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A Prime number is a natural number greater than 1 and divisible by 1 and itself only, for example: 2, 3, 5, 7, etc.

Method 1: Using conditional statements

In the below example, the number called MyNum is checked for prime number by dividing it with all natural numbers starting from 2 to N - 1.

#include <iostream>
using namespace std;

int main() {
  int MyNum = 17;
  int n = 0;
  for(int i = 2; i < MyNum; i++) {
    if(MyNum % i == 0){
      n++;
      break;
    }
  }
  if (n == 0) {
    cout<<MyNum<<" is a prime number.";
  } else {
    cout<<MyNum<<" is not a prime number.";
  }
  return 0;
}

The above code will give the following output:

17 is a prime number.

Method 2: Using function

In the below example, a function called primenumber() is created which takes a number as argument and checks it for prime number by dividing it with all natural numbers starting from 2 to N/2.

#include <iostream>
using namespace std;

static void primenumber(int);

static void primenumber(int MyNum) {
  int n = 0;
  for(int i = 2; i < (MyNum/2+1); i++) {
    if(MyNum % i == 0){
      n++;
      break;
    }
  }
  if (n == 0){
    cout<<MyNum<<" is a prime number.";
  } else {
    cout<<MyNum<<" is not a prime number.";
  }
}
int main() {
  primenumber(21);
  return 0;
}

The above code will give the following output:

21 is not a prime number.

Method 3: Optimized Code

  • Instead of checking the divisibility of given number from 2 to N/2, it is checked till square root of N. For a factor larger than square root of N, there must the a smaller factor which is already checked in the range of 2 to square root of N.
  • Except from 2 and 3, every prime number can be represented into 6k ± 1.
#include <iostream>
using namespace std;

static void primenumber(int);

static void primenumber(int MyNum) {
  int n = 0;
  if (MyNum == 2 || MyNum == 3){
    cout<<MyNum<<" is a prime number.";
  } else if (MyNum % 6 == 1 || MyNum % 6 == 5) {
      for(int i = 2; i*i <= MyNum; i++) {
        if(MyNum % i == 0){
          n++;
          break;
        }
      }
      if (n == 0){
        cout<<MyNum<<" is a prime number.";
      } else {
        cout<<MyNum<<" is not a prime number.";
      }
  } else {
    cout<<MyNum<<" is not a prime number.";
  }
}
int main() {
  primenumber(21);
  return 0;
}

The above code will give the following output:

21 is not a prime number.