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在原始印刷行上不包括数字2并包括数字99。 我是否需要更改find​​Prime（）上的内容？ 我尝试使用索引，但情况变得更糟。

    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);
  }
}

它正在显示此内容，而不是在程序的开头没有数字2，而在最后一行没有数字99。

Sieve size must be at least 2.
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 
3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97 99

Answer 1

您的toString()方法从num = 2（将num + 1附加到输出）开始循环。 您的循环应从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();
    }
}

另外，您的代码设置了numbers[1] = false 。 那应该是numbers[1] = true 。

您还将循环播放，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，需要指出的主要问题是您不需要max。 您的findPrimes（）看起来不错，但很难阅读且很难验证其正确性。 您的toString（）方法应该遍历每个元素，如果该元素为true，则将其添加到列表中。

另外1不是质数，因此numbers[1] = false; 是应该的。 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);
    }
}