# Check Prime Number in C#

In programming, writing algorithms to find positive integers greater than `1` which do not have any other factors except `1` or itself gives us the prime numbers. This tutorial will teach you three solutions to check prime numbers in C#.

## User Input to Check Prime Numbers in a Given Range in `C#`

In C#, the most efficient way to find prime numbers in a given range is by writing an algorithm using `for` loops and `if` conditions. The following C# program can help you understand algorithms to find prime numbers in a given range.

Code Example

``````using System;

static void checkPrime(int InputN)
{
int n = 0;

// algorithm to check prime number
for(int i = 2; i < (InputN/2+1); i++)
{
if(InputN % i == 0)
{
n++;
break;
}
}

if (n == 0)
{
Console.Write(InputN + " ");
}
}

static void Main(string[] args)
{
Console.WriteLine("Enter a number to check prime numbers in the given range: ");
int iNum = Convert.ToInt32(iN);

Console.WriteLine("Prime numbers less than "+ iNum + " are: ");

// loop for finding every prime number in the given range
for(int i = 2; i < iNum + 1; i++)
{
checkPrime(i);
}
}
}
``````

Output:

``````Enter a number to check prime numbers in the given range:
30
Prime numbers less than 30 are:
2 3 5 7 11 13 17 19 23 29
``````

## Use `isPrime` Boolean to Check Prime Number in `C#`

Use the `isPrime` Boolean to check whether the user input number is a prime number or not. In the case of a prime number, the value of `isPrime` will be `true` otherwise `false`.

It’s called trial division and consists of a `for` loop and `if-else` condition.

Code Example"

``````using System;

{
public static void Main(string[] args)
{

Console.WriteLine ("Enter a number: ");
bool isPrime = true;
for(var i = 2; i <= Math.Sqrt(n); i++)
{
if (n % i == 0)
{
isPrime = false;
break;
}
}
if (isPrime == true)
{
Console.WriteLine("It's a prime number.");
}
else
{
Console.WriteLine("It's not prime number");
}
}
}
``````

Output:

``````Enter a number: 19
It's a prime number.
``````

The `for` loop defines an integer `n` as a prime number if `n > 1` and `n` is not divisible by `i`. The `for` loop output will either be a composite number or a prime number.

An `if` condition will further check if `n` is divisible or not by any other number. If it is divisible, the value of `isPrime` will be `false`; otherwise, `true`.

After executing the `for` loop, the number will be prime if the value of `isPrime` is `true`.

## Use Recursion to Check Prime Number in `C#`

It is a native solution to find a prime number in C#. A C# algorithm checks if a number between `2` to `n - 1` divides `n`.

If it finds any number that divides, it will return `false` meaning `n` as a user-defined number is not a prime number.

Code Example:

``````using System;

class CheckPrime
{
static int i = 2;

// checks if a number is prime
static bool isPrime(int n)
{
// check this number
// you can modify this number or make the user set its value thorugh `Console.Readline();`
n = 7;

// corner cases
if (n == 0 || n == 1) {
return false;
}

// Checking Prime
if (n == i)
return true;

// base cases
if (n % i == 0) {
return false;
}
i++;
return isPrime(n);
}

static void Main() {
if(isPrime(35))
{
Console.WriteLine("It is a prime number.");
}
else{
Console.WriteLine("It is not a prime number.");
}
}
}
``````

Output:

``````It is a prime number.
``````

Hassan is a Software Engineer with a well-developed set of programming skills. He uses his knowledge and writing capabilities to produce interesting-to-read technical articles.

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