为什么我的质数java程序不打印出质数？

Question

static void primeNumbers(int n)
    {
         System.out.println("All the prime numbers up to "+n+" are -->\n");

        boolean isPrime = true; 
        for(int i = 2; i < n; i++)
        {
            for (int j = 1; j < i; j++)
            {
                if(i % j == 1)
                {
                    isPrime = true; 
                    System.out.print(j);
                }               

                if(i % j == 0)
                {
                    isPrime = false; 
                    System.out.print(" ");
                }
            }
        }

    }

In this code I am trying to list all the prime numbers up to 'n' which is the number the user inputs. I am confused as to how to fix this to produce all of the prime numbers up to n.

Answer 1

You want to test if i is dividable by any number in the range of [2..i-1] and if it is, then it is not a prime number. Your j -loop starts at 1, that is the first error.

Knowing when a number is dividable by another is tested like this: if (i % j == 0) { (if the division remainder is zero) where in your code you test for equality with 1 instead.

Third thing is that you make your decision in the first iteration (when j is 1) and always print something in your inner loop. You need to take the logic out of the inner loop. Only if i is not dividable by any of the j s then it is prime.

Here is a modified version of your code:

static void primeNumbers(int n) {
    System.out.println("All the prime numbers up to "+n+" are -->\n");

    for (int i = 2; i < n; i++) {
        boolean isPrime = true;
        for (int j = 2; j < i; j++) {
            if(i % j != 0) {
                isPrime = false;
            }
        }
        if (isPrime) {
            System.out.print(i, " ");
        }
    }
}

Answer 2

if (i % j == 0) {
    isPrime = false;
    System.out.print(" ");
}

This if statement is always true for any i because j starts at 1. As a result, you print a space for every number < n and >= 2

Also, isPrime may be an unnecessary variable.

Answer 3

Your algorithm, although slow, seems to be correct on most parts except for the fact that you are not handling the boundary conditions well. Eg 2 is a prime number, you are running iterations only till n-1, you are not reseting the isPrime flag for every number, and a few small things. Here's a revised working version. Try to compare it with yours and understand where you didn't get it right.

    int n = 10;
    System.out.println("All the prime numbers up to "+n+" are -->\n");

    boolean isPrime = true; 
    for(int i = 2; i <= n; i++)
    {
        for (int j = 2; j < i; j++)
        {   
            if(i % j == 0)
            {
                isPrime = false; 
                break;
            }
        }
        if(isPrime) {
            System.out.println(i);
        }
        isPrime = true;
    }

Answer 4

Don't re-set isPrime to true in your second for loop, as soon as the number is not prime once, we know it is not prime forever. Just set it to true once every iteration of the outer loop, and do your final "Is prime" check outside.

static void isPrime(int n)
    {
         System.out.println("All the prime numbers up to "+n+" are -->\n");

        for(int i = 2; i <= n; i++)
        {
            boolean isPrime = true;
            for (int j = 2; j < i; j++)
            {

                if(i % j == 0)
                {
                    isPrime = false;
                    System.out.println(i + " is not prime because " + i + " is divisible by " + j );
                    break;
                }
            }
            if (isPrime){
              System.out.println(i + " is prime.");
            }
        }

    }

I've added a couple prints to this code so that you can see why each part of the logic is doing what it does -- compare it to your code so that you can understand why yours doesn't actually search for primes.

Answer 5

I think is a logical problem in your script. You should count with a loop all the number from 2 to n-1 which are = to 0 with a test

n%i = 0

If the counter = 0 then no divider then prime number