# C++ Prime number program

In this lesson, you'll learn how to verify whether a given number is a prime number or not in C++, and you'll obtain code to do it.

## What is the definition of a prime number ?

A prime number is any number that is bigger than one and should be divided by one or divided by itself. Since prime numbers could not be divided by any other integer, so only the same number or 1 should be used to do so.

For Example : 2, 3, 5, 7 and so on.

You might be wondering why the number 2 is considered a prime number. Since 2 seems to be the only prime number that is also even, it is an exception. Only two integers are prime and successive natural numbers! In addition, the number 2 is the smallest prime number.

### Checking for a prime number :

If a number n, is divisible by any integer between 2 to one less than (n-1), it is not a prime number, otherwise it is.

Example :

``````#include <iostream>
using namespace std;

int main()
{
/* Defining and initializing a variable */
int num, j, k = 0;

/* Getting the input from the user */
cout << "Enter a number : "; cin>>num;

for (j = 1; j <= num; j++)
{
if (num % j == 0)
{
k++;
}
}
if (k == 2)
{
cout << "It’s a Prime number" << endl;
}
else
{
cout << "It’s not a Prime number" << endl;
}
return 0;
}
``````

Output :

``````Enter a number : 5
It’s a Prime number
``````

#### Explanation :

To begin, you must contain the iostream header file using the "include" directive followed by a #, which indicates that the header file must be processed before compilation, hence the name pre-processor directive. You may now utilize the namespace statement within a program to resolve naming conflicts.

Next, define three integer type variables, 'num', j and 'k,' and set k to 0. Using the cout""; command, inform the user to input any natural number num. The 'cin' command takes a value from the input device (in this case, the keyboard) and stores it in the variable num. You must now create a for - loop that will count from 1 to num. And within this loop, we verify whether dividing num by j yields a result of 0 or not. If the condition is met, the value of c is increased.

Now, if k == 2, it prints "It’s a Prime number" and if k is not equal to 2, it publishes "It’s not a Prime number". Finally, we get the required output.

Example 2 :

``````#include <iostream>
using namespace std;
int main()
{
int low, high, fl, temp;
cout << "Please enter ant two numbers to find the prime numbers between those numbers : "<< endl;
cin >> low >> high;

if ( low > high) {    //the numbers will be swapped when the low number will be greater than the high number. //
temp = low;
low = high;
high = temp;
}
cout << "The Prime numbers between " << low << " and " << high << " are : "<< endl;

while ( low < high)
{
fl = 0;
for ( int x = 2; x <= low/2; ++x)
{
if ( low % x == 0)
{
fl = 1;
break;
}
}
if ( fl == 0)
cout << low << " ";
++low;
}
return 0;
}
``````

Output :

``````Please enter ant two numbers to find the prime numbers between those numbers :
11
20
The Prime numbers between 11 and 20 are :
11 13 17 19
``````

#### Explanation :

We used integers as a low number, a high number, a temp variable, and a flag fl in the code above. We begin by taking two integers as input, one of which is low and the other high. If the low number is greater than the high number, the numbers will be switched using a temp variable before proceeding with the function. Now, the while loop will continue until low is less than high, and the for loop will continue to calculate prime numbers between them, resulting in printing all the prime number between low and high.

### Summary :

Prime number reasoning may be applied in any computer language, not only C++. This reasoning may be used to locate a group of prime numbers according to criteria in seconds without spending any time in computer programming, from a small collection of numbers to a large number of numbers.