Here is the code

import java.util.Scanner;

public class oneDimensionArray {
    public static void main (String[] args) {
        finding(2,100);
    }
    public static void finding(int start, int end) {
        int[] primeNumbers = new int[end];

        for (int n = start; n < end; n  ) {
            boolean check = true;
            int tryNum = 2;
            while (tryNum <= n / 2) {
                if (n % tryNum == 0) {
                    check = false;
                    break;
                }
                tryNum  = 1;
            }
            if (check) {
                primeNumbers[n] = n;
            }
        }

        for (int i : primeNumbers) {
            System.out.print(i   ", ");
        }
    }
}

The result successfully shows all prime numbers, but there are a lot of zeros substituting the non-prime numbers, just like this: 0, 0, 2, 3, 0, 5, 0, 7, 0, 0, 0, 11, 0, 13, 0, 0, 0, 17, 0, 19, 0, 0, 0, 23, 0, 0, 0, 0, 0, 29, 0, 31, 0, 0, 0, 0, 0, 37, 0, 0, 0, 41, 0, 43

I wonder where is the problem with my code, and how everything is caused.

CodePudding user response：

Why you take new array it's allocate memory space...

public class MyClass {
   public static void main (String[] args) {
        finding(100);
    }
    
    public static void finding(int end) {
    int count = 0;
    for (int n = 2; n < end; n  ) {
        count = 0;
        int tryNum = 1;
        while (tryNum <= n) {
            if (n % tryNum == 0) {
                count  ;
            }
            tryNum  ;
        }
        
        if(count==2)
           System.out.print(n "  ");     
        }
    }
}

CodePudding user response：

You should add a variable to store the number of primes found and do this.

public static void finding(int start, int end) {
    int[] primeNumbers = new int[end];

    int count = 0;
    for (int n = start; n < end; n  ) {
        boolean check = true;
        int tryNum = 2;
        while (tryNum <= n / 2) {
            if (n % tryNum == 0) {
                check = false;
                break;
            }
            tryNum  = 1;
        }
        if (check) {
            primeNumbers[count  ] = n;
        }
    }

    for (int i = 0; i < count;   i) {
        System.out.print(primeNumbers[i]   ", ");
    }
}

output:

2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 

CodePudding user response：

In java, every cell in an int array is initialled to 0, so when you print the array the output is indeed, bunch of zeros. to fix it, you can use a simple if statement to check of the value of a cell is not 0 like that:

for (int i : primeNumbers) {
   if (i != 0) {
     System.out.println(i   ", ");
   }
}