How to validate input in a do...while loop and then re-ask question if necessary?

Question

So far I have tried to make a do...while loop where it asks two questions. One which is how many miles and the second which is how much does the package weigh. If the number of miles is equal to 0 or less it should output an error and then re-ask the question. Once validated it should move onto the weight question with the same requirements, and if the weight is invalid it should only repeat the weight question since the miles question is already valid.

This is the code that I have so far:

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

You can use multiple while loops, one for each input.

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

You need additional loops to read from cin until the input is valid. And you need to ensure that cin is even successful in reading a value before you perform range checking on that value.

I would suggest doing the read+validation in a separate function, eg:

#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";