C++ Program to check Prime Number

• Difficulty Level : Easy
• Last Updated : 05 Dec, 2018

Given a positive integer, check if the number is prime or not. A prime is a natural number greater than 1 that has no positive divisors other than 1 and itself. Examples of first few prime numbers are {2, 3, 5,

Examples:

Input:  n = 11
Output: true

Input:  n = 15
Output: false

Input:  n = 1
Output: false
 // A school method based C++ program to check if a// number is prime#include using namespace std;  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;}  // Driver Program to test above functionint main(){    isPrime(11) ? cout << " true\n" : cout << " false\n";    isPrime(15) ? cout << " true\n" : cout << " false\n";    return 0;}
Output:
true
false

Time complexity of this solution is O(n)

Optimized School Method

 // A optimized school method based C++ program to check// if a number is prime#include using namespace std;  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;}  // Driver Program to test above functionint main(){    isPrime(11) ? cout << " true\n" : cout << " false\n";    isPrime(15) ? cout << " true\n" : cout << " false\n";    return 0;}
Output:
true
false

Please refer complete article on Primality Test | Set 1 (Introduction and School Method) for more details!

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