Indicates if a character is classed as a digit in the program's
    current locale.

    Format

      #include  <ctype.h>

      int isdigit  (int character);

1 - Argument

 character

    An object of type int. The value of character must be
    representable as an unsigned char or must equal the value of the
    macro EOF. If it has any other value, the behavior is undefined.

2 - Return Values

    nonzero            If a decimal digit.
    0                  If not a decimal digit.