isdigit not working as I expected

[programgirl]

Hello all!

I have a specific code question. I'm trying to ensure that the character entered is a digit. If it is a digit, the program should continue. If it's not a digit, an error message should occur and a prompt for another input. Here is the code, my questions are in the comments...

  #include <iostream>
#include <iomanip>
#include <cctype>		//prototypes for character functions
using namespace std;

//I've edited out the non pertinent code...
int main ()
{
int isDigit;
int userNum;

    cout << "Enter an integer. Enter -999 to quit: ";
    cin  >> userNum;
	
	isDigit = (isdigit (userNum));  //if the user enters a number, shouldn't this return true?  It's returning false even when a number is entered...
	while ( isDigit == false )
		{
		cout << "Your input is not valid.  Please enter a number from 0-9: ";
		cin >> userNum;
                isDigit = (isdigit (userNum)); 
		}

Thanks!

[KyleMiles]

Please supply us with the rest of your code... It may be the way that you edited it, but this makes your program seem recursive...

- Kyle

[Peter87]

isdigit works with single characters and returns true for '0' to '9'.

[vlad from moscow]

Function isdigit is applied to object of the type char. Though its argument is declared as int nevertheless it is used with characters that is it considers only the lowerst byte of the argument and checks whether its value is between '0' and '9' inclusively.


Consider an example

#include <iostream>
#include <iomanip>
#include <cctype>

int main()
{
   for ( char c = '0'; c <= '9'; c++ )
   {
      std::cout << c << " is digit " << std::boolalpha << ( std::isdigit( c ) != 0 ) << std::endl;
   }
}

[KyleMiles]

Oh, sorry about that... I failed to realize isDigit vs. isdigit. Peter87 is correct.

[Chervil]

//I've edited out the non pertinent code...

I think the declaration of userNum is extremely pertinent to the question.

[programgirl]

Thank you very much KyleMiles, vlad and Peter87, that makes sense. Chervil you are right that declaration is pertinent, my mistake for not adding to the code. I've corrected it for the post.