and check for prime. If it is a prime number, print the number.



Below is the implementation of the above approach:

C++

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// C++ program to print all primes
// less than N
#include <bits/stdc++.h>
using namespace std;
  
// function check whether a number
// is prime or not
bool isPrime(int n)
{
    // Corner case
    if (n <= 1)
        return false;
  
    // Check from 2 to n-1
    for (int i = 2; i < n; i++)
        if (n % i == 0)
            return false;
  
    return true;
}
// Function to print primes
void printPrime(int n)
{
    for (int i = 2; i <= n; i++) {
        if (isPrime(i))
            cout << i << " ";
    }
}
// Driver Code
int main()
{
    int n = 7;
    printPrime(n);
}

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Python3

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# Python3 program to print 
# all primes less than N
  
# Function to check whether 
# a number is prime or not .
def isPrime(n):
      
    # Corner case
    if n <= 1 :
        return False
  
    # check from 2 to n-1
    for i in range(2, n):
        if n % i == 0:
            return False
  
    return True
  
# Function to print primes
def printPrime(n):
    for i in range(2, n + 1):
        if isPrime(i):
            print(i, end = " ")
  
# Driver code
if __name__ == "__main__" :
    n = 7
    # function calling
    printPrime(n)
      
# This code is contributed 
# by Ankit Rai

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Java

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// Java program to print 
// all primes less than N
class GFG 
{
// function check whether 
// a number is prime or not
static boolean isPrime(int n)
{
// Corner case
if (n <= 1)
    return false;
  
// Check from 2 to n-1
for (int i = 2; i < n; i++)
    if (n % i == 0)
        return false;
  
return true;
}
  
// Function to print primes
static void printPrime(int n)
{
for (int i = 2; i <= n; i++) 
{
    if (isPrime(i))
        System.out.print(i + " ");
}
}
  
// Driver Code
public static void main(String[] args) 
{
    int n = 7;
    printPrime(n);
}
}
  
// This code is contributed
// by ChitraNayal

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C#

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// C# program to print 
// all primes less than N
using System;
  
class GFG 
{
// function check whether 
// a number is prime or not
static bool isPrime(int n)
{
      
    // Corner case
    if (n <= 1)
        return false;
      
    // Check from 2 to n-1
    for (int i = 2; i < n; i++)
        if (n % i == 0)
            return false;
      
    return true;
}
      
// Function to print primes
static void printPrime(int n)
{
for (int i = 2; i <= n; i++) 
{
    if (isPrime(i))
        Console.Write(i + " ");
}
}
  
// Driver Code
public static void Main() 
{
    int n = 7;
    printPrime(n);
}
}
  
// This code is contributed 
// by ChitraNayal

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PHP

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<?php 
// PHP program to print 
// all primes less than N
  
// function check whether 
// a number is prime or not
function isPrime($n)
{
    // Corner case
    if ($n <= 1)
        return false;
  
    // Check from 2 to n-1
    for ($i = 2; $i < $n; $i++)
        if ($n % $i == 0)
            return false;
  
    return true;
}
  
// Function to print primes
function printPrime($n)
{
    for ($i = 2; $i <= $n; $i++) 
    {
        if (isPrime($i))
            echo $i . " ";
    }
}
  
// Driver Code
$n = 7;
printPrime($n);
  
// This code is contributed 
// by ChitraNayal
?>

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Output:

2 3 5 7

Time Complexity: O(N * N)

A better approach is based on the fact that one of the divisors must be smaller than or equal to √n. So we check for divisibility only till √n.

C++

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// C++ program to print all primes
// less than N
#include <bits/stdc++.h>
using namespace std;
  
// function check whether a number
// is prime or not
bool isPrime(int n)
{
    // Corner cases
    if (n <= 1)
        return false;
    if (n <= 3)
        return true;
  
    // This is checked so that we can skip
    // middle five numbers in below loop
    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;
}
  
// Function to print primes
void printPrime(int n)
{
    for (int i = 2; i <= n; i++) {
        if (isPrime(i))
            cout << i << " ";
    }
}
// Driver Code
int main()
{
    int n = 7;
    printPrime(n);
}

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Java

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// Java program to print 
// all primes less than N
import java.io.*;
  
class GFG
{
  
// function check
// whether a number
// is prime or not
static boolean isPrime(int n)
{
    // Corner cases
    if (n <= 1)
        return false;
    if (n <= 3)
        return true;
  
    // This is checked so 
    // that we can skip
    // middle five numbers
    // in below loop
    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;
}
  
// Function to print primes
static void printPrime(int n)
{
    for (int i = 2; i <= n; i++)
    {
        if (isPrime(i))
            System.out.print(i + " ");
    }
}
  
// Driver Code
public static void main (String[] args)
{
    int n = 7;
    printPrime(n);
}
}
  
// This code is contributed
// by anuj_67.

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C#

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// C# program to print 
// all primes less than N
using System;
  
class GFG
{
  
// function check
// whether a number
// is prime or not
static bool isPrime(int n)
{
    // Corner cases
    if (n <= 1)
        return false;
    if (n <= 3)
        return true;
  
    // This is checked so 
    // that we can skip
    // middle five numbers
    // in below loop
    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;
}
  
// Function to print primes
static void printPrime(int n)
{
    for (int i = 2; i <= n; i++)
    {
        if (isPrime(i))
            Console.Write(i + " ");
    }
}
  
// Driver Code
public static void Main ()
{
    int n = 7;
    printPrime(n);
}
}
  
// This code is contributed 
// by ChitraNayal

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Python3

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# function to check if the number is 
# prime or not 
def isPrime(n) :
    # Corner cases
    if (n <= 1) :
        return False
    if (n <= 3) :
        return True
   
    # This is checked so that we can skip 
    # middle five numbers in below loop
    if (n % 2 == 0 or n % 3 == 0) :
        return False
   
    i = 5
    while(i * i <= n) :
        if (n % i == 0 or n % (i + 2) == 0) :
            return False
        i = i + 6
   
    return True 
  
# print all prime numbers 
# less than equal to N 
def printPrime(n):
    for i in range(2, n + 1):
        if isPrime(i):
            print (i, end =" "
   
n = 7            
printPrime(n) 

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PHP

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<?php 
// PHP program to print 
// all primes less than N
  
// function check whether 
// a number is prime or not
function isPrime($n)
{
    // Corner cases
    if ($n <= 1)
        return false;
    if ($n <= 3)
        return true;
  
    // This is checked so that
    // we can skip middle five
    // numbers in below loop
    if ($n % 2 == 0 || $n % 3 == 0)
        return false;
  
    for ($i = 5; 
         $i * $i <= $n; $i = $i + 6)
        if ($n % $i == 0 || 
            $n % ($i + 2) == 0)
            return false;
  
    return true;
}
  
// Function to print primes
function printPrime($n)
{
    for ($i = 2; $i <= $n; $i++) 
    {
        if (isPrime($i))
            echo $i . " ";
    }
}
  
// Driver Code
$n = 7;
printPrime($n);
  
// This code is contributed 
// by ChitraNayal
?>

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Output:

2 3 5 7

Time Complexity: O(N3/2)

The best solution is to use Sieve of Eratosthenes. The time complexity is O(√N * loglog(N))



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