isdigit() always passes the check

Question

Hello i want to check in my program if the user instead of typing a digit if he typed something that is not a digit.

so i did this function

void ValidationController::cinError(int *variable){

    if(!isdigit(*variable)){
        cin.clear();
        cin.ignore(256, '\n');
        cout <<*variable<<endl;
        *variable=0;
        cout <<*variable<<endl;
    }
}

i call the function like this:

int more;
cin >>more;
cinError(&more);

So my problem is that everytime i give a digit it acts like i didn't.It goes inside if and makes the variable equal to zero.What am i missing here?

Answer 1

Leaving aside the fact that you are using isdigit incorrectly, it's too late to check for isdigit anyway, because you are reading an int. In this situation it is the stream's >> operator that looks for digits, not your code.

If you want to validate the user input, read the data into a string, and then use isdigit on its components, like this:

string numString;
getline(cin, numString);
for (int i = 0 ; i != numString.length() ; i++) {
    if (!isdigit((unsigned char)numString[i])) {
        cerr << "You entered a non-digit in a number: " << numString[i] << endl;
    }
}
// Convert your validated string to `int`