LISP - Characters

In LISP, characters are represented as data objects of type character.
You can denote a character object preceding #\ before the character itself. For example, #\a means the character a.

Space and other special characters can be denoted by preceding #\ before the name of the character. For example, #\SPACE represents the space character.
The following example demonstrates this:

Example

Create a new source code file named main.lisp and type the following code in it.

(write 'a)
(terpri)
(write #\a)
(terpri)
(write-char #\a)
(terpri)
(write-char 'a)

When you execute the code, it returns the following result:

A
#\a
a
*** - WRITE-CHAR: argument A is not a character

Special Characters

Common LISP allows using the following special characters in your code. They are called the semi-standard characters.

• #\Backspace
• #\Tab
• #\Linefeed
• #\Page
• #\Return
• #\Rubout

Character Comparison Functions

Numeric comparison functions and operators, like, < and > do not work on characters. Common LISP provides other two sets of functions for comparing characters in your code.
One set is case-sensitive and the other case-insensitive.
The following table provides the functions:

Case Sensitive Functions	Case-insensitive Functions	Description
char=	char-equal	Checks if the values of the operands are all equal or not, if yes then condition becomes true.
char/=	char-not-equal	Checks if the values of the operands are all different or not, if values are not equal then condition becomes true.
char<	char-lessp	Checks if the values of the operands are monotonically decreasing.
char>	char-greaterp	Checks if the values of the operands are monotonically increasing.
char<=	char-not-greaterp	Checks if the value of any left operand is greater than or equal to the value of next right operand, if yes then condition becomes true.
char>=	char-not-lessp	Checks if the value of any left operand is less than or equal to the value of its right operand, if yes then condition becomes true.

Example

Create a new source code file named main.lisp and type the following code in it.

; case-sensitive comparison
(write (char= #\a #\b))
(terpri)
(write (char= #\a #\a))
(terpri)
(write (char= #\a #\A))
(terpri)
   
;case-insensitive comparision
(write (char-equal #\a #\A))
(terpri)
(write (char-equal #\a #\b))
(terpri)
(write (char-lessp #\a #\b #\c))
(terpri)
(write (char-greaterp #\a #\b #\c))

When you execute the code, it returns the following result:

NIL
T
NIL
T
NIL
T
NIL