虽然循环不会终止？

Question

我想知道为什么这个 while 循环不允许我的程序终止？

据我了解（尽管我很可能是错的）条件while (cin >> line) 检查我的输入流是否有一个字符串，然后运行我的循环，直到在输入中找不到其他字符串。然而，在测试我的代码后，我得到了正确的输出，但我的循环从未终止任何关于为什么的想法？

#include <cstdlib>
#include <iostream>
#include <cctype>

using namespace std;

int main() {

string roman_digits[] = {"", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX"};
string roman_tens  [] = {"", "X", "XX", "XXX", "XL", "L", "LX", "LXX", "LXXX", "XC"};
string roman_hundreds [] = {"", "C", "CC", "CCC", "CD", "D", "DC", "DCC", "DCCC", "CM"};
string roman_thousands [] = {"", "M","MM", "MMM"};
string line;
char c;


cout << "Type in a Roman numeral: ";

// Loops through inputted Roman Numerals.    
while (cin >> line){
    int i = 0;

    // Loops through a Roman numeral and changes it to uppercase.
    while(line[i]){
        c = line[i];
        c = (toupper(c));
        line[i] = c;
        i++;
    }


// Loops through checking roman numeral with the thousands array and if there is a match prints out the equivalent arabic number.
    for (int i = 0; i < 10; i++){
       if (roman_thousands[i] == line){
           cout << "The Arabic equivalent of " 
                << line <<" is: " << i << 0 << 0 << 0 << endl;
        }
    }
 // Loops through checking roman numeral with the hundreds array and if there is a match prints out the equivalent arabic number.
    for (int i = 0; i < 10; i++){
        if (roman_hundreds[i] == line){
            cout << "The Arabic equivalent of " << line << " is: " << i << 0 << 0 << endl;
        }
    }
     // Loops through checking roman numeral with the tens array and if there is a match prints out the equivalent arabic number.
    for (int i = 0; i < 10; i++){
        if (roman_tens[i] == line){
            cout << "The Arabic equivalent of " << line << " is: " << i << 0 << endl;
        }
    }
     // Loops through checking roman numeral with the digits array and if there is a match prints out the equivalent arabic number.
    for (int i = 0; i < 10; i++){
        if (roman_digits[i] == line){
            cout << "The Arabic equivalent of " << line << " is: " << i << endl;
        }

    }
 }


  return 0;


}

Answer 1

程序始终等待您添加更多输入，因此它不会终止。有几种方法可以解决这个问题：

• 让程序查找特定关键字，例如“quit”或“exit”，甚至只是一个空格，然后键入该关键字以终止。这很简单，但不是很优雅。
• 发送“流结束”指示符作为输入中的唯一内容。在 linux 和 unix 中，您只需键入 Ctrl-D，这将表明您已关闭标准输入。正如一些 cmets 所说，Ctrl-Z 是 Windows 文件说明符的结尾，如果您使用它的话。

Answer 2

你的程序永远不会结束，因为你的外循环永远运行。

可能的修复：

while (cin >> line) 
{
   int i = 0;
   if (line == "quit") break;

   while(line[i])
   {
     c = line[i];
     c = (toupper(c));
     line[i] = c;
     i++;
   }
   // run for loops
}

那么你将不得不调用所有这些 for 循环。可能最好将它们放在一个函数中。

Answer 3

您的程序表现出未定义的行为，因为它在此循环中读取到数组的末尾：

for (int i = 0; i < 10; i++){
   if (roman_thousands[i] == line){
       cout << "The Arabic equivalent of " 
            << line <<" is: " << i << 0 << 0 << 0 << endl;
    }
}