Ascertains whether a given character is a letter of the alphabet
int isalpha ( int c );
The function isalpha( ) tests whether its character argument is a letter of the alphabet. If the character is alphabetic, isalpha( ) returns a nonzero value (that is, TRue); if not, the function returns 0 (false).
Which characters are considered alphabetic depends on the current locale setting for the localization category LC_CTYPE, which you can query or change using the setlocale( ) function.
In the C locale, which is the default locale setting, the alphabetic characters are those for which isupper( ) or islower( ) returns true. These are the 26 lowercase and 26 uppercase letters of the Latin alphabet, which are the letters in the basic source and execution character sets (see "Character Sets" in Chapter 1).
Accented characters, umlauts, and the like are considered alphabetic only in certain locales. Moreover, other locales may have characters that are alphabetic, but are neither upper- nor lowercase, or both upper- and lowercase.
In all locales, the isalpha( ) classification is mutually exclusive with iscntrl( ), isdigit( ), ispunct( ), and isspace( ).
See the example for isprint( ) in this chapter.
The corresponding C99 function for wide characters, iswalpha( ); isalnum( ), isblank( ), iscntrl( ), isdigit( ), isgraph( ), islower( ), isprint( ), ispunct( ), isspace( ), isupper( ), isxdigit( ), setlocale( )