while循環不終止？

Question

我想知道爲什麼這個while循環不允許我的程序終止？

據我所知（雖然我很可能是錯的）條件while (cin >> line)檢查我的輸入流爲一個字符串，然後運行我的循環，直到輸入中找不到其他字符串。然而，在測試我的代碼後，我得到正確的輸出，但是我的循環從未終止任何想法，爲什麼？

#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

該程序總是等待你添加更多的輸入，所以它不會終止。有幾種方法可以解決這個問題：

• 讓程序查找「quit」或「exit」之類的特定關鍵字，甚至只是一個空格，然後鍵入終止。這很簡單，但不是很優雅。
• 發送「流結束」指示符作爲輸入中的唯一內容。在linux和unix中，只需鍵入Ctrl-D，這將表明您已關閉標準輸入。正如一些評論所說，如果你使用的是Ctrl-Z，那麼它就是Windows文件說明符的結尾。

Answer 2

你的程序永遠不會結束，因爲你的外循環永遠運行。

可能的解決辦法：

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 
} 

，那麼你將不得不調用所有的for循環。可能最好把它們放在一個函數中。

Answer 3

你的程序表現出不確定的行爲，因爲它讀取過去的陣列在這個循環的結束：

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