isalpha—alphabetic character predicate #include <ctype.h>
int isalpha(int c);
Description
isalpha is a macro which classifies ASCII integer values by table
lookup. It is a predicate returning non-zero when c represents an
alphabetic ASCII character, and 0 otherwise. It is defined only if
c is representable as an unsigned char or if c is EOF.
You can use a compiled subroutine instead of the macro definition by
undefining the macro using `#undef isalpha'.
Returns
isalpha returns non-zero if c is a letter (A–Z or
a–z).
Portability
isalpha is ANSI C.
No supporting OS subroutines are required.