isalpha in ctype.h

[Hascall John Paul]

In article <collinsa.669647114 at p4.cs.man.ac.uk> collinsa at p4.cs.man.ac.uk (Adrian Collins) writes:
}In <1991Mar20.112543.5515 at ericsson.se> etxnisj at eos8c21.ericsson.se (Niklas Sjovall) writes:
}>I want to use a macro defined in ctype.h ... i don't fully understand it.
}>#define	isalpha(c)	((_ctype_+1)[c]&(_U|_L))
}>It's the part (_ctype_+1)[c] i don't understand.

}For some reason the first entry in the array isn't used for holding
}character type information (beats me why) ...

     The is????? macros are defined over the set (-1 ... 255) hence
the need to offset by 1 to `align' with C's "start at 0" arrays (-1
is for EOF).  This is so stuff like the following works correctly.

          do {
             c = getchar();
                 :
             if (isalpha(c)) fribbles(c);
                 :
          } while (c != EOF);
--
John Hascall                        An ill-chosen word is the fool's messenger.
Project Vincent
Iowa State University Computation Center                       john at iastate.edu
Ames, IA  50011                                                  (515) 294-9551