Re: Can a cin >> be in a while()?

Hakusa@gmail.com wrote:


The value of the expression (cin >> basePrice) is (a reference to) the
stream cin, which easily converts into a boolean value indicating
whether the stream is in a good state, i.e. (roughly) whether the last
operation on the stream succeeded.

Therefore,

    while (cin >> basePrice, basePrice <= 0)

is the easiest fix that retains your original intention. Here, (cin >>
basePrice) is evaluated first, and (basePrice <= 0) is evaluated next
and becomes the value of the total expression.

In addition, good programs should always anticipate unexpected input
(such as non-digits) or unexpected end of input. Therefore, I propose
the following structure:

    while (true) {
        cout << "What is the base price of the product? ";
        if (cin >> basePrice && basePrice > 0) {
            cout << "A valid input: " << x << '\n';
            // process basePrice
            break;
        }
        else if (cin)
            cout << "Please enter a positive number. ";
        else if (!cin.eof()) {
            cout << "Please enter a valid number. ";
            cin.clear();
            cin.ignore(numeric_limits<streamsize>::max(), '\n');
        }
        else {
            cout << '\n';
            cout << "End-of-input. Good-bye.\n";
            break;
        }
        cout << "Try again.\n";
    }

Alternatively, you may want to process basePrice *after* the while loop,
in which case you have to test again with "if (cin)" after the loop
because you reach the same point when an end-of-input is reached.

You may also want to make the structure into a separate function:

bool get_base_price(int& x)
{
    while (true) {
        cout << "What is the base price of the product? ";
        if (cin >> x && x > 0) {
            cout << "A valid input: " << x << '\n';
            return true;
        }
        else if (cin)
            cout << "Please enter a positive number. ";
        else if (!cin.eof()) {
            cout << "Please enter a valid number. ";
            cin.clear();
            cin.ignore(numeric_limits<streamsize>::max(), '\n');
        }
        else {
            cout << '\n';
            cout << "End-of-input. Good-bye.\n";
            return false;
        }
        cout << "Try again.\n";
    }
    // not reached
}

int main()
{
    int basePrice;
    if (get_base_price(basePrice)) {
        // process basePrice
    }
}

You may find some strange behaviour as you enter "-1 3" or "3x" in an
interactive environment, especially if you have successive questions.
It is less confusing when you ignore the rest of the line after reading
an item:

bool get_base_price(int& x)
{
    while (true) {
        cout << "What is the base price of the product? ";
        if (cin >> x && x > 0) {
            cout << "A valid input: " << x << '\n';
            cin.ignore(numeric_limits<streamsize>::max(), '\n');
            return true;
        }
        else if (cin)
            cout << "Please enter a positive number. ";
        else if (!cin.eof()) {
            cout << "Please enter a valid number. ";
            cin.clear();
        }
        else {
            cout << '\n';
            cout << "End-of-input. Good-bye.\n";
            return false;
        }
        cin.ignore(numeric_limits<streamsize>::max(), '\n');
        cout << "Try again.\n";
    }
    // not reached
}

Or a more rigorous method is to validate the input is to get one lineful
of input as a string and scan it. Boost::lexical_cast can help you with
this:

#include <boost/lexical_cast.hpp>

bool get_base_price(int& x)
{
    string line;
    while (true) {
        cout << "What is the base price of the product? ";
        if (getline(cin, line))
            try {
                x = boost::lexical_cast<int>(line);
                if (x > 0) {
                    cout << "A valid input: " << x << '\n';
                    return true;
                }
                else
                    cout << "Please enter a positive number. ";
            }
            catch (boost::bad_lexical_cast&) {
                cout << "Please enter a valid number. ";
            }
        else {
            cout << '\n';
            cout << "End-of-input. Good-bye.\n";
            return false;
        }
        cout << "Try again.\n";
    }
    // not reached
}

--
Seungbeom Kim

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