c++ Input Validation printing twice?

Question

Don't know why it is printing twice. This is just practice I am trying to use cin.clear() / cin.ignore though. Trying to write a menu with functions inside switch statements.

#include <iostream>
#include <iomanip>
using namespace std;

void showMenu();
void showFees(double, int);

int main()
{
    int choice;
    int months;

    const int ADULT_CHOICE = 1;
    const int CHILD_CHOICE = 2;
    const int SENIOR_CHOICE = 3;
    const int QUIT_CHOICE = 4;

    const double ADULT = 40.0;
    const double CHILD = 20.0;
    const double SENIOR = 30.0;

    do
    {
        showMenu();
        cin >> choice;

        while (choice < ADULT_CHOICE || choice > QUIT_CHOICE)
        {
            cout << "Please enter a valid menu choice: ";
            cin.clear();
            cin.ignore(INT_MAX, '\n');
        }



        if (choice != QUIT_CHOICE)
        {
            cout << "For how many months? ";
            cin >> months;

            switch (choice)
            {
            case ADULT_CHOICE:
                showFees(ADULT, months);
                break;
            case CHILD_CHOICE:
                showFees(CHILD, months);
                break;
            case SENIOR_CHOICE:
                showFees(SENIOR, months);
                break;
            }

        }

    } while (choice != QUIT_CHOICE);

    system("pause");
    return 0;
}

void showMenu()
{
    cout << "\nHealth Club Membership Menu" << endl << endl;
    cout << "1. Standard Adult Membership" << endl;
    cout << "2. Child Membership" << endl;
    cout << "3. Senior Citizen Membership" << endl;
    cout << "4. Quit the Program" << endl << endl;
    cout << "Enter your choice: ";
}

void showFees(double memberRate, int months)
{
    cout << "The total charges are $" << (memberRate * months) << endl;
}

This is what it's printing. I'm fairly new c++, so I'm not really sure what's wrong. Is it the spacing? No clue.

Health Club Membership Menu

1. Standard Adult Membership
2. Child Membership
3. Senior Citizen Membership
4. Quit the Program

Enter your choice: 0
Please enter a valid menu choice: Please enter a valid menu choice: 0
Please enter a valid menu choice:

Answer 1

The input stream looks like this:

0\n

You first read the 0, and now the stream is

\n

The 0 is invalid, so you print "Please enter a valid menu choice: " and skip over the newline.
The stream is now empty.

Since the value of choice hasn't changed, it's still invalid and you print "Please enter a valid menu choice: " again.

This time the input stream is empty, so the ignore has to wait for the next newline, and that's why you're getting two outputs only the first time.

If you instead had done the output after you were done with the input processing:

cin.clear();
cin.ignore(INT_MAX, '\n');
cout << "Please enter a valid menu choice: ";

the output woldn't have been repeated.

A different bug is that the loop never exits, even if you input a valid value.

The reason for this is that you never read a new value for choice.

You need to add

cin >> choice;

at the end of the validation loop.

Answer 2

Problem is here:

  while (choice < ADULT_CHOICE || choice > QUIT_CHOICE)
        {
            cout << "Please enter a valid menu choice: ";
            cin.clear();
            cin.ignore(INT_MAX, '\n');
        }

Try this code. You must read value to variable choice again.

  while (choice < ADULT_CHOICE || choice > QUIT_CHOICE)
        {
            cout << "Please enter a valid menu choice: " << endl;
            cin >> choice;
            cout << endl;
        }

Answer 3

You have forgotten to read input again:

cin >> choice;

        while (choice < ADULT_CHOICE || choice > QUIT_CHOICE)
        {
            cout << "Please enter a valid menu choice: ";
            cin.clear();
            cin.ignore(INT_MAX, '\n');
        }

You read your input once. Then you go into the while check choice, You do cin.clear() and cin.ignore() and once again your loop executes as no new data has been passed: This should do the job:

cin >> choice;

        while (choice < ADULT_CHOICE || choice > QUIT_CHOICE)
        {
            cout << "Please enter a valid menu choice: ";
            cin.clear();
            cin.ignore(INT_MAX, '\n');
            cin >> choice;
        }