C++ Program to Check Whether a Number is Prime or Not

In this C++ program, we check whether a number is prime or not. This program uses for loop, if-else statement and break statement. First of all, we need to know what is prime number.

Prime number: A number which is only divided 1 and itself is called prime number. A prime number cannot be divided by any other number except 1 and itself.

C++ Program to Check Whether a Number is Prime or Not

/*C++ Program to Check Whether a Number is Prime or Not*/

#include<iostream>

using namespace std;

int main()

{

    int a,i;

    cout<<endl<<"Enter a number : ";

    cin>>a;

    int flag=0;

    for(i=2;i<=a/2;i++)

    {

        if(a%i==0)

        {

            flag++;

            break;

        }

    }

    if(flag==1)

        cout<<endl<<a<<" is not a prime number";

    else

        cout<<endl<<a<<" is prime number";

    return 0;

}

OUTPUT:

Enter a number : 9

9 is not a prime number

In this program, first we read a number from keyboard using cin>>a; statement. After reading number, for loop is used to find whether a number is prime or not. 

Inside the for loop, variable i is incremented from 2 to a/2 (a is number to check whether prime or not) and if a is completely divided by i between any number from 2 to a/2, then flag (variable) is incremented and break statement causes the loop to terminate.

Outside the for loop, if-else statement is used to check the status to variable flag. If flag is 1, the number is not prime number and if flag is 0, number is prime number.     

C++ Program to Check Whether a Number is Prime or Not Using Function

/*C++ Program to Check Whether a Number is Prime or Not Using Function*/

#include<iostream>

using namespace std;

void checkprime(int x);

int main()

{

    int a,i;

    cout<<endl<<"Enter a number : ";

    cin>>a;

    checkprime(a);

    return 0;

}

void checkprime(int x)

{

    int i,flag=0;

    for(i=2;i<=x/2;i++)

    {

        if(x%i==0)

        {

            flag++;

            break;

        }

    }

    if(flag==1)

        cout<<endl<<x<<" is not a prime number";

    else

        cout<<endl<<x<<" is prime number";

}

OUTPUT:

Enter a number : 11

11 is prime number