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Character Class Tests in C


The <ctype.h> is a header file in C which is used to declare functions for testing characters. This header file of the C standard library is used to declare multiple functions which are beneficial for not only testing characters but also mapping them.

In the <ctype.h>, each function that has the argument is an int. What this means is, that all the functions accept int as a parameter, whose value must be EOF (End of File) or representable as an unsigned char, and the return value is an int.

Note: If the argument c satisfies the described conditions, then the functions return non-zero (true) and in case it does not satisfy the conditions, the functions return zero(false).

Library Functions

S. No. Function Description
1 isalnum(c)It stands for is_alphanumeric() and it checks whether the passed character is alphanumeric or not.
2isalpha(c)It is an acronym to test a character as alphabetic and it checks whether the passed character is alphabetic.
3iscntrl(c)Short for control character and it checks whether the passed character is a control character.   (Control character)
4isdigit(c)This function is used to check whether the passed character is a decimal digit.   (decimal digit)
5isgraph(c)This function is used to check whether the passed character has graphical representation using locale.   (printing character except space)
6islower(c)This function is used to check whether the passed character is a lowercase letter.   (lower-case letter)
7isprint(c)This function is used to check whether the passed character is printable.   (printing character except space)
8ispunct(c)This function is used to check whether the passed character is a punctuation character.   (It prints characters with the exception of space or letter or digit)
9isspace(c)This function is used to check whether the passed character is white-space.   (space, form feed, newline, carriage return, tab, vertical tab)
10isupper(c)This function is used to check whether the passed character is an uppercase letter.   (upper-case letter)
11isxdigit(c)This function is used to check whether the passed character is a hexadecimal digit.   (hexadecimal digit)



In C programming, the seven bit ASCII character set consists of ;

  • The printing characters which lies in the range : 0 x 20 (‘  ‘) to 0 x 7E (‘ ~ ‘)
  • And, The control characters which lies in the range : 0 (NULL) to 0 x 1F(US), and 0x7F(DEL)

Note: There are a few special cases, and it consists of two functions that converts the case of letter (either uppercase or lowercase):

int tolower (int c)Converts c to lowercase
int toupper (int c)Converts c to uppercase

Here, one thing to keep a note of is,

  • If ‘c’ is a lowercase letter, toupper(c) returns the corresponding upper-case letter; otherwise, it returns c.
  • If ‘c’ is an uppercase letter, tolower(c) returns the corresponding lower-case letter; otherwise, it returns c.

Character Classes

S. No. Function Description
1Digitsset of whole numbers (range: 0 to 9)    { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 }.
2Hexadecimal digitsset of whole numbers + alphabets ranging from A to F or a to f (both cases)   { 0 1 2 3 4 5 6 7 8 9 A B C D E F a b c d e f }.
3LettersIt is a set or collection of letters from both cases. In simpler terms, it is a set of lowercase and uppercase letters.   The range of letters lies from: (a to z) and (A to Z)   { a b c d e f g h i j k l m n o p q r s t u v w x y z } + {A B C D E F G H I J K L M N O P Q R S T U V W X Y Z }  
4Lowercase lettersIt is a set of lowercase letters which lies in the range from (a to z)    { a b c d e f g h i j k l m n o p q r s t u v w x y z }.
5Uppercase lettersIt is a set of uppercase letters which lies in the range from: (A to Z)    {A B C D E F G H I J K L M N O P Q R S T U V W X Y Z }.
6Alphanumeric charactersIt is a set of Digits (0-9) , Lowercase letters (a-z) and Uppercase letters (A-Z).   { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 + a to z + A to Z }
7Alphabetic charactersset of Lowercase letters and Uppercase letters.
8Punctuation charactersset of @ # * ! "  $ % & ' ( )  + , - . / : ; < = > ? @ [ \ ] ^ _ ` { | } ~
9Printable charactersset of Alphanumeric characters, Space characters. and Punctuation characters   {set of Digits (0-9) , Lowercase letters (a-z) and Uppercase letters (A-Z) + punctuation characters + space characters }  
10Control charactersoctal codes 000 through 037, and 177 (DEL).
11Graphical characters set of Alphanumeric characters and Punctuation characters.   {set of Digits (0-9), Lowercase letters (a-z) and Uppercase letters (A-Z) + punctuation characters}
12Blank charactersspaces and tabs.
13Space charactersset of tab, newline, vertical tab, return, and space etc.