在Lisp中对char列表进行排序

Question

Is there a way to sort a list made of only characters in alphabetic order (from "a" to "z"), such as: (list cmayk) => (list ackmy)?

I didn't found a sort like that, so i thought this could solve the problem:

cl-prompt> (setq q (mapcar 'string '(c m a y k))) => ("C" "M" "A" "Y" "K")
cl-prompt> (sort q 'string<) => ("A" "C" "K" "M" "Y")

At this point i'm stuck, because by using the char converter the output will not be the list i want. Example:

char (string 'a) 0 => #\A

And i will have a list made of: (#\\A #\\C #\\K #\\M #\\Y), which is not the result i want.

Answer 1

One can convert characters to their ASCII numbers, sort the numbers and convert back to characters. Following is code in Racket, a Lisp/Scheme derivative:

(define l (list #\c #\m #\a #\y #\k))
(map integer->char
     (sort (map char->integer l) <))

Output:

'(#\a #\c #\k #\m #\y)

Answer 2

In the case of (cmayk) , you have a list of symbols. In the case of (#\\A #\\C #\\K #\\M #\\Y) , you have a list of characters.

If you want to sort a list of symbols according to the symbol names, you can do (I am assuming the list is bound to the variable foo below):

(setq foo (sort foo #'string< :key #'symbol-name))

Note that the :key #'symbol-name could be argued to be superfluous, as string< actually operates on "string designators", but it is much clearer to a human reader that you are indeed intending to sort the list based on symbol names if you say so explicitly.