C＃使用一种方法来检查数字是素数，不是素数还是负数

Question

For a school practice school assignment we have to create a little program that uses a bool method to calculate if a number is prime or not. It also has to check if a number is negative. If 0 is entered it will stop asking numbers and ends the program. (I've added a screenshot of the assignment)

I've managed to make the Prime checking part. When i enter a negative number it will display: Negative number, after that it will still calculate if it's a prime yes or no. The same with ending the program. It first calculates if 0 is a prime number and then ends it.

How do i fix that?

This is my code:

static void Main(string[] args)
    {

        int entered_number = 1;


        while (entered_number != 0)
        {

            //ask for user entry
            Console.Write("enter a number: ");
            entered_number = int.Parse(Console.ReadLine());

            if (entered_number < 0)
            {
                Console.WriteLine("Number is negative");
            }

            //variable for subprogram
            bool prime = IsPrimeNumber(entered_number);

            //output
            if (prime == true)
            {
                Console.WriteLine("Number is Prime");
            }

            else if (prime == false)
            {
                Console.WriteLine("Number is not Prime");
            }

        }

        Console.WriteLine("End of program");
        Console.ReadKey();


    }
    //subprogram
    static bool IsPrimeNumber(int entered_number)
    {
        //count
        int count = 0;

            for (int i = 1; i <= entered_number; i++)
            {
                if (entered_number % i == 0)
                {
                    count++;
                }
            }

            if (count == 2)
            {
                return true;
            }

            else if (count != 2 && entered_number != 0)
            {
                return false;
            }

        return false;
    }

If someone knows how please let me know.

Yours sincerely,

Max van Kekeren

Answer 1

Ask the user for input at least once before the while loop. This will ensure that if the first number they enter is 0, the program will end as expected. Currently it uses 1 the first time running, then asks for user input and runs all the code anyway. As for your if (entered_number < 0) statement, rather put all the code as an else to this if statement so that if the number is negative your program just asks the user for input again. See example below (I left out the first suggestion, you need to do some of the coding yourself :p):

while (entered_number != 0)
{

    //ask for user entry
    Console.Write("enter a number: ");
    entered_number = int.Parse(Console.ReadLine());

    if (entered_number < 0)
    {
        Console.WriteLine("Number is negative");
    }
    else 
    {
        //variable for subprogram
        bool prime = IsPrimeNumber(entered_number);

        //output
        if (prime == true)
        {
            Console.WriteLine("Number is Prime");
        }

        else if (prime == false)
        {
            Console.WriteLine("Number is not Prime");
        }
    }
}

Console.WriteLine("End of program");
Console.ReadKey();

Answer 2

use do while , your code will be executed at least once.

 int entered_number = 0;
        do
        {
            //ask for user entry
            Console.Write("enter a number: ");
            entered_number = int.Parse(Console.ReadLine());

            if (entered_number < 0)
            {
                Console.WriteLine("Number is negative");
            }
            else if (entered_number > 0)
            {
                Console.WriteLine(IsPrimeNumber(entered_number) ? "Number is Prime" : "Number is not Prime");
            }
            else
            {
                break;
            }
        }
        while (entered_number != 0);

        Console.WriteLine("End of program");
        Console.ReadKey();