isdigit()               Test for Digit (Macro)

 #include   <ctype.h>;

 int        isdigit(c)
 int        c;                           Integer value to be tested

    isdigit() tests whether 'c' is a digit in the set '0' - '9'.

       Returns:     True (nonzero) if the integer satisfies the test
                    condition or false (zero) if it does not.  The result
                    is undefined if 'c' is other than an ASCII character
                    or EOF.

         Notes:     isdigit() is a macro.

   -------------------------------- Example ---------------------------------

    The following statements open a file, read all the characters in, and
    total only those characters that are digits.

           #include <ctype.h>
           #include <stdio.h>             /* for fopen */

           FILE *stream;
           int ch;

           main()
           {
               int totl = 0;

               if ((stream = fopen("input.dat","r+")) != NULL) {
                   while (!feof(stream))
                       if (isdigit(ch = getc(stream)))
                           printf("item cost: $%d.00\n",ch);
               }
           }