EECS168:badInput

Recovering from bad user input

You can stand up to even the worst user using a combination of method in std::cin. First, we need to detected if the users input triggered the fail bit (i.e. we ask for an int and they type a char). Then we clear out the input stream because we don't know what they typed. Finally we let them try again. We can trap them in a loop until their input does not cause the failbit to trigger.

Example of getting a good int:

#include <iostream>
#include <limits>
 
int main()
{
                         
    int n = 0;
    std::cout << "Input a number: ";
    std::cin >> n;
    
    while ( std::cin.fail() )
    {
        std::cin.clear(); // unset failbit

        // skip bad input up to the next newline
        std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');

        std::cout << "Sorry, your input did not seem to be an int. Try again: ";   
        std::cin >> n;
    }

    //Flush out anything left in the stream (e.g. if they type 2.5 for an int
    //  the .5 would still be there
    std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');

    std::cout << "You entered: " << n;
}