c++ - Integer validation for input

Question

I tried to prompt user for input and do the validation. For example, my program must take in 3 user inputs. Once it hits non-integer, it will print error message and prompt for input again. Here is how my program going to be look like when running :

Enter number: a

Wrong input

Enter number: 1

Enter number: b

Wrong input

Enter number: 2

Enter number: 3

Numbers entered are 1,2,3

And here is my code:

double read_input()
{
    double input;
    bool valid = true;
    cout << "Enter number: " ;
    while(valid){
        cin >> input;
        if(cin.fail())
        {
            valid = false;
        }
    }
    return input;
}

My main method:

int main()
{
double x = read_input();
double y = read_input();
double z = read_input();
}

When my first input is non-integer, the program just exits by itself. It does not ask for prompting again. How could I fixed it? Or am I supposed to use a do while loop since I asking for user input.

Thanks in advance.

Answer 1

When the reading fails, you set valid to false, so the condition in the while loop is false and the program returns input (which is not initialized, by the way).

You also have to empty the buffer before using it again, something like:

#include <iostream>
#include <limits>

using namespace std;

double read_input()
{
    double input = -1;
    bool valid= false;
    do
    {
        cout << "Enter a number: " << flush;
        cin >> input;
        if (cin.good())
        {
            //everything went well, we'll get out of the loop and return the value
            valid = true;
        }
        else
        {
            //something went wrong, we reset the buffer's state to good
            cin.clear();
            //and empty it
            cin.ignore(numeric_limits<streamsize>::max(),'
');
            cout << "Invalid input; please re-enter." << endl;
        }
    } while (!valid);

    return (input);
}