Question

我是java的新手，我正在制作一个简单的计算器，但如果其中一个选项（add，sub，mul或div）没有被输入，我希望代码能够回到起点。如果正确的方法是循环语句，我将如何改变我的代码？

import java.util.Scanner;
public class Calc {

    public static void main(String[] args) {

    Scanner math = new Scanner(System.in);
     double fnum, snum, answer;
     System.out.println("Enter add for addition");
     System.out.println("Enter sub for subtraction");
     System.out.println("Enter mul for multiplication");
     System.out.println("Enter div for division");
        String txt = math.nextLine();
        switch (txt) {
        case "add":
        // Code for addition
            break;
        case "sub":
            //code for subtraction
            break;
        case "mul":
        // Code for multiplication.
           break;
        case "div":
            //just code for division
            break;
        default:
            System.err.println("Please enter a valid option");
        // How do i make it go back to the beginning of the switch
    }
    }   
}

Answer 1

您可以使用do{}while(<condition>)例如：

boolean check = false;
do {
    String txt = math.nextLine();
    switch (txt) {
        case "add":
            System.out.println("You selected ADDITION");
            System.out.println("Enter first number");
            fnum = math.nextDouble();
            System.out.println("Enter second number");
            snum = math.nextDouble();
            answer = fnum + snum;
            System.out.println(answer);
            check = true;//change your boolean to true
            break;
        case "sub":
            System.out.println("You selected SUBTRACTION");
            System.out.println("Enter first number");
            fnum = math.nextDouble();
            System.out.println("Enter second number");
            snum = math.nextDouble();
            answer = fnum - snum;
            System.out.println(answer);
            check = true;//change your boolean to true
            break;
        case "mul":
            System.out.println("You selected MULTIPLICATION");
            System.out.println("Enter first number");
            fnum = math.nextDouble();
            System.out.println("Enter second number");
            snum = math.nextDouble();
            answer = fnum * snum;
            System.out.println(answer);
            check = true;//change your boolean to true
            break;
        case "div":
            System.out.println("You selected DIVISION");
            System.out.println("Enter first number");
            fnum = math.nextDouble();
            System.out.println("Enter second number");
            snum = math.nextDouble();
            answer = fnum / snum;
            System.out.println(answer);
            check = true;//change your boolean to true
            break;
        default:
            System.err.println("Please enter a valid option");
    }
} while (!check);

Answer 2

如果输入有效，您可以将其包装到do while循环和break中，例如：

boolean valid = true;
do{
    String txt = math.nextLine();
    switch (txt) {
    case "add":
        System.out.println("You selected ADDITION");
        System.out.println("Enter first number");
        fnum = math.nextDouble();
        System.out.println("Enter second number");
        snum = math.nextDouble();
        answer = fnum + snum;
        System.out.println(answer);
        break;
    case "sub":
        System.out.println("You selected SUBTRACTION");
        System.out.println("Enter first number");
        fnum = math.nextDouble();
        System.out.println("Enter second number");
        snum = math.nextDouble();
        answer = fnum - snum;
        System.out.println(answer);
        break;
    case "mul":
        System.out.println("You selected MULTIPLICATION");
        System.out.println("Enter first number");
        fnum = math.nextDouble();
        System.out.println("Enter second number");
        snum = math.nextDouble();
        answer = fnum * snum;
        System.out.println(answer);
        break;
    case "div":
        System.out.println("You selected DIVISION");
        System.out.println("Enter first number");
        fnum = math.nextDouble();
        System.out.println("Enter second number");
        snum = math.nextDouble();
        answer = fnum / snum;
        System.out.println(answer);
        break;
    default:
        System.err.println("Please enter a valid option");
        valid = false;
    }
}while(!valid);

Answer 3

据我所知，你想循环你的情况，直到用户给出＆＃34; sum，div，sub，mul＆＃34; 你可以用do {} while（）;。

来做

    do{

        Input of user

        Case 1 answer=true;

        Case 2 answer=true;

        Default answer=false;

    }while(!answer);

Answer 4

图案：

    boolean condition = true;
    while(condition){

        //Input of user
        switch(input){
            Case 1:
                //somecode
                break;
            Case 1:
                //somecode
                break;
            case "exit":
                condition = false;
                break;
            default:
                //wrong input
                break;
        }
    }

工作代码：

package com.example;

    import java.util.Scanner;

    public class MyClass {
        public static void main(String... args) {
            Scanner math = new Scanner(System.in);
            double fnum, snum, answer;
            boolean exit = true;
            while (exit) {
                System.out.println("Enter add for addition");
                System.out.println("Enter sub for subtraction");
                System.out.println("Enter mul for multiplication");
                System.out.println("Enter div for division");
                System.out.println("Enter ext for exit");

                String txt = math.nextLine();
                switch (txt) {
                    case "add":
                        System.out.println("You selected ADDITION");
                        System.out.println("Enter first number");
                        fnum = math.nextDouble();
                        System.out.println("Enter second number");
                        snum = math.nextDouble();
                        answer = fnum + snum;
                        System.out.println(answer);
                        break;
                    case "sub":
                        System.out.println("You selected SUBTRACTION");
                        System.out.println("Enter first number");
                        fnum = math.nextDouble();
                        System.out.println("Enter second number");
                        snum = math.nextDouble();
                        answer = fnum - snum;
                        System.out.println(answer);
                        break;
                    case "mul":
                        System.out.println("You selected MULTIPLICATION");
                        System.out.println("Enter first number");
                        fnum = math.nextDouble();
                        System.out.println("Enter second number");
                        snum = math.nextDouble();
                        answer = fnum * snum;
                        System.out.println(answer);
                        break;
                    case "div":
                        System.out.println("You selected DIVISION");
                        System.out.println("Enter first number");
                        fnum = math.nextDouble();
                        System.out.println("Enter second number");
                        snum = math.nextDouble();
                        answer = fnum / snum;
                        System.out.println(answer);
                        break;
                    case "ext":
                        exit = false;
                        break;

                    default:
                        System.err.println("Please enter a valid option");
                }
            }

        }
    }

需要帮助重置switch语句

4 个答案: