In this post, we will learn **how to find prime numbers using C++** Programming language.

A **prime number** is a natural number greater than **1 **that is not a product of two smaller natural numbers. For example: **27** is not a prime number because **27 = 3 x 9**. While **17** is a prime number because there are only two factors of **17**: **1** and **17** itself.

We will find the prime numbers by checking the number of factors of a particular number. If** factors > 2 **then, it’s not a prime number.

So, without further ado, let’s begin this tutorial.

**C++ Program to Find Prime Number**

// C++ Program to Find Prime Number #include <iostream> using namespace std; int main(){ int num, i; bool flag = true; // Asking for input cout << "Enter any positive integer: "; cin >> num; for (i = 2; i < num; ++i){ if (num % i == 0){ flag = false; break; } } if (flag == true) cout << num << " is a prime number."; else cout << num << " is not a prime number."; return 0; }

**Output 1**

```
Enter any positive integer: 17
17 is a prime number.
```

**Output 2**

```
Enter any positive integer: 91
91 is not a prime number.
```

**How Does This Program Work ?**

int num, i; bool flag = true;

In this program, we have declared **two int data type** variables named **i **and **num**.

// Asking for input cout << "Enter any positive integer: "; cin >> num;

Then, the user is asked to enter a positive integer.

for (i = 2; i < num; ++i){ if (num % i == 0){ flag = false; break; } }

Then, we check whether the entered number is divisible by any number greater than **2** or not. If there is any number which is divisible by **num**, then **flag** value changes to **false**.

if (flag == true) cout << num << " is a prime number."; else cout << num << " is not a prime number.";

If** flag = true**, then the entered number is a **prime number **otherwise the entered number is not a prime number.

**C++ Program to Find Prime Number Using While Loop**

// C++ Program to Find Prime Number #include <iostream> using namespace std; int main(){ int num, i; bool flag = true; // Asking for input cout << "Enter any positive integer: "; cin >> num; i = 2; while(i <= num/2){ if(num % i == 0){ flag = false; break; } i++; } if (flag == true) cout << num << " is a prime number."; else cout << num << " is not a prime number."; return 0; }

**Output 1**

```
Enter any positive integer: 97
97 is a prime number.
```

**Output 2**

```
Enter any positive integer: 25
25 is not a prime number.
```

**Conclusion**

I hope after going through this post, you understand **how to find prime numbers using C++** Programming language.

If you have any doubt regarding the program, feel free to contact us in the comment section. We will be delighted to guide you.

**Also Read:**

SheriffWhy do u change the flag to false

Am confused