计算素数时没有输出

Question

i want to print first 100 prime numbers. so, i created an array of int 100. I added first prime, bag[0] = 2, then for the following numbers, I tried to write an algorithm. It will start from 3 and goes on until array is full. Every number is decided to be if it is prime by whether it is divisible by the previous elements in array and if it is prime then it will be added to array.

here is my code:

public class Trial02 
{   
    public static void main( String[] args) 
    {       
        int[] bag = new int[100];
        bag[0] = 2; //first element of prime array
        int valid = 1;
        int i;
        boolean result = true;
        String str = "";

        //starting from 3 it checks if a number is prime until array is full
        for( i=3; valid<bag.length; i++)
        {
            //it checks if previous primes in array are divisible by current number until coming to current number
            for(int k=0; k<valid; k++)
            {
                if( i % bag[k] == 0)
                    result = false;
            }

            if( result == true) //if it is prime it is added to array
            {
                bag[valid] = i;
                valid ++;
            }
        }

        //printing results
        for(int m=0; m < bag.length; m++)
            str = str + bag[m] + " ";
        System.out.println("zaa xd");
        System.out.println(str);
    }

}

but it don't give any output, just a blank. I couldn't find where my mistake is. Thanks in advance.

Answer 1

你从来没有真正检查一个数字是否是素数（ result没有设置在任何有用的地方）

Answer 2

It looks like you need to reset result = true; inside the first for loop. Your code as posted sets result = false and then never changes it.

Answer 3

The most obvious error is that your boolean result = true; is outside the loop: once set to false , it never gets set back to true . You do not see any output because your program never stops.

As a side note, you do not need to check all primes all the way to the last one you've discovered: you can stop once you reach the square root of the candidate prime, ie i*i > bag[k] . You are not going to notice any effect when your limit is 100, but if you try 100000, it would help a lot more.

Answer 4

You have a number of logical mistakes in your code.

First you have a for loop that has a finalization case unrelated to its indexer. While this is valid, it makes the code harder to understand.

More importantly, result is only ever set to false, not true, so thus the loop will run forever, as valid is never changed.

Answer 5

Your logic for determining the first 100 prime numbers is incorrect. And number of logical errors are present as indicated by others. I have re-written your code but not tested. I guess it will work:

public class Trial02 
{   
    public static void main( String[] args) 
    {       
    int[] bag = new int[100];
    bag[0] = 2; //first element of prime array
    int valid = 1;
    int i;
    boolean isPrime = true;
    String str = "";

    //starting from 3 it checks if a number is prime until array is full
    for( i=3; valid<bag.length; i++)
    {
        isPrime = true;
        for (int k = 2; k < i; k++)
        {
            if (i % k == 0)
            {
                isPrime = false;
                break;
            }
        }

        if (isPrime == true)
        {
            bag[valid++] = i;
        }
    }

    //printing results
    for(i=0; i < bag.length; i++)
        str = str + bag[i] + " ";
    System.out.println("zaa xd");
    System.out.println(str);
    }

}