如何正确使用goto语句 [英] How to use goto statement correctly

问题描述

我正在上高中AP计算机科学课。

我决定把一个 goto 语句扔进我们的一个实验室，只是为了玩，但我得到了这个错误。

 线程main中的异常java.lang.Error：未解决的编译问题：
令牌上的语法错误goto ，断言预期
重启无法解析为变量
在Chapter_3.Lab03_Chapter3.Factorial.main（Factorial.java:28）
  

我在Stackoverflow上找到了一个 goto 的问题，以了解如何正确地做到这一点，并且我完全按照在其中一个答案中。我真的不明白为什么编译器想要一个断言语句（至少这是我想的那样），我也不知道如何使用断言。它似乎想要重新启动部分 goto restart; 作为变量，但重启只是一个标签，将程序拉回到第10行，以便用户可以输入有效的 int 。如果它想重新启动变量，我该怎么做？

  import java.util。*; 
 
公共类Factorial 
 {
 public static void main（String [] args）
 {
 int x = 1; 
 int factValue = 1; 
 Scanner userInput = new Scanner（System.in）; 
 restart：
 System.out.println（请输入非零，非负值以进行析因化。）; 
 int factInput = userInput.nextInt（）; 
 
 while（factInput< = 0）
 {
 System.out.println（输入非零，非负值的因子化。）; 
 factInput = userInput.nextInt（）; 
} 
 
 if（x< 1）//这是执行上述while循环的另一种方式，我只是想玩得开心。 
 {
 System.out.println（您输入的号码无效。请再试一次。）; 
 goto restart; 
} 
 while（x< = factInput）
 {
 factValue * = x; 
 x ++; 
} 
 System.out.println（factInput +！=+ factValue）; 
 userInput.close（）; 
} 
} 
  

解决方案

As所有答案已经指出 goto - Java 中的保留字，并且未在该语言中使用。

restart：被称为标识符后跟冒号。

<如果您希望实现类似的行为，那么您需要注意以下几点：

  outer：//应该放在循环之前
 loopingConstructOne {//我们可以在外部之前有语句但不在标签和循环之间的语句
 inner：
 loopingConstructTwo {
继续; //这将循环到loopingConstructTwo的顶部并继续。 
休息; //这会打破循环的构造2。 
继续外围; //这将转到外部标签并重新进入loopingConstructOne。 
突破外部; //这会打破循环的构造方法。 
继续内在; //这将表现得类似于继续。 
打破内心; //这将与break类似
} 
} 
  

我不确定是否应该说类似，因为我已经拥有。

I am taking my high school AP Computer Science class.

I decided to throw a goto statement into a one of our labs just to play around, but I got this error.

Exception in thread "main" java.lang.Error: Unresolved compilation problems: 
    Syntax error on token "goto", assert expected
    restart cannot be resolved to a variable
at Chapter_3.Lab03_Chapter3.Factorial.main(Factorial.java:28)

I went to a goto question on Stackoverflow to find out how to do it properly, and I did exactly as was demonstrated in one of the answers. I really don't understand why the compiler wants an assert statement (at least that's what I assume it wants), nor do I have any idea how to use assert. It seems to want the restart part of goto restart; to be a variable, but restart is just a label that pulls the program back up to line 10 so that the user can enter a valid int. If it wants restart to be a variable, how do I do that?

import java.util.*;

public class Factorial 
{
    public static void main(String[] args) 
    {
        int x = 1;
        int factValue = 1;
        Scanner userInput = new Scanner(System.in);
        restart:
        System.out.println("Please enter a nonzero, nonnegative value to be factorialized.");
        int factInput = userInput.nextInt();

        while(factInput<=0)
        {
            System.out.println("Enter a nonzero, nonnegative value to be factorialized.");
            factInput = userInput.nextInt();
        }

        if(x<1)//This is another way of doing what the above while loop does, I just wanted to have some fun.
        {
            System.out.println("The number you entered is not valid. Please try again.");
            goto restart;
        }
        while(x<=factInput)
        {
            factValue*=x;
            x++;
        }
        System.out.println(factInput+"! = "+factValue);
        userInput.close();
    }
}

解决方案

As already pointed out by all the answers goto - a reserved word in Java and is not used in the language.

restart: is called an identifier followed by a colon.

Here are a few things you need to take care of if you wish to achieve similar behavior -

outer:                  // Should be placed exactly before the loop
loopingConstructOne  {  // We can have statements before the outer but not inbetween the label and the loop          
    inner:
    loopingConstructTwo {
        continue;       // This goes to the top of loopingConstructTwo and continue.
        break;          // This breaks out of loopingConstructTwo.
        continue outer; // This goes to the outer label and reenters loopingConstructOne.
        break outer;    // This breaks out of the loopingConstructOne.
        continue inner; // This will behave similar to continue.
        break inner;    // This will behave similar to break.
    }
}

I'm not sure of whether should I say similar as I already have.

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