Loop until integer input is in required range fails to work with non-digit character inputs

Question

I'm having a problem with what should be incredibly simple code. I want to take in an integer between 1 and 3 with error checking. It works fine for checking for numbers that are too large or too small, but when a alpha/number combination is entered, it gets stuck in an infinite loop. Suggestions?

#include <iostream>
using namespace std;

int main(int argc, char *argv[]){
    int input;

    cout << "\nPlease enter a number from 1 to 3:" << endl;
    cout << "-> ";
    cin >> input;

    while(input< 1 || input> 3){
        cout << "\n---------------------------------------" << endl;
        cout << "\n[!] The number you entered was invalid." << endl;
        cout << "\nPlease re-enter a number from 1 to 3" << endl;
        cout << "-> ";
        cin >> input;
    }

    cout << "You chose " << input << endl;
}

Answer 1

The problem is that:

cin >> input;

Will cause the bad bit to be set when you try and read a non numeric value. After that happens any attempt to use the operator>> is silently ignored.

So the way to correct for this is to test if the stream is in a good state and if not then reset the state flags and try and read again. But note that the bad input (that caused the problem) is still on the input so you need to make sure you throw it away as well.

if (cin >> input)
{
    // It worked (input is now in a good state)
}
else
{
    // input is in a bad state.
    // So first clear the state.
    cin.clear();

    // Now you must get rid of the bad input.
    // Personally I would just ignore the rest of the line
    cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');

    // now that you have reset the stream you can go back and try and read again.
}

To prevent it getting stuck (which is caused by the bad bit being set) read into a string then use a string stream to parse user input. I also prefer this method (for user interactive input) as it allows for easier combination of different styles of reading (ie combining operator>> and std::getline() as you can use these on the stringstream).

#include <iostream>
#include <sstream>
#include <string>
// using namespace std;
// Try to stop using this.
// For anything other than a toy program it becomes a problem.

int main(int argc, char *argv[])
{
    int          input;

    std::string  line;
    while(std::getline(std::cin, line))   // read a line at a time for parsing.
    {
        std::stringstream linestream(line);
        if (!(linestream >> input))
        {
             // input was not a number
             // Error message and try again
             continue;
        }
        if ((input < 1) || (input > 3))
        {
             // Error out of range
             // Message and try again
             continue;
        }
        char errorTest;
        if (linestream >> errorTest)
        {
             // There was extra stuff on the same line.
             // ie sobody typed 2x<enter>
             // Error Message;
             continue;
        }

        // it worked perfectly.
        // The value is now in input.
        // So break out of the loop. 
        break;
    }
}

Answer 2

#include <iostream>
#include <string>

using namespace std;

int validatedInput(int min = 1, int max = 3)
{
    while(true)
    {
        cout << "Enter a number: ";
        string s;
        getline(cin,s);
        char *endp = 0;
        int ret = strtol(s.c_str(),&endp,10);
        if(endp!=s.c_str() && !*endp && ret >= min && ret <= max)
            return ret;
        cout << "Invalid input. Allowed range: " << min << "-" << max <<endl;
    }
}

int main(int argc, char *argv[])
{
    int val = validatedInput();
    cout << "You entered " << val <<endl;
    return 0;
}