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Programs that work with characters and strings often need to classify a
character—is it alphabetic, is it a digit, is it whitespace, and so
on—and perform case conversion operations on characters. The
functions in the header file ctype.h are provided for this
purpose.
Since the choice of locale and character set can alter the
classifications of particular character codes, all of these functions
are affected by the current locale. (More precisely, they are affected
by the locale currently selected for character classification—the
LC_CTYPE category; see Locale Categories.)
The ISO C standard specifies two different sets of functions. The
one set works on char type characters, the other one on
wchar_t wide characters (see Extended Char Intro).