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();
}

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();

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 :)

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