C++ program to check if a given number is a prime number or not

In this article, you will learn and get code on checking whether a given number, given by the user at run-time, is a prime number or not in C++. The program is created in the following ways:

Check if a number is prime or not without using a function or a class
Check if a number is prime or not using a user-defined function
Using class and object, determine whether a number is a prime number

What is a prime number?

A prime number is a number that can only be divisible by 1 and the number itself. That is, if a number has no factors except 1 and the number itself, then it is called a prime number. For example, 13, 7, 17, 19, 23, etc.

Examine the prime number in C++

To check whether the number is a prime number or not in C++ programming, you have to ask the user to enter a number first, and then check and print as shown in the program given below:

If a number, n, is divisible by any number from 2 to one less than the number (n-1), then it is not prime; otherwise, it is a prime number.

#include<iostream>
using namespace std;
int main()
{
    int num, i, chk=0;
    cout<<"Enter a Number: ";
    cin>>num;
    for(i=2; i<num; i++)
    {
        if(num%i==0)
        {
            chk++;
            break;
        }
    }
    if(chk==0)
        cout<<"\nIt is a Prime Number";
    else
        cout<<"\nIt is not a Prime Number";
    cout<<endl;
    return 0;
}

This program was built and runs under the Code::Blocks IDE. Here is its sample run:

Now supply any number as input, say 13, and press the ENTER key to check whether it is a prime or not, as shown here:

The dry run of the above program (with user input 13) goes like this:

Initially chk=0 and num=13 (the user entered 13).
Inside the for loop, the initialization part executes first and only once. Therefore i=2
Checks whether the value of i is less than the value of num or not.
The condition evaluates to be true because 2 (the value of i) is less than 13 (the value of num), therefore program flow goes inside the loop.
Inside the loop, there is an if statement that checks whether the value of num is divisible by the current value of i or not.
Note: If the value of i ever divides num, just increment the value of count and exit the loop using the break keyword.
Because 13 is not divisible by 2, the condition evaluates to be false, therefore program flow doesn't go inside the if's body.
Now the program flow goes to the update part of the for loop and increments the value of i. Now i=3. Checks for the condition again to see if i<num or 3<13 evaluates to true or false.
Condition again evaluates to be true, therefore program flow again goes inside the loop.
In this manner, each time you enter the loop, check whether the num's value is divisible by the i's value.
Continue the process until the condition of the for loop evaluates to be false.
After exiting from the loop, check for the value of chk and print the message.
That is, if chk holds its initial value (0), then the given number is a prime number, otherwise it is not a prime number.

In C++, use a function to check the prime number

Now let's create the same-purpose program using a user-defined function called checkPrimeNumber(). It takes the number entered by the user as an argument and returns a value. Initialize its return value to a variable, say chk, so check its value and print the message accordingly.

#include<iostream>
using namespace std;
int checkPrimeNumber(int);
int main()
{
    int num, chk;
    cout<<"Enter a Number: ";
    cin>>num;
    chk = checkPrimeNumber(num);
    if(chk==0)
        cout<<"\nIt is a Prime Number";
    else
        cout<<"\nIt is not a Prime Number";
    cout<<endl;
    return 0;
}
int checkPrimeNumber(int num)
{
    int i, chk=0;
    for(i=2; i<num; i++)
    {
        if(num%i==0)
        {
            chk++;
            return chk;
        }
    }
    return chk;
}

It will produce the same output as the previous program. Here is its sample run with user input as 16:

You can also prefer the program given below for the same purpose as the previous one. In this program, rather than returning a value to the main() function, this program checks and prints directly from inside the function:

#include<iostream>
using namespace std;
void checkPrimeNumber(int);
int main()
{
    int num;
    cout<<"Enter a Number: ";
    cin>>num;
    checkPrimeNumber(num);
    cout<<endl;
    return 0;
}
void checkPrimeNumber(int num)
{
    int i, chk=0;
    for(i=2; i<num; i++)
    {
        if(num%i==0)
        {
            chk++;
            break;
        }
    }
    if(chk==0)
        cout<<"\nIt is a Prime Number";
    else
        cout<<"\nIt is not a Prime Number";
}

In C++, check the prime number using the class

This is the last program that uses classes and objects to check for the prime number. The question is: write a program in C++ to check whether an input number is a prime number or not using classes. Here is its answer:

#include<iostream>
using namespace std;
class fresherearth
{
    private:
        int num, i, chk;
    public:
        int getData();
        int checkPrimeNumber(int);
};
int fresherearth::getData()
{
    cout<<"Enter a Number: ";
    cin>>num;
    return num;
}
int fresherearth::checkPrimeNumber(int num)
{
    int i, chk=0;
    for(i=2; i<num; i++)
    {
        if(num%i==0)
        {
            chk++;
            return chk;
        }
    }
    return chk;
}
int main()
{
    fresherearth c;
    int num, chk=0;
    num = c.getData();
    chk = c.checkPrimeNumber(num);
    if(chk==0)
        cout<<"\nIt is a Prime Number";
    else
        cout<<"\nIt is not a Prime Number";
    cout<<endl;
    return 0;
}

The brief explanation of the above code is:

A fresherearth (class) object c is created.
Using this object, we have called the function getData() of the class fresherearth.
and its return value gets initialized to num inside the main() function.
Because the getdata() function receives a value from the user and returns it, the number entered by the user is basically initialized to num.
Now we've passed num to checkPrimeNumber() function. This function checks whether it is a prime or not. It returns 0 (if num is a prime number) or 1 (if num is a composite number), and its return value gets initialized to chk inside the main() function.
Finally, we have checked the value of chk and printed the message.

The same program in different languages

C++ Quiz

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