为什么正数 return 的增量为负数？ (Java)

Question

我有一个列出素数的程序。我从一个正整数开始，它返回一个包含 negative 数字（不应该这样做）的数组（应该这样做）。问题是代码中的任何地方都没有递减运算符，只有递增运算符和模数函数。这是完整的代码和输出供您参考，完整注释以便于理解：

package calculations;

import java.util.ArrayList;

public class PrimeNumbers {
 public static void main(String[] args) {
    
     //Declaring variables to be used...
     ArrayList<Integer> listOfPrimes = new ArrayList<Integer>();
     listOfPrimes.add(2);
     int numOfPrimesToList = 10;
     int possiblePrimeCounter  = 0;
     
     /* We start testing at the number 2 and increment until 
      * the list of primes we have listed is equal to the
      * number of primes want to list. 
      */
     for(int atNum = 2; listOfPrimes.size() < numOfPrimesToList; atNum++) {
        
         /* While the index of the number to divide from 
          * is less than the length of our list, we do 
          * the following:
          */
         for(int intIndexToCheck = 0; intIndexToCheck < listOfPrimes.size(); intIndexToCheck++) {
            
             /* If the modulus of the number we are testing 
              * and the one are dividing by is greater than
              * zero, we increment its possibility of being
              * a prime by one. 
              */
             if(atNum % listOfPrimes.get(intIndexToCheck) < 0) {
                 possiblePrimeCounter++;
                
                 /* If its chance of being a prime is 1 (that is, the
                  * times it has been given a chance is equal to the
                  * total number of possible chances it is given), then
                  * we can consider it to be a prime.
                  */
                 if(possiblePrimeCounter == listOfPrimes.size()) {
                     //We then add this number to the list of primes.
                     listOfPrimes.add(atNum);
                    
                     possiblePrimeCounter = 0;
                 }
             }
         }
     }
     //Finally, we print out the list of primes.
     System.out.println(listOfPrimes);
 }
}

Returns:

[2, -2147483647, -2147483645, -2147483644, -2147483643, -2147483641, -2147483640, -2147483639, -2147483638, -2147483637]

Answer 1

发生的事情称为整数溢出。当 int 成为最大值 (Integer.MAX_VALUE) 时，如果您尝试递增，它将返回到它的最小值 (Integer.MIN_VALUE)，这就是您得到的数字。

Answer 2

问题是你的代码没有按照你的评论所说的那样做。你有

         /* If the modulus of the number we are testing 
          * and the one are dividing by is greater than
          * zero, we increment its possibility of being
          * a prime by one. 
          */
         if(atNum % listOfPrimes.get(intIndexToCheck) < 0) {

它说您检查大于是否为零，但实际上它检查的是小于零。更改行以实际执行评论所说的

         if(atNum % listOfPrimes.get(intIndexToCheck) > 0) {

由于您正在测试小于零的余数，而这对于正数永远不会发生，因此您不断增加数字，直到它溢出并变为负数，此时余数确实变得小于零。

对于您的后续问题，如果您的数字不是素数，则 possiblePrimeCounter 不会重置，因此 possiblePrimeCounter 会继续测试下一个数字。此外，在添加一个素数之后，您还需要进行另一项测试并检查它是否可以被自身整除（因为您刚刚增加了列表的大小）。

一个简单的解决方法是

     possiblePrimeCounter = 0; // Need to reset before trying each new number
     for(int intIndexToCheck = 0; intIndexToCheck < listOfPrimes.size(); intIndexToCheck++) {

或者使循环更高效，一旦我们知道数字不是素数就没有必要继续测试。

     possiblePrimeCounter = 0;

     for(int intIndexToCheck = 0; intIndexToCheck < listOfPrimes.size(); intIndexToCheck++) {
         if(atNum % listOfPrimes.get(intIndexToCheck) !=0) {
             possiblePrimeCounter++;
             if(possiblePrimeCounter == listOfPrimes.size()) {
                 //We then add this number to the list of primes.
                 listOfPrimes.add(atNum);
                
                 possiblePrimeCounter = 0;
             }
         } else {
             break; // Not prime
         }
     }

为什么正数 return 的增量为负数？ (Java)

Why does an increment by positive one return a negative? (Java)

java

increment

primes

modulus