sort with :key in Lisp

Sorts an alist of cheese wheels by weight and by name, copying first because sort is destructive.

;; SORT is destructive: it rearranges the sequence you hand it, so
;; pass a COPY-LIST whenever the original still matters.
(defvar *wheels*
  '(("gouda" . 4200) ("brie" . 900) ("comte" . 12500)))

;; :KEY pulls the part to compare out of each element, and the
;; predicate then sees only that part.
(defvar *by-weight* (sort (copy-list *wheels*) #'< :key #'cdr))
(defvar *by-name* (sort (copy-list *wheels*) #'string< :key #'car))
(defvar *heaviest* (car (last *by-weight*)))

(format t "by weight ~a~%" (mapcar #'car *by-weight*))
(format t "by name   ~a~%" (mapcar #'car *by-name*))
(format t "heaviest  ~a at ~a g~%" (car *heaviest*) (cdr *heaviest*))

How it works

1. (sort (copy-list *wheels*) #'< :key #'cdr) orders by weight and protects *wheels* from being rearranged.
2. (sort (copy-list *wheels*) #'string< :key #'car) swaps in a string predicate and reads the other half of each cons.
3. (car (last *by-weight*)) takes the heaviest wheel off the sorted end.

Keywords and builtins used here

• car
• cdr
• defvar
• format
• last
• mapcar
• sort

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Step 3 of 3 in Higher-order, step 15 of 27 in Language basics.

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