Question

很抱歉，如果问过这个问题（可能很多次），但我找不到答案。我正在学习Java，我已经编写了一个简单的代码。它要求2个数字，比较它并使用If来给出一些结果。它运行良好，但我尝试循环它。当我将第一个数字值设为10时，循环是循环的。这不会像这样工作，因为“首先不能解析为变量”因为它在循环内部。好的我明白了，但是有什么办法可以让我在内部循环中使用变量吗？

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

但是有什么办法可以让我在内部循环中使用变量吗？

没有。你必须在循环之外声明它。

您也可以声明一个布尔变量，并使用它{}为First。类似的东西：

public static void main(String[] args) {
    boolean check = false;
    do {    
        [...]
        check = First == 10;
    }

    while(check); 

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

但是正如你所看到的，你需要在循环之外声明它。

Answer 2

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

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.");
}

Answer 3

首先，请使用lowerCamelCase命名变量。第一个字母总是小写。

不，你不能在循环中声明一个变量并将其用作条件。使用变量时，需要了解定义它的范围。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，然后在循环中使用它。

int first = 0;

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

Answer 4

不，你必须在循环之外声明变量

你可以这样做

public class Loop {
public static void main(String[] args) {
    int first = 0; //declare first outside of the loop
    do {    
        //...
        first = Integer.parseInt(FirstNum); // parse int to string
        //....
    }
    while(first == 10); 

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

Answer 5

您应首先声明您的扫描程序在循环之外。

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。如果您在first == 10期间循环，则表示如果＆＃34;首先＆＃34;您的程序将退出不同于10。

Answer 6

注意：在ANY循环内声明一个变量（for do，while do-while）是一种非常糟糕的编程习惯，应该避免使用。

但是对于你的问题： but is there any way I can make this work with variable inside loop?

如果符合您的要求，您可以使用下面的break语句。

public static void main(String[] args) {
 do {    
      int First = Integer.parseInt(FirstNum); // changing String to int

      if (First == 10)
      {  break; }

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

最好使用while循环，如下所示：

   while (true)
   {    
      int First = Integer.parseInt(FirstNum); // changing String to int

      if (First == 10)
      {  break; }

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

参考： Break DO While Loop Java?

Answer 7

解决这个问题，不推荐。正如你发布的那样 但是有什么方法可以让我在内部循环中使用变量吗？

import java.util.Scanner;

class Ideone
{
    public static void main (String[] args) throws java.lang.Exception
    {
        Scanner sc = new Scanner(System.in);
        do {
            System.out.println("Enter first no. ");
            int first = sc.nextInt();
            System.out.println("Enter second no. ");
            int second = sc.nextInt();
            if(first == second) {
                System.out.printf("First and second both are equal i.e: %d\n", first);
            } else {
                System.out.printf("First: %d and second: %d both are different\n", first, second);
            }
            if(10 == first) {
                break;
            }
        } while(true);
        System.out.println("Done, since value of first is 10");
    }
}

Java“while” - 循环内部的变量

7 个答案: