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’).