Using C isdigit for error checking

Question

While using the boolean check for the int num this loop doesn't work. The lines after it go unrecognized. Enter and integer like 60 and it just closes. Did I use isdigit wrong?

int main()
{
    int num;
    int loop = -1;

    while (loop ==-1)
    {
        cin >> num;
        int ctemp = (num-32) * 5 / 9;
        int ftemp = num*9/5 + 32;
        if (!isdigit(num)) {
            exit(0);  // if user enters decimals or letters program closes
        }

        cout << num << "°F = " << ctemp << "°C" << endl;
        cout << num << "°C = " << ftemp << "°F" << endl;

        if (num == 1) {
            cout << "this is a seperate condition";
        } else {
            continue;  //must not end loop
        }

        loop = -1;
    }
    return 0;
}

Answer 1

When you call isdigit(num), the num must have the ASCII value of a character (0..255 or EOF).

If it's defined as int num then cin >> num will put the integer value of the number in it, not the ASCII value of the letter.

For example:

int num;
char c;
cin >> num; // input is "0"
cin >> c; // input is "0"

then isdigit(num) is false (because at place 0 of ASCII is not a digit), but isdigit(c) is true (because at place 30 of ASCII there's a digit '0').

Answer 2

isdigit only checks if the specified character is a digit. One character, not two, and not an integer, as num appears to be defined as. You should remove that check entirely since cin already handles the validation for you.

http://www.cplusplus.com/reference/clibrary/cctype/isdigit/

Answer 3

If you're trying to protect yourself from invalid input (outside a range, non-numbers, etc), there are several gotchas to worry about:

// user types "foo" and then "bar" when prompted for input
int num;
std::cin >> num;  // nothing is extracted from cin, because "foo" is not a number
std::string str;
std::cint >> str;  // extracts "foo" -- not "bar", (the previous extraction failed)

More detail here: Ignore user input outside of what's to be chosen from