Program to find Prime Numbers Between given Interval

// C# program to find the prime numbers
// between a given interval
using System;

class GFG{

// Driver code    
public static void Main(string[] args) 
{ 
    
    // Declare the variables
    int a, b, i, j, flag;

    // Ask user to enter lower value of interval
    Console.WriteLine("Enter lower bound of " +
                      "the interval: ");
                      
    // Take input
    a = int.Parse(Console.ReadLine()); 

    // Ask user to enter upper value of interval
    Console.WriteLine("\nEnter upper bound " + 
                      "of the interval: ");
                      
    // Take input
    b = int.Parse(Console.ReadLine()); 

    // Print display message
    Console.WriteLine("\nPrime numbers between " +  
                      "{0} and {1} are: ", a, b);

    // Traverse each number in the interval
    // with the help of for loop
    for(i = a; i <= b; i++)
    {
        
        // Skip 0 and 1 as they are
        // neither prime nor composite
        if (i == 1 || i == 0)
            continue;

        // flag variable to tell
        // if i is prime or not
        flag = 1;

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

        // flag = 1 means i is prime
        // and flag = 0 means i is not prime
        if (flag == 1)
            Console.WriteLine(i);
    }
}
}

// This code is contributed by jana_sayantan

// C# program to find the prime numbers
// between a given interval
using System;
class GFG
{
  // Driver code
  static public void Main()
  {

    // Declare the variables
    int a, b, i, j, flag;

    // Ask user to enter lower value of interval
    Console.Write(
      "Enter lower bound of the interval: ");
    a = Convert.ToInt32(
      Console.ReadLine()); // Take input

    // Ask user to enter upper value of interval
    Console.Write(
      "\nEnter upper bound of the interval: ");
    b = Convert.ToInt32(
      Console.ReadLine()); // Take input

    // Print display message
    Console.Write("\nPrime numbers between " + a
                  + " and " + b + " are: ");

    // Explicitly handling the cases when a is less than
    // 2
    if (a == 1)
    {
      Console.Write(a + " ");
      a++;
      if (b >= 2)
      {
        Console.Write(a + " ");
        a++;
      }
    }
    if (a == 2)
    {
      Console.Write(a + " ");
    }

    // MAKING SURE THAT a IS ODD BEFORE WE BEGIN
    // THE LOOP
    if (a % 2 == 0) 
    {
      a++;
    }

    // NOTE : WE TRAVERSE THROUGH ODD NUMBERS ONLY
    for (i = a; i <= b; i = i + 2)
    {

      // flag variable to tell
      // if i is prime or not
      flag = 1;

      // WE TRAVERSE TILL SQUARE ROOT OF j only.
      // (LARGEST POSSIBLE VALUE OF A PRIME FACTOR)
      for (j = 2; j * j <= i; ++j)
      {
        if (i % j == 0) 
        {
          flag = 0;
          break;
        }
      }

      // flag = 1 means i is prime
      // and flag = 0 means i is not prime
      if (flag == 1) 
      {
        Console.Write(i + " ");
      }
    }
  }
}

// This code is contributed by rag2127

// C# program to find the prime numbers
// between a given interval using Sieve of Eratosthenes
using System;

public class GFG
{
    public static void SieveOfEratosthenes(int srt, int n)
    {
        // Create a boolean array "prime[srt to n]" and
        // initialize all entries it as true. A value in
        // prime[i] will finally be false if i is Not a
        // prime, else true.
        bool[] prime = new bool[n + 2];
        Array.Fill(prime, true);
        prime[0] = false;
        prime[1] = false;

        for (int p = 2; p * p <= n; p++)
        {
            // If prime[p] is not changed, then it is a
            // prime
            if (prime[p] == true)
            {
                // Update all multiples of p greater than or
                // equal to the square of it. Numbers which
                // are multiples of p and are less than p^2
                // are already marked.
                for (int i = p * p; i <= n; i += p)
                {
                    prime[i] = false;
                }
            }
        }

        // Print all prime numbers
        for (int p = srt; p <= n; p++)
        {
            if (prime[p])
            {
                Console.Write(p + " ");
            }
        }
    }

    // Driver Code
    public static void Main(string[] args)
    {
        int srt = 1;
        int end = 10;
        SieveOfEratosthenes(srt, end);
    }
}

// This code is contributed by Pushpesh Raj