Java do-while循环中正负数计数器统计结果异常问题

使用do-while循环实现正负数输入统计功能，规则为输入0时终止程序，分别统计输入的正数、负数个数，0不计入统计。测试时依次输入4个正数、3个负数后输入0，预期输出正数数量4、负数数量3，实际运行输出正数3个、负数3个，尝试将后置自增运算符替换为前置自增运算符后问题仍存在。

实际运行输入输出记录 #

Enter an integer, the input end if it is 0: 4
Enter an integer, the input end if it is 0: 3
Enter an integer, the input end if it is 0: 2
Enter an integer, the input end if it is 0: 1
Enter an integer, the input end if it is 0: -4
Enter an integer, the input end if it is 0: -3
Enter an integer, the input end if it is 0: -2
Enter an integer, the input end if it is 0: 0
num of postives 3 and num of negatives 3 <----- output wrong

原始问题代码 #

import java.util.Scanner;

public class App {
    public static void main(String[] args) throws Exception {
        /**
         * my pesducode
         * enter an integer, input ends if 0: 
         * while input != 0
         *     if input > 0
         *         postives = postiveCount++
         *     else
         *         negatives = negativeCount++
         *     total+= input
         *  end loop
         * totalNumsEnter = postive + negatives
         * avg = total/ totalNumsEnter
         */
        Scanner input = new Scanner(System.in);
        int num = 0;
        int postiveCount = 0;
        int postives = 0;
        int negatives = 0;
        int negativeCount = 0;
        do{
            System.out.print("Enter an integer, the input end if it is 0: ");
            num = input.nextInt();
            
            if(num > 0){
               postives = postiveCount++;
            }
            else{
                negatives = negativeCount++;
            }
        }while( num != 0);
        System.out.printf("num of postives %d and num of negatives %d", postives,negatives);
        input.close();
    }
}

代码存在两处核心逻辑错误：

• 冗余定义重复计数变量：同时声明了postiveCount/postives、negativeCount/negatives两组功能完全一致的计数变量，多余的跨变量赋值操作直接导致计数传递错误。
• 计数逻辑与边界判断缺失：
  1. 执行postives = postiveCount++时，后置自增运算符会先返回postiveCount的当前值完成赋值，再对postiveCount做自增。最后一次输入正数时，postiveCount自增后的值为4，但赋值给postives的是自增前的3，这是正数统计少1的直接原因。就算替换为前置自增，双变量赋值的逻辑依然会存在计数不同步的问题。
  2. do-while循环是先执行循环体再判断终止条件，输入0时会先进入循环体走到else分支，若不做过滤会错误给负数计数加1，本次测试中负数结果恰好正确属于赋值逻辑掩盖下的巧合。

import java.util.Scanner;

public class App {
    public static void main(String[] args) throws Exception {
        Scanner input = new Scanner(System.in);
        int num;
        int positives = 0;
        int negatives = 0;
        do{
            System.out.print("Enter an integer, the input end if it is 0: ");
            num = input.nextInt();
            // 输入0直接跳过计数
            if(num == 0){
                break;
            }
            if(num > 0){
               positives++;
            } else {
                negatives++;
            }
        }while( num != 0);
        System.out.printf("num of positives %d and num of negatives %d", positives,negatives);
        input.close();
    }
}

修改点说明 #

• 删除冗余的postiveCount、negativeCount变量，直接用输出对应的positives、negatives作为计数器，避免多变量赋值带来的数值不同步问题
• 读取输入后增加0值判断，遇到终止符直接跳过计数逻辑，避免边界值污染统计结果
• 修正原代码中postives的拼写错误为标准命名positives

内容的提问来源于stack exchange，提问作者user18904944