循环中的Java“while”变量

Question

抱歉，如果有人问这个问题（可能很多次），但我找不到答案。我正在学习Java，并编写了一个简单的代码。它需要两个数字，比较并使用If给出一些结果。它运行得很好，但我试着循环它。gram在循环中，而我给出的第一个数值是10。这不会像这样工作，因为“First无法解析为变量”，因为它位于循环中。好吧，我明白了，但有没有办法让这个循环中的变量起作用呢

import java.util.Scanner; 

public class Loop {

    public static void main(String[] args) {
        do {    
            System.out.println("give me 2 numbers: ");
            String FirstNum, SecNum;
            Scanner FirstRead = new Scanner(System.in);
            Scanner SecRead = new Scanner(System.in);
            FirstNum = FirstRead.nextLine();
            SecNum = SecRead.nextLine();
            int First = Integer.parseInt(FirstNum); // changing String to int
            int Second = Integer.parseInt(SecNum); // changing String to int

            // Playing with loop

            if (First == Second) {
                System.out.println("First " + First + " is same as "+ Second);
            }
            else {
                System.out.println("Number " + First + " is different then " + Second);
            }   
        }
        while(First == 10); 
        System.out.print("We're done here, because first number is 10");
    }
}

Answer 1

首先，请使用lowerCamelCase来命名变量。第一个字母总是小写的

不，不能在循环中声明变量并将其用作条件。使用变量时，需要知道定义变量的范围。如果在循环内定义它，它将仅在循环内可用。因此，你唯一能做的就是在循环外定义变量，然后在循环内使用它

int first = 0;

do {
    ...
    first = Integer.parseInt(FirstNum); // changing String to int
    ...
} while (first == 10);

Answer 2

你应该首先声明你的扫描仪在循环之外

import java.util.Scanner; 

public class Loop {
    public static void main(String[] args) {
        int first = 0;
        final Scanner scanner = new Scanner(System.in);
        do {    
            System.out.println("Give me 2 numbers: ");
            final String firstNum = scanner.nextLine();
            final String secNum = scanner.nextLine();
            first = Integer.parseInt(firstNum); // changing String to int
            final int second = Integer.parseInt(secNum); // changing String to int

            if (first == second) {
                System.out.println("First " + first + " is same as "+ second);
            } else {
                System.out.println("Number " + first + " is different then " + second);
            }   
        // Playing with loop
        } while(first != 10); 

        System.out.print("We're done here, because first number is 10");

        // Free resources (should be done in a try/finally clause)
        scanner.close();
    }
}

你的while条款错了。如果在first == 10时循环，这意味着如果“first”不同于10，程序将退出

Answer 3

正如其他人所说，您必须在循环范围之外声明条件变量。您还可以对您的程序应用其他一些改进：

public static void main(final String[] args) {
    // You need only one scanner for both numbers. I t can be reused, so
    // declare it outside the loop.
    final Scanner scanner = new Scanner(System.in);
    // Also the number variables can be reused.
    int first, second;

    do {
        System.out.println("Give me two numbers:");

        // Scan the integers directly without parsing the lines manually.
        first = scanner.nextInt();
        second = scanner.nextInt();

        if (first == second) {
            System.out.println("The first number (" + first + ") is the same as the second (" + second + ").");
        } else {
            System.out.println("The first number (" + first + ") is different from the second (" + second + ").");
        }
    } while (10 != first);

    scanner.close();

    System.out.println("We are done here, because the first number is 10.");
}