Question

I am reading input for my program in a loop using getline.

    string temp(STR_SIZE, ' ');
    string str_num(STR_SIZE, ' ');
    ...
    getline(cin, temp, '\n');

After which, I use a function to find the next delimiter(white space) and assign all the characters before the white space to str_num. Looks something like this:

    str_num.assign(temp, 0, next_white_space(0));

I have verified that this works well. The next step in my solution would be to convert str_num to an int(this part also works well), but I should check to make sure each character in str_num is a digit. Here’s the best of what I’ve tried:

    if(!isdigit(str_num[0] - '0')) {
        cout << "Error: Not an appropriate value\n";
        break; /* Leave control structure */
    }

For some reason, This always prints the error message and exits the structure.
Why is that?
I’ve used operator[] for string objects before, and it seemed to work well. But, here, it’s totally messing me up.

Thanks.

Answer 1

std::isdigit takes a char’s integer value and checks it.

So, remove the – ‘0’ and just pass str_num[index] to isdigit().

Note: because this function comes from C, the old style of treating chars as integers shows through in the method taking an int. However, chars can promote to int values, so a char becomes an int just fine and this works.