Skip to main content

char-upcase, char-downcase

char-upcase, char-downcase Function

Syntax:

char-upcase character → corresponding-character

char-downcase character → corresponding-character

Arguments and Values:

character, corresponding-character—a character .

Description:

If character is a lowercase character , char-upcase returns the corresponding uppercase character . Otherwise, char-upcase just returns the given character.

If character is an uppercase character , char-downcase returns the corresponding lowercase character . Otherwise, char-downcase just returns the given character.

The result only ever differs from character in its code attribute; all implementation-defined attributes are preserved.

Examples:

(char-upcase #\a) → #\A 
(char-upcase #\A) → #\A 
(char-downcase #\a) → #\a 
(char-downcase #\A) → #\a 
(char-upcase #\9) → #\9 
(char-downcase #\9) → #\9 
(char-upcase #\@) → #\@ 
(char-downcase #\@) → #\@ 
;; Note that this next example might run for a very long time in 
;; some implementations if CHAR-CODE-LIMIT happens to be very large 
;; for that implementation. 
(dotimes (code char-code-limit) 
  (let ((char (code-char code))) 
    (when char 
      (unless (cond ((upper-case-p char) (char= (char-upcase (char-downcase char)) char)) ((lower-case-p char) (char= (char-downcase (char-upcase char)) char)) 
		    (t (and (char= (char-upcase (char-downcase char)) char) 
			    (char= (char-downcase (char-upcase char)) char)))) 
	(return char))))) 
→ NIL

Exceptional Situations:

Should signal an error of type type-error if character is not a character .

See Also:

upper-case-p, alpha-char-p, Section 13.1.4.3 (Characters With Case), Section 13.1.10 (Documen tation of Implementation-Defined Scripts)

Notes:

If the corresponding-char is different than character, then both the character and the corresponding char have case.

Since char-equal ignores the case of the characters it compares, the corresponding-character is always the same as character under char-equal.

Expanded Reference: char-upcase, char-downcase​

Converting letter case​

char-upcase converts a lowercase letter to uppercase. char-downcase converts an uppercase letter to lowercase.

(char-upcase #\a)
=> #\A
(char-upcase #\z)
=> #\Z
(char-downcase #\A)
=> #\a
(char-downcase #\Z)
=> #\z

Characters without case are returned unchanged​

Non-alphabetic characters and characters that are already the target case are returned as-is.

(char-upcase #\A)
=> #\A
(char-downcase #\a)
=> #\a
(char-upcase #\5)
=> #\5
(char-downcase #\5)
=> #\5
(char-upcase #\@)
=> #\@
(char-downcase #\Space)
=> #\Space

Case conversion round-trip​

Converting up then down (or down then up) always returns to the original character for characters with case.

(char-downcase (char-upcase #\a))
=> #\a
(char-upcase (char-downcase #\Z))
=> #\Z
(char-downcase (char-upcase #\5))
=> #\5

Building a simple string case converter​

char-upcase and char-downcase operate on individual characters. You can use them with map to convert entire strings.

(map 'string #'char-upcase "Hello, World!")
=> "HELLO, WORLD!"

(map 'string #'char-downcase "Hello, World!")
=> "hello, world!"

Swapping case of each character​

(defun swap-case (char)
  (cond ((upper-case-p char) (char-downcase char))
        ((lower-case-p char) (char-upcase char))
        (t char)))

(map 'string #'swap-case "Hello, World!")
=> "hELLO, wORLD!"