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   Early toolsmiths writing in C under Unix began developing idioms at a rapid rate to classify characters into different types. For example, in the ASCII character set, the following test identifies a letter:
    if ('A' <= c && c <= 'Z' || 'a' <= c && c <= 'z')
   However, this idiom does not necessarily work for other character sets such as EBCDIC.
    Pretty soon, programs became thick with tests such as the one above, or worse, tests almost like the one above. A programmer can write the same idiom several different ways, which slows comprehension and increases the chance for errors.

   The <ctype.h> contains prototypes for a dozen character classification functions. All of these functions except isdigit and isxdigit are locale-specific; their behavior may change if the locale changes.[clarification needed]

isalnum    test for alphanumeric character
isalpha    test for alphabetic character
isblank    test for blank character (new in C99)
iscntrl    test for control character
isdigit    test for digit. Not locale-specific.
isgraph    test for graphic character, excluding the space character.
islower    test for lowercase character
isprint    test for printable character, including the space character.

ispunct    test for punctuation character
isspace    test for any whitespace character
isupper    test for uppercase character
isxdigit    test for hexadecimal digit. Not locale-specific.

Return the converted character unless it is not alphabetic.

tolower    convert character to lowercase
toupper    convert character to uppercase

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