Question

我将所有代码放入一个 do-while 语句中，这样它就会一直循环，直到用户输入 3 退出。出于某种原因，当我关闭 do 语句时，它看不到变量 userInput。我试过改变很多东西，但没有一个奏效。所以问题出在 while (userInput !=3); 它抛出 userInput cannot be resolved to a variable

import java.util.Scanner;
public class Lab4 {

    public static void main(String[] args) {
    
    

do {
        Scanner in = new Scanner (System.in);
        
        System.out.print("Please choose your choice from the following menu");
        
        System.out.print("\n1) Print through all integer numbers between two given integers");
        System.out.print("\n2) Display a right triangular pattern of stars");
        System.out.println("\n3) Quit");

        int userInput = in.nextInt();
     
        if (userInput == 1) {
                System.out.print("Enter the start number: ");
                int firstInteger = in.nextInt();
                System.out.print("Enter the second number: ");
                int secondInteger = in.nextInt();
            
        while (firstInteger <= secondInteger) {
                System.out.print(firstInteger + " ");
                firstInteger++;
                
                
            }
                    
        }else if (userInput == 2) {
                System.out.print("Enter the height: ");
                int triangleHeight = in.nextInt();
                
            }
            
          }  while (userInput != 3); 
            
        }
     
      }

Answer 1

userInput 必须在循环外声明，while 条件才能看到它。

试试这个：

import java.util.Scanner;

public class Lab4 {

    public static void main(String[] args) {
        
        int userInput;

        do {
            Scanner in = new Scanner(System.in);

            System.out.print("Please choose your choice from the following menu");

            System.out.print("\n1) Print through all integer numbers between two given integers");
            System.out.print("\n2) Display a right triangular pattern of stars");
            System.out.println("\n3) Quit");

            userInput = in.nextInt();

            if (userInput == 1) {
                System.out.print("Enter the start number: ");
                int firstInteger = in.nextInt();
                System.out.print("Enter the second number: ");
                int secondInteger = in.nextInt();

                while (firstInteger <= secondInteger) {
                    System.out.print(firstInteger + " ");
                    firstInteger++;

                }

            } else if (userInput == 2) {
                System.out.print("Enter the height: ");
                int triangleHeight = in.nextInt();

            }

        } while (userInput != 3);

    }

}

Answer 2

userInput 定义在循环块内部，因此不能从外部访问它，包括循环的条件。一种解决方案是将声明移出其中：

int userInput = -1;
do {
    // code...
    userInput = in.nextInt();
    // more code...
} while (userInput != 3);

Java do-while 循环看不到结束语句

2 个答案: