Question

所以......我在实验室里遇到了一些问题。我已经有一段时间了，但我的计划并没有做我期望它做的事情。我目前正在用Java编写一个Sieve of Eratosthenes程序。不幸的是，它没有给出素数列表的预期输出。我不能，为了我的生活，弄清楚出了什么问题......这里的任何人都可以给我一个指针，指出我可能搞砸了什么吗？

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：

从2到50的素数是： 7 13 19 25 31 39 43 49

Answer 1

在嵌套的for循环中，您可以j开始prime。但是，考虑素数2：它位于索引0，而不是索引2.如果你在j开始prime-first，事情就应该解决了。另外，正如评论者指出的那样，您的打印循环应该从索引0而不是first开始。

有些人帮助弄清楚我的Eratosthenes筛子出了什么问题

1 个答案: