c++ isalnum endless loop

Question

Greetings!

Lets cut the excessive intro this time and get straight to the point.

I have a problem in C++ using the isalnum method.

the code:

int playAgainst = 0;
do
{
    cout << "Who do you want to play against?(1/2)\n";
    cout << "1: Human player\n";
    cout << "2: Computer player\n";
    cin >> playAgainst
} while(!isalnum(playAgainst) && playAgainst != 0);

As seen in the code, I'm providing the user with a choice. Play against human or play against a computer.

What I want is, as long as the user enters anything else then an integer value(cin >> playAgainst) to repeat the question. However, If i enter a char, or string value, it keeps looping endlessly. I am not 100% sure, but it would be obvious, if the problem is, that the non int value is already saved as the value for playAgainst.. How can I check in this bit of code if the input value is int before saving it?

Or is the only possibility to save as a char/string and then check?

If the latter is the case, a new problem arises. isalnum only accepts int as parameter, atleast from what I know. How will I check if that string or char is an int?

Thank you for taking the time to read. And hopefully Ill be accepting a reply as answer soon ^^

Thanks everyone for the answers. I have gotten what I wanted, and everything has been solved.

The reason I chose for the accepted answer, is well... because initially, it made my code work the way I want it to. I want to accept multiple answers though..

Answer 1

Remove the isalnum() check. cin >> playAgainst will convert anything except a number to zero, which will be caught by the second check. Note that this is only an option because zero is not a valid input.

isalnum() is useful if you're interpreting the characters yourself, but in this case the stream has already done that for you.

Answer 2

Make playAgainst a char and compare against '0', not 0. Right now, the user has to enter the ASCII (or whatever your character set is) code for the character '1' or '2'.

isalnum won't work on ints outside the valid range of char, except for EOF. (The fact that it takes an int argument is a leftover from C, which has different integer promotions rules than C++. to accomodate for EOF.)

Answer 3

This is how the compiler will implement isalnum:

int isalnum (int ch)
{
  return (ch >= 'a' && ch <= 'z') || 
         (ch >= 'A' && ch <= 'Z') || 
         (ch >= '0' && ch <= '9');
}

So you might as well write that code snippet in your own code, and it will be equivalent with an inlined version of isalnum.

Answer 4

It's because you don't clear the buffer. When the input is invalid, you need to clear it from the buffer, and then you can move on to the next input, else you're trying to extract the same input every time (which fails, because it's the same bad input), and thus enter an infinite loop.

Answer 5

The problem is that you're inputing an int, and not a char. And if the text in the input isn't an int, then the input fails. In which case, playAgainst isn't modified, and the failure is memorized in std::cin until you explicitly clear the error. And inputting from a stream in an error state is a no-op. What you probably want to do is

1. Input a single character: if you don't want to skip spaces, using `std::cin.get( ch )` or `ch = std::cin.get()`. (In the latter case, `ch` should be an `int`, since it must also handle `EOF`. On the other hand, you can use `::isalnum` on it directly, which you can't do if `ch` is a `char`.
2. Fully check for valid input: not just `::isalnum`, but rather whether the input is a legal selector in your list. Something along the lines of:

       ch != EOF && std::find( legalChars.begin(), legalChars.end(), (char)ch ) != legalChars.end()
   

3. In case of error, clear any remaining input, say with:

              std::cin.ignore(INT_MAX, '\n');
   

In practice, you'll probably want to treat EOF differently from an erroneous command. (If you don't clear the input after EOF, you won't be able to read anything else. But presumably, if you got EOF, it's because the user gave up, and doesn't want to try any more.)

Finally, it's probably preferrable to keep all of the information in a common location, using a table of:

struct Command
{
    char        op;
    char const* prompt;
    void (*     func)();
};

You then loop over a table of these to output the prompt, search it to see if the character was legal, and finally, call the function on the entry you found. Or define an abstract base class, a concrete class deriving from it for each command, and use an std::map<char, AbstractBase*> for the mapping, etc. Very C++, but perhaps a bit overkill for such a simple case.

Answer 6

Why not use isdigit().