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isalpha

Defined in header <ctype.h> 
int isalpha( int ch );

Checks if the given character is an alphabetic character, i.e. either an uppercase letter (ABCDEFGHIJKLMNOPQRSTUVWXYZ), or a lowercase letter (abcdefghijklmnopqrstuvwxyz).

In locales other than "C", an alphabetic character is a character for which isupper() or islower() returns true or any other character considered alphabetic by the locale. In any case, iscntrl(), isdigit(), ispunct() and isspace() will return false for this character.

The behavior is undefined if the value of ch is not representable as unsigned char and is not equal to EOF.

Parameters

ch - character to classify

Return value

Non-zero value if the character is an alphabetic character, zero otherwise.

Example

Demonstrates the use of isalpha with different locales (OS-specific).

#include <ctype.h>
#include <stdio.h>
#include <locale.h>
 
int main(void)
{
    unsigned char c = '\xdf'; // German letter ß in ISO-8859-1
 
    printf("isalpha('\\xdf') in default C locale returned %d\n", !!isalpha(c));
 
    setlocale(LC_CTYPE, "de_DE.iso88591");
    printf("isalpha('\\xdf') in ISO-8859-1 locale returned %d\n", !!isalpha(c));
}

Possible output:

isalpha('\xdf') in default C locale returned 0
isalpha('\xdf') in ISO-8859-1 locale returned 1

References

  • C17 standard (ISO/IEC 9899:2018):
    • 7.4.1.2 The isalpha function (p: 145)
  • C11 standard (ISO/IEC 9899:2011):
    • 7.4.1.2 The isalpha function (p: 200-201)
  • C99 standard (ISO/IEC 9899:1999):
    • 7.4.1.2 The isalpha function (p: 181-182)
  • C89/C90 standard (ISO/IEC 9899:1990):
    • 4.3.1.2 The isalpha function

See also

(C95)
 checks if a wide character is alphabetic
(function)
C++ documentation for isalpha
ASCII values characters

iscntrl
iswcntrl.

isprint
iswprint.

isspace
iswspace.

isblank
iswblank.

isgraph
iswgraph.

ispunct
iswpunct.

isalnum
iswalnum.

isalpha
iswalpha.

isupper
iswupper.

islower
iswlower.

isdigit
iswdigit.

isxdigit
iswxdigit.
decimal hexadecimal octal
0–8 \x0–\x8 \0–\10 control codes (NUL, etc.) ≠0 0 0 0 0 0 0 0 0 0 0 0
9 \x9 \11 tab (\t) ≠0 0 ≠0 ≠0 0 0 0 0 0 0 0 0
10–13 \xA–\xD \12–\15 whitespaces (\n, \v, \f, \r) ≠0 0 ≠0 0 0 0 0 0 0 0 0 0
14–31 \xE–\x1F \16–\37 control codes ≠0 0 0 0 0 0 0 0 0 0 0 0
32 \x20 \40 space 0 ≠0 ≠0 ≠0 0 0 0 0 0 0 0 0
33–47 \x21–\x2F \41–\57 !"#$%&'()*+,-./ 0 ≠0 0 0 ≠0 ≠0 0 0 0 0 0 0
48–57 \x30–\x39 \60–\71 0123456789 0 ≠0 0 0 ≠0 0 ≠0 0 0 0 ≠0 ≠0
58–64 \x3A–\x40 \72–\100 :;<=>?@ 0 ≠0 0 0 ≠0 ≠0 0 0 0 0 0 0
65–70 \x41–\x46 \101–\106 ABCDEF 0 ≠0 0 0 ≠0 0 ≠0 ≠0 ≠0 0 0 ≠0
71–90 \x47–\x5A \107–\132 GHIJKLMNOP
QRSTUVWXYZ 		0 ≠0 0 0 ≠0 0 ≠0 ≠0 ≠0 0 0 0
91–96 \x5B–\x60 \133–\140 [\]^_` 0 ≠0 0 0 ≠0 ≠0 0 0 0 0 0 0
97–102 \x61–\x66 \141–\146 abcdef 0 ≠0 0 0 ≠0 0 ≠0 ≠0 0 ≠0 0 ≠0
103–122 \x67–\x7A \147–\172 ghijklmnop
qrstuvwxyz 		0 ≠0 0 0 ≠0 0 ≠0 ≠0 0 ≠0 0 0
123–126 \x7B–\x7E \172–\176 {|}~ 0 ≠0 0 0 ≠0 ≠0 0 0 0 0 0 0
127 \x7F \177 backspace character (DEL) ≠0 0 0 0 0 0 0 0 0 0 0 0

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