### 3.2 `isalpha`—alphabetic character predicate

Synopsis

         #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 when `isascii`(c) is true or 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.