java中1到100之间的素数

Question

Guys I have been trying to figure out why this code is not showing the appropriate results for almost 5 hours!!! It's should be a really easy task but it's driving me crazy. I do have to get the project to work with the included code and not another similar code. The issue that I have is that is not showing the prime numbers in the bottom of my software. From my understanding this code should display the prime numbers 1-100 and have the all in lines of 5 numbers except for the last one. Some guidance is greatly appreciated.

public class Prime numbers 1-100{
public static void main (String[] args) {

    int number = 100;

    // Assume the number is prime
    boolean isPrime = true; // Is the current number prime?

    // Test if number is prime
    for (int divisor = 2; divisor <= number / 2; divisor++) {
        if (number % divisor == 0) { // If true, number is not prime
            isPrime = false; // Set isPrime to false
            break; // Exit for loop
        }
        // print prime numbers
        if(isPrime)
            System.out.println("Prime numbers between 1 and " + number);
    }
}

}

Answer 1

Your code should have nested loops and an array which stores found prime numbers to be shown at the end.

To print them at the end you should first print your statement, then use a loop to print the array containing prime numbers found.

Answer 2

If you want to print all the prime numbers from 1 to 100 then you must iterate through all these numbers (1st loop) and for each of them iterate through all its possible divisors (nested loop):

int number = 100;

boolean isPrime = false;

System.out.println("2");
System.out.println("3");

for (int i = 5; i <= number; i++) {
    for (int divisor = 2; divisor <= Math.sqrt(i); divisor++) {
        isPrime = !(i % divisor == 0);
        if (!isPrime)
            break;
    }

    if (isPrime)
        System.out.println("" + i);
}

Note that for 2 and 3 the condition could not be applied so they are printed first.
I used @selbie's optimization hint <= sqrt(number)

Answer 3

public class Prime{
public static void main (String[] args) {
    boolean isPrime = true; 
    // iterate over all numbers from 1 to 100
    for (int number = 1; number <= 100; number++) {
        // We want to believe it's a prime
        isPrime = true;
        // 1 is not a prime
        if (number == 1) isPrime = false;
        // iterate over all numbers which are lower then 'number'
        for (int divisor = 2; divisor < number && isPrime; divisor++) {
            // if we can divide by it, it's not a prime
            if (number % divisor == 0) {
                isPrime = false;
            }
        }
        // print if it's a prime
        if (isPrime) System.out.println(number+", ");
    }
}}

Answer 4

I want to thank everyone advice and tips. And motivation for pushing me. I was able to get the program to work. See below

public class primes {
public static void main (String[] args) {
    final int NUMBER_OF_PRIMES = 26; // Number of primes to display
    final int NUMBER_OF_PRIMES_PER_LINE = 5; // Display 5 numbers per line
    int count = 0; // Count the number of prime numbers
    int number = 1; // A number to be tested for primeness

    System.out.println("The prime numbers between 1 and 100 are \n");

    // Repeatedly find prime numbers
    while (count < NUMBER_OF_PRIMES) {
        // Assume the number is prime
        boolean isPrime = true; // Is the current number prime?

        // Test whether number is prime
        for (int divisor = 2; divisor <= number / 2; divisor++) {
            if (number % divisor == 0) { // If true, number is not prime
                isPrime = false; // Set isPrime to false
                break; // Exit the for loop
            }
        }

        // Display the prime number and increase the count
        if (isPrime) {
            count++; // Increase the count

            if (count % NUMBER_OF_PRIMES_PER_LINE == 0) {
                // Display the number and advance to the new line
                System.out.println(number);
            }
            else
                System.out.print(number + " ");     
            }

        //Check if the next number is prime
        number++;
        }
    }
}