Using functions isdigit : char functions « Data Types « C++ Tutorial

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2. Data Types
3. char functions

#include <iostream>
using std::cout;
using std::endl;

#include <cctype>
using std::isalnum;
using std::isalpha;
using std::isdigit;
using std::isxdigit;

int main()
{
   cout << "According to isdigit:\n"
      << ( isdigit( '8' ) ? "8 is a" : "8 is not a" ) << " digit\n"
      << ( isdigit( '#' ) ? "# is a" : "# is not a" ) << " digit\n";

   return 0;
}

According to isdigit:
8 is a digit
# is not a digit