int isdigit(int chr);

tests if a character a digit

#include <iostream> /* including standard library */
#include <string>
#include <cctype>
 
using namespace std;
int main ( void )
{
  int i;
  string str="Test\n-!&%1";
  for (i=0; i <= 11;i++)
    {
     if ( isdigit(str[i]) != 0 )
      {
       cout << "Character " << i << " is a digit" << endl;
      }
    }
  return 0;
}

  Character 9 is a digit