如何重新提示以允许用户继续转换值？

Question

我需要将一个数值转换成罗马数字，我遇到了一些麻烦。当我测试我的代码并且用户输入一个数字时，代码会将它转换成一个罗马数字，但是我无法获得提示之后再次出现；在一个不正确的整数放置之后，它会很好地重新提示，而不是在一个正确的整数之后。我不知道如何使用户能够继续转换值，直到他们选择-1退出。下面是我到目前为止想出的。

import java.util.Scanner;

public class arabicToRoman {
static String a = "";

public static void main (String[] args){

    Scanner input = new Scanner(System.in);
    System.out.println("Enter a number between 1 and 3999 (-1 to quit): ");

    while (!input.hasNext("-1")) {
        String a = input.next(); 

    try {
        Integer number = Integer.parseInt(a); 

        if ((number <= 3999) && (number > 0)) {
            System.out.println(arabicToRoman(number)); 

        } else if (number > 3999) {
            System.out.println("Error: number must be between 1 and 3999");

        } else if (number == 0) {
            System.out.println("Error: The Romans did not have a way to represent negative numbers or zero.");

        } else {
            System.out.println("Error: The Romans did not have a way to represent negative numbers or zero.");

        }
    } catch (NumberFormatException e) {
        System.out.println("You did not enter a number!");
    }
}  
}

public static String arabicToRoman(int arabic) {    
    while (arabic >= 1000){
        a += "M";
        arabic -= 1000;
    }
    while (arabic >= 900){
        a += "CM";
        arabic -= 900;
    }
    while (arabic >= 500){
        a += "D";
        arabic -= 500;
    }
    while (arabic >= 400){
        a += "CD";
        arabic -= 400;
    }
    while (arabic >= 100){
        a += "C";
        arabic -= 100;
    }
    while (arabic >= 90){
        a += "XC";
        arabic -= 90;
    }
    while (arabic >= 50){
        a += "L";
        arabic -= 50;
    }
    while (arabic >= 40){
        a += "XL";
        arabic -= 40;
    }
    while (arabic >= 10){
        a += "X";
        arabic -= 10;
    }
    while (arabic >= 9){
        a += "IX";
        arabic -= 10;
    }
    while (arabic >= 5){
        a += "V";
        arabic -= 5;
    }
    while (arabic >= 4){
        a += "IV";
        arabic -= 4;
        }
    while (arabic >= 1){
        a += "I";
        arabic -= 1;
    }
    return a;
}
}

输入输入后，它似乎显示了结果，但之后没有提示，用户可以输入其他内容并读取它；如果是另一个数字，则结果将添加到前面的结果中。

Answer 1

您的代码无法正常工作，因为满足return条件时调用了((number <= 3999) && (number > 0))语句。此return终止循环。

相反，您可能想调用方法arabicToRoman(int arabic)，只需打印出结果。

我还会考虑重组您的代码，使其在sentinel value循环条件下使用while，并编辑代码，以便在给出非数字答案时它不会崩溃。现在编写代码时，这样做将生成一个java.util.InputMismatchException。

Scanner input = new Scanner(System.in);
System.out.println("Enter a number between 1 and 3999 (-1 to quit):");

while (!input.hasNext("-1")) {

    String a = input.next(); // gets the next item from the Scanner

    try {
        Integer number = Integer.parseInt(a); // tries to 'cast' the String to an Integer

        if ((number <= 3999) && (number > 0)) {
            System.out.println(arabicToRoman(number)); // prints out the result

        } else if (number > 3999) {
            System.out.println("Error: number must be between 1 and 3999");

        } else if (number == 0) {
            System.out.println("Error: The Romans did not have a way to represent negative numbers or zero.");

        } else {
            System.out.println("Error: The Romans did not have a way to represent negative numbers or zero.");

        }
    } catch (NumberFormatException e) {
        System.out.println("You did not enter a number!");
    }
}

编辑:修复连接问题

所编写的程序正在将结果存储在一个state variable中，该state variable通过对该结果的调用而持久化。这就是为什么每次调用该方法时，它都会打印附加到前一个结果的当前结果。您希望编辑代码以不将a存储为状态变量，并返回arabicToRoman(int arabic)方法而不更改现有数据。在方法中声明变量将允许在方法终止时将其释放。

为了解决这个问题：

首先，在类中的方法之外删除这一行：

static String a = "";

然后，编辑您的arabicToRoman(int arabic)方法如下所示：

public static String arabicToRoman(int arabic) {
    String a = ""; // note this String declaration at the start of the method

    while (arabic >= 1000) {
        a += "M";
        arabic -= 1000;
    }
    while (arabic >= 900) {
        a += "CM";
        arabic -= 900;
    }
    while (arabic >= 500) {
        a += "D";
        arabic -= 500;
    }
    while (arabic >= 400) {
        a += "CD";
        arabic -= 400;
    }
    while (arabic >= 100) {
        a += "C";
        arabic -= 100;
    }
    while (arabic >= 90) {
        a += "XC";
        arabic -= 90;
    }
    while (arabic >= 50) {
        a += "L";
        arabic -= 50;
    }
    while (arabic >= 40) {
        a += "XL";
        arabic -= 40;
    }
    while (arabic >= 10) {
        a += "X";
        arabic -= 10;
    }
    while (arabic >= 9) {
        a += "IX";
        arabic -= 10;
    }
    while (arabic >= 5) {
        a += "V";
        arabic -= 5;
    }
    while (arabic >= 4) {
        a += "IV";
        arabic -= 4;
    }
    while (arabic >= 1) {
        a += "I";
        arabic -= 1;
    }
    return a;
}

我再次测试这一点，并可以确认它产生了正确的行为，正如您所描述的。

为了清晰起见这就是最终产品的样子。。