我如何让这个 for 循环只打印素数？

Question

What I need it to do is print all of the prime numbers starting at 1 and ending at the input, if the input is also a prime number. Here's my code now:

static void primeNumbers(int n) {

boolean isPrime;

System.out.println("All the prime numbers up to " + n + " are -->");
for (int prime = 2; prime < n; prime = prime++) {
  if (n % prime == 0) {
    isPrime = false;
  }
  if(isPrime == true){
    System.out.println(prime);
  }
}
}

My teacher said that I need to make a nested for loop, but I just don't know what to put in it. I'm also getting an error saying that my last use of isPrime hasn't been initialized.

Answer 1

You need to actually check for primality, and not just see if the number is a factor of n:

static boolean isPrime(int n) {
    if (n < 2) {
        return false;
    }
    if (n % 2 == 0) {
        return n == 2;
    }
    for (int k = 3; k * k <= n; k += 2) {
        if (n % k == 0) {
            return false;
        }
    }
    return true;
}

static void primeNumber(int n) {
    System.out.println("All the prime numbers up to " + n + " are -->");
    for (int num = 2; num < n; num ++) {
        if (isPrime(num)) {
            System.out.println(num);
        }
    }
}

Answer 2

You will need to do something like this for your program to work.

static void primeNumbers(int n) {

    boolean isPrime = true;

    System.out.println("All the prime numbers up to " + n + " are -->");
    for (int prime = 2; prime < n; prime++) {
        if (n % prime == 0) {
            isPrime = false;
        }else{
            isPrime = true;
        }
        if(isPrime){
            System.out.println(prime);
        }
    }
}

Answer 3

It is necessary to initialize isPrime.

And, Eratosthenes' s sieve is famous algorithm for get prime numbers.

I think the URL below will help you.

https://www.geeksforgeeks.org/java-program-for-sieve-of-eratosthenes/

Answer 4

here is one of the solutions

 public static void main(String[] args) {
        int n = 23;
        boolean[] isPrime = new boolean[n + 1];

        for (int i = 1; i <= n; i++) {
            isPrime[i] = true;
        }

        for (int i = 2; i <= n / i; i++) {
            if (isPrime[i]) {
                for (int j = i; j <= n / i; j++) {
                    isPrime[i * j] = false;
                }
            }
        }

        for (int i = 1; i <= n; i++) {
            if (isPrime[i]) {
                System.out.println(i);
            }
        }
    }