LISP - Characters


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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.

Live Demo
(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.

Live Demo
; 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


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