How can I validate the input is only number digit?

Question

I wont to check whether the age is number or not in this example it will accept 1E as number while it will not accept E1, I wont to accept only digit
not any symbol or alphabet

 int main()
    {
      int age= 0;
        std::cin >> age; 
        if (std::cin.fail())
        {
        std::cout << "I failed, try again ..." << std::endl;
        std::cin.clear(); // reset the failed state
    }

@knivil, If trying to store the integer result, regex is kind of overkill. A call to `std::stoi` is enough with proper error checking. — chris, Feb 27 '17 at 14:31
Read a line from `std::cin` into a `std::string` (e.g. using `std::getline()`. Then check the string by whatever criteria you choose to determine if it contains a number. If it does, extract the number from the string (e.g. using a `std::istrstream` to help). — Peter, Feb 27 '17 at 14:31
If you want to verify the entire line of input, you will have to read a string with `getline` and check the content. The input `1E` is valid if your next statement is `cin >> ch;`. That would get you `1` in `age` and `'E'` in `ch`. — Bo Persson, Feb 27 '17 at 14:31
Not exactly the same question but this answer may be of help: https://stackoverflow.com/questions/24504582/how-to-test-whether-stringstream-operator-has-parsed-a-bad-type-and-skip-it/27004240#27004240 — Galik, Feb 27 '17 at 14:34
Nobody should throw an exception on verifying user input. And `std::stoi("31337 with words") is 31337` is not the thing I would suggest here. — knivil, Feb 27 '17 at 14:42

score 0 · Answer 1 · answered Feb 27 '17 at 14:30

Really cin and the extraction operators are no good for parsing user input. They're OK for inputting automatically formatted records, but not input that is likely to have free form errors in it.

If you use line-based input, then call the function strtol(), you can find digits. strtol() also handles minus signs and numbers which are too large to fit into an integer. It's essentially the safe integer parser. Also, on error, you can easily discard the whole line and ask the user to re-enter, which is usually the desired behaviour.

How can I validate the input is only number digit?

1 Answers1