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，而它在主要打印行上包括数字 99。我需要更改我的 findPrime() 上的某些内容吗？我尝试使用索引，但变得更糟。

    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);
  }
}

它显示的是这个，而不是在程序的开头有数字 2 并且在程序的最后一行没有数字 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() 方法在 num = 2 处开始循环（将 num+1 附加到输出）。您的循环应该从 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。

你也在循环直到< max - 1。考虑循环直到< max。

Answer 2

我对您的代码进行了一些更改，要指出的重要一点是您不需要 max。您的 findPrimes() 看起来不错，但难以阅读且难以验证正确性。您的 toString() 方法应该遍历每个元素，如果该元素为真，则将其添加到列表中。

1 也不是素数，所以 numbers[1] = false; 应该是这样。 Why is 1 not a prime number?

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);
    }
}