Java 需要来自两个独立输入的标记值

Question

我刚刚遇到哨兵控制的迭代，我遇到了一些困难。我必须制作一个程序，从用户那里获取使用的里程数和加仑数，计算总英里数。我试图在用户完成时使用 -1 作为标记值退出循环，但是当我输入该值时，Java 不会终止程序。相反，它要求加仑的另一个值。我如何让它接受来自第一个输入的标记值？

示例：

输入英里或 -1 退出

-1

输入加仑

-1

终止

import java.util.Scanner;

public class Activity3_17 {

    public static void main(String[] args) {

        Scanner input = new Scanner(System.in);
        // processing phase
        int miles = 1;
        int gallons = 1;
        int totalMiles = 0;
        int totalGallons = 0;
        float mpg = 0;


        System.out.println("Enter miles or -1 to exit");
        miles = input.nextInt();

        System.out.println("Enter gallons");
        gallons = input.nextInt();

        while (miles != -1) {
            System.out.println("Enter miles or -1 to exit");
            miles = input.nextInt();

            System.out.println("Enter gallons or -1 to exit");
            gallons = input.nextInt();

            totalMiles = totalMiles + miles;
            totalGallons = totalGallons + gallons;


        }
        if (miles == -1) {
            System.out.print("Terminate");
        }
        else{
            mpg = (float) totalMiles / totalGallons;
            System.out.println(mpg);
            }

    }
}

Answer 1

Java 仅在每次完成时评估 while 循环。因此，您需要手动检查 miles 是否为 -1 并打破循环。

while (miles != -1) {
    System.out.println("Enter miles or -1 to exit");
    miles = input.nextInt();

    if (miles == -1) break;         

    System.out.println("Enter gallons or -1 to exit");
    gallons = input.nextInt();

    totalMiles = totalMiles + miles;
    totalGallons = totalGallons + gallons;
}

Answer 2

只需在 while 循环中添加 2 个附加条件（如果有 break）即可立即退出 while

public static void main(String[] args) {

    Scanner input = new Scanner(System.in);
    // processing phase
    int miles = 1;
    int gallons = 1;
    int totalMiles = 0;
    int totalGallons = 0;
    float mpg = 0;


    System.out.println("Enter miles or -1 to exit");
    miles = input.nextInt();

    System.out.println("Enter gallons");
    gallons = input.nextInt();

    while (miles != -1) {
        System.out.println("Enter miles or -1 to exit");
        miles = input.nextInt();
        if(miles == -1) break;
        System.out.println("Enter gallons or -1 to exit");
        gallons = input.nextInt();
       if(gallons == -1) break;

        totalMiles = totalMiles + miles;
        totalGallons = totalGallons + gallons;


    }
    if (miles == -1) {
        System.out.print("Terminate");
    }
    else{
        mpg = (float) totalMiles / totalGallons;
        System.out.println(mpg);
        }

}
}

Answer 3

您需要检查用户是否在 while 循环中输入了 -1。如果他们这样做了，请使用 break 退出循环，然后终止程序。

mpg 仍在循环中打印，但仅在进行检查之后。这确保了用户提供了有效的输入。

我决定将循环条件设为 true，因为如果 miles 或 gallons 为 -1，循环就会中断。

public static void main(String[] args) {
    Scanner input = new Scanner(System.in);

    int miles = 1;
    int gallons = 1;
    int totalMiles = 0;
    int totalGallons = 0;
    float mpg = 0;

    while (true) {
        System.out.println("Enter miles or -1 to exit");
        miles = input.nextInt();
        if (miles == -1) break;

        System.out.println("Enter gallons or -1 to exit");
        gallons = input.nextInt();
        if (gallons == -1) break;

        totalMiles = totalMiles + miles;
        totalGallons = totalGallons + gallons;
        mpg = (float) totalMiles / totalGallons;

        System.out.println(mpg);
    }
    input.close();
    System.out.print("Terminate");
}

Answer 4

谢谢大家的帮助。这是完成的工作代码：

public static void main(String[] args) {

    Scanner input = new Scanner(System.in);

    int miles = 0;
    int gallons = 0;
    int totalMiles = 0;
    int totalGallon = 0;
    double mpg = 0;
    double totalMpg = 0;

    while(miles != -1){
        System.out.println("Enter mileage or -1 to exit: ");
        miles = input.nextInt();
        if(miles == -1){
            break;
        }
        System.out.println("Enter gallons or -1 to exit: ");
        gallons = input.nextInt();
        mpg = (double) miles / gallons;
        System.out.printf("MPG: %.4f%n", mpg);
        if(gallons == -1){
            break;
        }
        totalMiles = totalMiles + miles;
        totalGallon = totalGallon + gallons;

    }
    if (miles == -1){
        totalMpg = (double) totalMiles / totalGallon;
        System.out.printf("Total used is %.4f%n", totalMpg);
    }


}

Java 需要来自两个独立输入的标记值

Java is requiring sentinel value from two separate inputs

java

java.util.scanner

sentinel