java - 有些帮助找出我的埃拉托斯特尼筛出了什么问题

Question

所以...我在实验室遇到了一些问题。我已经这样做了一段时间，但我的程序并没有按照我的预期进行。我目前正在用 Java 编写埃拉托斯特尼筛法程序。不幸的是，它没有给出素数列表的预期输出。我一生都无法弄清楚出了什么问题...这里有人能给我指出我可能在哪里搞砸了吗？

非常感谢!

import java.util.*;
import java.math.*;
public class primeSieve {

/**
 * @param args
 */ 

public static void main(String[] args){
    // TODO Auto-generated method stub
    Scanner kb = new Scanner(System.in);

    //Get user input
    System.out.println("Please enter the first number of the set of numbers to calculate primes (first number MUST be prime):");
    int first = kb.nextInt();

    System.out.println("Please enter the last number of the set of numbers to calculate primes (first number MUST be prime):");
    int last = kb.nextInt();

    List<Integer> primeList = new ArrayList<Integer>();
    List<Integer> numList = new ArrayList<Integer>();

    //Make array with values from 2 to the last value entered by user
    for(int i = 2; i <= last; i++){
        numList.add(i);
    }

    int size = numList.size();

    //Calculate primes
    for(int i = 0; i < size; i++)
    {   
        if(i != 0 && i % 2 == 0){continue;} //No point in checking even numbers
        if(numList.get(i) == 0){continue;}  //If a value has been removed, it's been set to 0. No need to check it.

        int prime = numList.get(i);         //The current number being worked on. Should be prime
        primeList.add(prime);               //The number is prime, put it into the prime list.
        for(int j = prime; j < size; j += prime) //Loop to remove multiples
        {
            numList.set(j, 0);  //Number is a multiple of the prime previously calculated. Set it to 0, not a prime.
        }
    }
    int primeSize = primeList.size();   
    System.out.println(primeSize);
    System.out.println("The prime numbers from " + first + " to " + last + " are:");
    for(int i = first; i < primeSize; i++)
    {
        System.out.println(primeList.get(i));
    }
}       

}

编辑:当我查找 2 到 50 之间的素数时，程序的输出是这样的:

从 2 到 50 的素数是:713192531394349

Answer 1

在嵌套的 for 循环中，您从 prime 处开始 j。但是，考虑素数 2:它位于索引 0，而不是索引 2。如果您在 prime-first 处启动 j，事情应该会成功。另外，正如评论者指出的那样，您的打印循环应该从索引 0 开始，而不是 first。