std::isalpha

 

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Defined in header <cctype>
int isalpha(int ch );

Checks if the given character is an alphabetic character as classified by the currently installed C locale. In the default locale, the following characters are alphabetic:

• uppercase letters ABCDEFGHIJKLMNOPQRSTUVWXYZ
• lowercase letters abcdefghijklmnopqrstuvwxyz

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

The behavior is undefined if the value of ch is not representable as unsignedchar 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.

Notes

Like all other functions from <cctype>, the behavior of std::isalpha is undefined if the argument's value is neither representable as unsignedchar nor equal to EOF. To use these functions safely with plain chars (or signedchars), the argument should first be converted to unsignedchar:

bool my_isalpha(char ch){return std::isalpha(static_cast<unsignedchar>(ch));}

Similarly, they should not be directly used with standard algorithms when the iterator's value type is char or signedchar. Instead, convert the value to unsignedchar first:

int count_alphas(conststd::string& s){returnstd::count_if(s.begin(), s.end(), // static_cast<int(*)(int)>(std::isalpha) // wrong// [](int c){ return std::isalpha(c); } // wrong// [](char c){ return std::isalpha(c); } // wrong[](unsignedchar c){return std::isalpha(c);}// correct);}

Example

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

Run this code

#include <cctype>#include <clocale>#include <iostream>   int main(){unsignedchar c ='\xdf';// German letter ß in ISO-8859-1   std::cout<<"isalpha(\'\\xdf\', default C locale) returned "<<std::boolalpha<<!!std::isalpha(c)<<'\n';   std::setlocale(LC_ALL, "de_DE.iso88591");std::cout<<"isalpha(\'\\xdf\', ISO-8859-1 locale) returned "<<static_cast<bool>(std::isalpha(c))<<'\n';   }

Possible output:

isalpha('\xdf', default C locale) returned false isalpha('\xdf', ISO-8859-1 locale) returned true

See also

isalpha(std::locale)	checks if a character is classified as alphabetic by a locale (function template)
iswalpha	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