# C# Program to Check Prime Number

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## What is Prime Number?

A positive integer that is divisible only by itself and 1.

Tips: It is recommended to use our online Prime Numbers calculator for better understanding.

## Examples

The following table provides few examples of prime numbers.

NumberDivisorResult
131, 13Prime Number
151, 3, 5, 15Not a Prime Number
471, 47Prime Number

## Using for loop

In the following example, we will check whether the given number (7) is a Prime number or not using for loop.

### Example

C# Compiler
using System; namespace myApp { class Program { static void Main(string[] args) { int num = 7; int i = 1; int count = 0; for(i=1; i<=num; i++) { if(num % i == 0) count++; } if(count == 2) Console.Write("{0} is a prime number", num); else Console.Write("{0} is not a prime number", num); } } }

### Output

7 is a prime number

## Using while loop

In the following example, we will check whether the given number (7) is a Prime number or not using while loop.

### Example

C# Compiler
using System; namespace myApp { class Program { static void Main(string[] args) { int num = 7; int i = 1; int count = 0; while(num >= i) { if(num % i == 0) count++; i++; } if(count == 2) Console.Write("{0} is a prime number", num); else Console.Write("{0} is not a prime number", num); } } }

### Output

7 is a prime number

## Using do while loop

In the following example, we will check whether the given number (7) is a Prime number or not using do while loop.

### Example

C# Compiler
using System; namespace myApp { class Program { static void Main(string[] args) { int num = 7; int i = 1; int count = 0; do{ if(num % i == 0) count++; i++; }while(i <= num); if(count == 2) Console.Write("{0} is a prime number", num); else Console.Write("{0} is not a prime number", num); } } }

### Output

7 is a prime number

## Prime Numbers between the Given Range

In the following example, we will find all the Prime numbers between 1 and 20.

### Example

C# Compiler
using System; namespace myApp { class Program { static void Main(string[] args) { int start = 1; int end = 20; int count = 0; int i = 1; Console.WriteLine("Prime numbers between 1 and 20:"); for(start=start; start<=end; start++) { for(i=1; i<=start; i++) { if(start % i == 0) count++; } if(count == 2) Console.Write(start +" "); count = 0; } } } }

### Output

Prime numbers between 1 and 20: 2 3 5 7 11 13 17 19

## Check Whether the Given Number is Prime or Composite

In the following example, we will check whether the given number is a Prime number or Composite number.

### Example

C# Compiler
using System; namespace myApp { class Program { static void Main(string[] args) { Console.Write("Enter a (int) number: "); int num = Convert.ToInt32(Console.ReadLine()); int i; int count = 0; for(i=1; i<=num; i++) { if(num % i == 0) count++; } if(count == 2) Console.Write("{0} is a prime number", num); else Console.Write("{0} is a composite number", num); } } }

### Output

Enter a (int) number: 11 11 is a prime number

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