需要帮助找出我的代码中的错误

Question

所以我需要帮助弄清楚为什么我的代码不包括数字 2 而它在主要印刷行上包括数字 99。我需要更改 findPrime() 上的某些内容吗？我尝试使用索引，结果变得更糟。    class Sieve {    private int max;    private boolean[] numbers;    public Sieve(int max) {        if (max < 2) {            throw new IllegalArgumentException();        }        this.max = max;        numbers = new boolean[max];        numbers[0] = false;        numbers[1] = false;        numbers[2] = true;        for (int i = 2; i < max-1; i++) {        numbers[i] = true;        }    }    public void findPrimes() {        for (int num = 2; num < max-1; num++) {            int multiples = num + num;            while (multiples < max-1) {                numbers[multiples-1] = false;                multiples += num;            }        }    }    @Override    public String toString() {        StringBuilder builder = new StringBuilder();        for (int num = 2; num < max; num++) {            if (numbers[num]) {                builder.append(num+1).append(" ");            }        }        return builder.toString();        }    }class Driver{//  MAIN. Find some primes.  public static void main(String [] args)  {    Sieve sieve = null;  //  We must initialize SIEVE or Java will cry.//  5 points. This must print "Sieve size must be at least 2." but without the//  quotes.    try    {      sieve = new Sieve(0);    }    catch (IllegalArgumentException oops)    {      System.out.println("Sieve size must be at least 2.");    }//  5 points. This must print nothing.    try    {      sieve = new Sieve(100);    }    catch (IllegalArgumentException oops)    {      System.out.println("Sieve size must be at least 2.");    }//  10 points. This must print integers from 2 to 99, separated by blanks.    System.out.println(sieve);//  10 points. This must print the prime numbers between 2 and 99, separated by//  blanks. They are:////  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    sieve.findPrimes();    System.out.println(sieve);  }}

Answer 1

您的toString()方法从 num = 2 开始循环（将 num+1 附加到输出）。你的循环应该从 1 开始。

public String toString() {

    StringBuilder builder = new StringBuilder();

    for (int num = 1; num < max; num++) { . // used to start at 2

        if (numbers[num]) {

            builder.append(num+1).append(" ");

        }

    }

    return builder.toString();

    }

}

加上你的代码集numbers[1] = false。那应该是numbers[1] = true。

您也在循环直到< max - 1. 考虑循环直到< max。

Answer 2

我对您的代码进行了一些更改，要指出的是您不需要 max。您的 findPrimes() 看起来不错，但难以阅读且难以验证其正确性。您的 toString() 方法应该迭代每个元素，如果该元素为真，则将其附加到列表中。

1 也不是素数，所以numbers[1] = false;它应该是。为什么1不是质数？

class Sieve {

    // private int max; // max is superfluous use numbers.length instead

    private boolean[] numbers;

    public Sieve(int max) {

        if (max < 2) {

            throw new IllegalArgumentException();

        }

        numbers = new boolean[max];

        numbers[0] = false;

        numbers[1] = false;

        numbers[2] = true;

        for (int i = 2; i <numbers.length; i++) {

            numbers[i] = true;

        }

    }

    public void findPrimes() {

        // a bit more compact and neater in my opinion

        for (int num=2; num<numbers.length; num++) {

            for (int multiple=num+num; multiple<numbers.length; multiple+=num) {

                numbers[multiple] = false;

            }

        }

    }

    @Override

    public String toString() {

        StringBuilder builder = new StringBuilder();

        // you should be iterating from 0 to 99

        for (int i=0; i<numbers.length; i++) {

            if (numbers[i]) {

                builder.append(i).append(" ");

            }

        }

        return builder.toString();

    }

}

class Driver

{

    //  MAIN. Find some primes.

    public static void main(String [] args)

    {

        Sieve sieve = null;  //  We must initialize SIEVE or Java will cry.

        //  5 points. This must print "Sieve size must be at least 2." but without the

        //  quotes.

        try

        {

            sieve = new Sieve(0);

        }

        catch (IllegalArgumentException oops)

        {

            System.out.println("Sieve size must be at least 2.");

        }

        //  5 points. This must print nothing.

        try

        {

            sieve = new Sieve(100);

        }

        catch (IllegalArgumentException oops)

        {

            System.out.println("Sieve size must be at least 2.");

        }

        //  10 points. This must print integers from 2 to 99, separated by blanks.

        System.out.println(sieve);

        //  10 points. This must print the prime numbers between 2 and 99, separated by

        //  blanks. They are:

        //

        //  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

        sieve.findPrimes();

        System.out.println(sieve);

    }

}