Common-Lisp: How to sort the characters of a string?

Question

I know how to do this in every other language that I know, but I'm just starting Lisp and not quite getting it. My idea of

• make a list of characters
• convert to ascii values
• sort
• convert back to characters
• convert back to a string

Seems heavy-handed. Is there some better way to do this? I'm trying to write a function that, given a string, returns a string with the letters sorted. So, for example:

gate => aegt
house => ehosu
door => door

This routine will be used as part of an anagram-finder.

Thanks!

Answer 1

In Common Lisp strings are sequences, and sort works on any sequence type, so it will do the trick.

Here's an example:

(let ((the-string (copy-seq "this is the string")))
  (sort the-string #'char-lessp))
;; => "   eghhiiinrsssttt"

And here's the Hyperspec entry for sort and stable-sort. Just pick your predicate (the second argument to sort) to get your desired sort order.

Note that I used copy-seq in the example because sort is destructive - it modified the string in-place.

Answer 2

The sort function takes a sequence, which a string already is, so your only problem is finding the right comparison function. Characters are not numbers, so you should use the character comparison functions, e.g. char>:

* (sort (copy-seq "hello") #'char>)

"ollhe"

Answer 3

A string is a sequence of characters. sort sorts sequences so it sorts a string like it sorts a list:

(setq tester (copy-seq "lkjashd")) =>  "lkjashd"
(stable-sort tester #'char-lessp) =>  "adhjkls"
tester => "adhjkls" ; NB: it mutates the string!