如何在 do...while 循环中验证输入，然后在必要时重新提问？

Question

到目前为止，我已经尝试做一个 do...while 循环，它会问两个问题。 一个是多少英里，第二个是包裹的重量。 如果英里数等于或小于 0，它应该输出一个错误，然后重新提问。 一旦通过验证，它应该转移到具有相同要求的重量问题，如果重量无效，它应该只重复重量问题，因为英里问题已经有效。

这是我到目前为止的代码：

int miles = 0;
int choice = 0;
double weight = 0.0;


do
{
    cout << "Enter the number of miles as a whole number: ";
    cin >> miles;
    if (miles > 0)
    {
        cout << "Enter the weight of the package in pounds: ";
        cin >> weight;

        if (weight > 0 && weight < 10)
        {
            cout << "etc etc etc";
        }
        else
        {
            cout << "\n\tError: Weight must be greater than zero and less than 10 pounds!\n" << endl;
        }

    }
    else
    {
        cout << "\n\tError: Miles must be greater than zero!\n" << endl;
    }

    cout << "Enter 1 to continue or 0 to quit: ";
    cin >> choice;
    cout << endl;

}
while (choice != 0); 
cout << "\nGood-bye!\n";

return 0;  

Answer 1

您可以使用多个 while 循环，每个输入一个。

int miles = 0;
double weight = 0.0;
bool correct = false;

while (!correct)
{
    cout << "Enter the number of miles as a whole number: " << std::endl;
    bool success = cin >> miles;
    if (!success || miles < 0) {
        cout << "Invalid miles value -- must be nonnegative." << std::endl;
    }
    else {
        correct = true;
    }
}

correct = false;

while (!correct)
{
    cout << "Enter the weight in pounds: " << std::endl;
    bool success = cin >> weight;
    if (!success || weight < 0 || weight > 10) {
        cout << "Invalid weight value -- must be between 0 and 10." << std::endl;
    }
    else {
        correct = true;
    }
}

// do calculation

cout << "\nGood-bye!\n";

return 0;

Answer 2

您需要额外的循环才能从cin读取，直到输入有效。 并且在对该值执行范围检查之前，您需要确保cin甚至成功读取该值。

我建议在单独的函数中进行读取+验证，例如：

#include <iostream>
#include <limits>

template<typename T>
T prompt(const char *msg, T maxValue = std::numeric_limits<T>::max())
{
    T value;

    do
    {
        cout << msg << ": ";

        if (!(cin >> value))
        {
            cout << "\n\tError: Invalid input!\n" << endl;
            cin.clear();
            cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
        }
        else if (value < 0)
        {
            cout << "\n\tError: Value must be greater than 0!\n" << endl;
        }
        else if (value >= maxValue)
        {
            cout << "\n\tError: Value must be less than " << maxValue << "!\n" << endl;
        }
        else
            break;
    }
    while (true);

    return value;
}

...

int miles;
double weight;
int choice;

do
{
    miles = prompt<int>("Enter the number of miles as a whole number");
    weight = prompt<double>("Enter the weight of the package in pounds", 10.0);
    choice = prompt<int>("Enter 1 to continue or 0 to quit", 2);
    cout << endl;
}
while (choice != 0); 

cout << "\nGood-bye!\n";