为什么这个循环不起作用？

Question

为什么这个循环不起作用？当答案不是 y 或 n 时，它会产生分段错误，但我认为我已经解决了这个问题，但它没有用。我该如何解决这个问题才能停止分段错误？我很困惑为什么它不起作用。

int main()
{
    string yes = "y";
    string uyes = "Y";
    string no = "n";
    string uno = "N";
    string answer = " ";    
    do
    {
        system("CLS");
        cout << "Welcome to the Red Fern Shipping Company where we ship your packages your way!\n";
        cout << "Do you want to ship a parcel? Y or N?\n";
        cin >> answer;
        do
        {
            if (answer == no || answer == uno)
            {
                cout << "Thank you! Come again!\n";
                system("PAUSE");
                return 0;
            }
            else if (answer == yes || answer == uyes)
            {
                break;
            }
            else
            {
                cin.clear();
                cin.sync();
                cout << "That answer is invalid and cannot work. Please enter a 'y' for yes or 'n' for no.\n";
            }
        } while (answer != yes && answer != uyes && answer != no && answer != uno);

    } while (answer == yes || answer == uyes);
    system("PAUSE");
    return 0;
}

Answer 1

'answer' 的值将永远不等于 yes/no 如果它是一次，因为 'cin >> answer' 没有在内部 do-while 中声明。

Answer 2

问题是内循环是无限的

        do
        {
            if (answer == no || answer == uno)
            {
                cout << "Thank you! Come again!\n";
                system("PAUSE");
                return 0;
            }
            else if (answer == yes || answer == uyes)
            {
                break;
            }
            else
            {
                cin.clear();
                cin.sync();
                cout << "That answer is invalid and cannot work. Please enter a 'y' for yes or 'n' for no.\n";
            }
        } while (answer != yes && answer != uyes && answer != no && answer != uno);

循环变量answer 内没有得到更新。您应该插入语句

std::cin >> answer;

在此内部循环中。

除了一个语句外，我会将所有 if 语句放在内部循环之外。例如

        bool valid_answer;
        do
        {
            cin >> answer;

            valid_answer = answer == yes || answer == uyes || 
                                   answer == no || answer == uno;
            if ( !valid_answer )
            {
                cin.clear();
                cin.sync();
                cout << "That answer is invalid and cannot work. Please enter a 'y' for yes or 'n' for no.\n";
            }
        } while ( !valid_answer );


    if (answer == no || answer == uno)
    {
        cout << "Thank you! Come again!\n";
        system("PAUSE");
        return 0;
    }

为什么这个循环不起作用？

Why doesn't this loop work?

c++

if-statement

do-while