Example to check whether an integer (entered by the user) is a prime number or not using for loop and if...else statement.

To understand this example, you should have the knowledge of following C++ programming topics:

A positive integer which is only divisible by 1 and itself is known as prime number.

For example: 13 is a prime number because it is only divisible by 1 and 13 but, 15 is not prime number because it is divisible by 1, 3, 5 and 15.

```
#include <iostream>
using namespace std;
int main()
{
int n, i;
bool isPrime = true;
cout << "Enter a positive integer: ";
cin >> n;
for(i = 2; i <= n / 2; ++i)
{
if(n % i == 0)
{
isPrime = false;
break;
}
}
if (isPrime)
cout << "This is a prime number";
else
cout << "This is not a prime number";
return 0;
}
```

**Output**

Enter a positive integer: 29 This is a prime number.

This program takes a positive integer from user and stores it in variable `n`.

Then, `for`

loop is executed which checks whether the number entered by user is perfectly divisible by `i` or not.

The foor loop initiates with an initial value of `i` equals to 2 and increasing the value of `i` in each iteration.

If the number entered by user is perfectly divisible by `i` then, *isPrime* is set to `false` and the number will not be a prime number.

But, if the number is not perfectly divisible by `i` until test condition `i <= n/2`

is true means, it is only divisible by 1 and that number itself.

So, the given number is a prime number.

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