how to show prime numbers in c#

[2aliraza]

hi

plz help me how to simple displaying 1 to 20 prime numbers
with for loop
plz help me

[kvprajapati]

If your application type is console then you have to use Console.WriteLine() method. If an app is winform then use UI elements (ListBox).

[2aliraza]

If your application type is console then you have to use Console.WriteLine() method. If an app is winform then use UI elements (ListBox).

plz if u have any demo code plz post here

[2aliraza]

i made this demo but i want without Boolean Condition

plz help me

{
            int x,y;
            bool a = true;
            for ( x = 1; x <=20; x++)
            {
                for ( y = 2; y <= 20; y++)
                {
                    if (x != y && x % y == 0)
                    {
                        a = false;
                        break;
                    }
                }
                if (a)
                {
                    Console.WriteLine(x);
                }
                a = true;
            }
            Console.Read();
        }

[ddanbe]

Write an IsPrime function.
also on line 6 you don't have to let go the variable y all the way to 20.
The square root of 20 will do.
See :

int x, y;
            bool a = true;
            int SQRT = (int)Math.Sqrt(20); // SQRT=4
            for (x = 1; x <= 20; x++)
            {
                for (y = 2; y <= SQRT; y++)
                {
                    if (x != y && x % y == 0)
                    {
                        a = false;
                        break;
                    }
                }

                if (a)
                {
                    Console.WriteLine(x);
                }
                a = true;
            }
            Console.Read();

[Momerath]

Using enumerator

using System;
using System.Collections.Generic;


namespace ConsoleApplication1 {
    class Program {
        static void Main() {
            foreach (int p in Primes(2000)) {
                Console.WriteLine("{0}", p);
            }

            Console.ReadLine();
        }

        static IEnumerable<int> Primes(int limit) {
            if (limit > 1) {
                yield return 2;
                List<int> primes = new List<int>();
                int v = 3;
                while (v <= limit) {
                    Boolean prime = true;
                    foreach (int p in primes) {
                        if (v % p == 0) {
                            prime = false;
                            break;
                        }
                    }
                    if (prime) {
                        primes.Add(v);
                        yield return v;
                    }
                    v += 2;
                }
            }
        }

    }
}

You can speed up generation a bit by adding in the Sqrt stuff:

using System;
using System.Collections.Generic;


namespace ConsoleApplication1 {
    class Program {
        static void Main() {
            foreach (int p in Primes(200000)) {
                Console.WriteLine("{0}", p);
            }

            Console.ReadLine();
        }

        static IEnumerable<int> Primes(int limit) {
            if (limit > 1) {
                yield return 2;
                List<int> primes = new List<int>();
                int v = 3;
                while (v <= limit) {
                    Boolean prime = true;
                    int sv = (int)Math.Sqrt(v);
                    for (int i = 0; i < primes.Count && primes[i] <= sv; i++) {
                        if (v % primes[i] == 0) {
                            prime = false;
                            break;
                        }
                    }
                    if (prime) {
                        primes.Add(v);
                        yield return v;
                    }
                    v += 2;
                }
            }
        }

    }
}

But you'll only need that if you are doing really large lists. And if you are doing those, there are better methods of prime generation :)

[2aliraza]

thanks