# std::isalpha

From cppreference.com
< cpp‎ | string‎ | byte

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 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.

### Notes

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

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

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

    int count_alphas(const std::string& s)
    {
        return std::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
                             [](unsigned char 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()
    {
        unsigned char 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)

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`

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/string/byte/isalpha&oldid=152952"