寻找素数C＃

Question

程序应该要求一个数字，并打印介于1到用户输入的数字之间的所有质数……为什么不起作用？

bool isPrime = true;
int primes = 0;
Console.WriteLine("Enter a number");
int N = int.Parse(Console.ReadLine());
for (int i = 2; i <= N; i++)
{
    for (int j = 2; j <= Math.Sqrt(i); j++)
    {
        if (i % j == 0)
        {
            isPrime = false;
        }  
    }
    if (isPrime)
    {
        Console.WriteLine(i + " is a prime number");
        primes++;
    }
}
Console.WriteLine("Between 1 to " + N + " there are " + primes + " prime numbers");

Answer 1

您已将布尔值排除在循环之外。 因此，一旦它为假，在其他循环中就永远不会为真，这会导致问题。

        int primes = 0;
        Console.WriteLine("Enter a number");
        int N = int.Parse(Console.ReadLine());
        for (int i = 2; i <= N; i++)
        {
           bool isPrime = true;
            for (int j = 2; j <= Math.Sqrt(i); j++)
            {
                if (i % j == 0)
                {
                    isPrime = false;
                }  

            }
            if (isPrime)
            {
                Console.WriteLine(i + " is a prime number");
                primes++;
            }
        }
        Console.WriteLine("Between 1 to " + N + " there are " + primes + " prime numbers");

Answer 2

首先定义这个类：

public static class PrimeHelper
{
    public static bool IsPrime(int candidate)
    {
        if ((candidate & 1) == 0)
        {
            if (candidate == 2)
            {
            return true;
            }
            else
            {
            return false;
            }
        }
        for (int i = 3; (i * i) <= candidate; i += 2)
        {
            if ((candidate % i) == 0)
            {
            return false;
            }
        }
        return candidate != 1;
    }
}

然后在您的应用程序中调用它：

var finalNumber = int.Parse(Console.ReadLine());
for (int i = 0; i < finalNumber; i++)
{
    bool prime = PrimeHelper.IsPrime(i);
    if (prime)
    {
    Console.Write("Prime: ");
    Console.WriteLine(i);
    }
}