Character Class Tests in C

Introduction

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.
2	isalpha(c)	It is an acronym to test a character as alphabetic and it checks whether the passed character is alphabetic.
3	iscntrl(c)	Short for control character and it checks whether the passed character is a control character.   (Control character)
4	isdigit(c)	This function is used to check whether the passed character is a decimal digit.   (decimal digit)
5	isgraph(c)	This function is used to check whether the passed character has graphical representation using locale.   (printing character except space)
6	islower(c)	This function is used to check whether the passed character is a lowercase letter.   (lower-case letter)
7	isprint(c)	This function is used to check whether the passed character is printable.   (printing character except space)
8	ispunct(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)
9	isspace(c)	This function is used to check whether the passed character is white-space.   (space, form feed, newline, carriage return, tab, vertical tab)
10	isupper(c)	This function is used to check whether the passed character is an uppercase letter.   (upper-case letter)
11	isxdigit(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
1	Digits	set of whole numbers (range: 0 to 9)    { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 }.
2	Hexadecimal digits	set 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 }.
3	Letters	It 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 }
4	Lowercase letters	It 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 }.
5	Uppercase letters	It 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 }.
6	Alphanumeric characters	It 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 }
7	Alphabetic characters	set of Lowercase letters and Uppercase letters.
8	Punctuation characters	set of @ # * ! "  $ % & ' ( )  + , - . / : ; < = > ? @ [ \ ] ^ _ ` { | } ~
9	Printable characters	set 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 }
10	Control characters	octal codes 000 through 037, and 177 (DEL).
11	Graphical characters	set of Alphanumeric characters and Punctuation characters.   {set of Digits (0-9), Lowercase letters (a-z) and Uppercase letters (A-Z) + punctuation characters}
12	Blank characters	spaces and tabs.
13	Space characters	set of tab, newline, vertical tab, return, and space etc.