Facebook Page Twitter Page LinkedIn Page
× C# Examples


A Prime number is a natural number greater than 1 and divisible by 1 and itself only, for example: 2, 3, 5, 7, etc.

Objective: Write a C# code to find all prime numbers less than a given number.

Method 1: Using method to find prime number

In the below example, a method called primenumber() is created which takes a number as argument and checks it for prime number by dividing it with all natural numbers starting from 2 to N/2.

using System;

class MyProgram {
  static void primenumber(int MyNum) 
  {
    int n = 0;
    for(int i = 2; i < (MyNum/2+1); i++)
    {
      if(MyNum % i == 0){
        n++;
        break;
      }
    }
    if (n == 0){
      Console.Write(MyNum + " "); 
    }
  }

  static void Main(string[] args) {
    int x = 50;
    Console.WriteLine("Prime numbers less than "+ x + " are: ");
    for(int i = 2; i < x + 1; i++)
    {
      primenumber(i);
    }
  }
}

The above code will give the following output:

Prime numbers less than 50 are:
2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 

Method 2: Optimized Code

  • Instead of checking the divisibility of given number from 2 to N/2, it is checked till square root of N. For a factor larger than square root of N, there must the a smaller factor which is already checked in the range of 2 to square root of N.
  • Except from 2 and 3, every prime number can be represented into 6k ± 1.
using System;

class MyProgram {
  static void primenumber(int MyNum) 
  {
    int n = 0;
    if (MyNum == 2 || MyNum == 3){
      Console.Write(MyNum + " ");
    } else if (MyNum % 6 == 1 || MyNum % 6 == 5) {
        for(int i = 2; i*i <= MyNum; i++)
        {
          if(MyNum % i == 0){
            n++;
            break;
          }
        }
        if (n == 0){
          Console.Write(MyNum + " ");
        } 
    }
  }

  static void Main(string[] args) { 
    int x = 100;
    Console.WriteLine("Prime numbers less than "+ x + " are: ");
    for(int i = 2; i < x + 1; i++)
    {
      primenumber(i);
    }
  }
}

The above code will give the following output:

Prime numbers less than 100 are:
2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97