Alists & plists in Lisp

Looks up an area code in an association list and a field in a property list, and says when each shape fits.

;; An association list is a list of key/value conses, and ASSOC hands
;; back the first pair whose key matches.
(defvar *area-codes*
  '((503 . "Portland") (206 . "Seattle") (415 . "San Francisco")))

(defvar *city* (cdr (assoc 206 *area-codes*)))
(defvar *missing* (assoc 999 *area-codes*))

;; A property list is a flat run of alternating keys and values that
;; GETF reads. Plists suit small records with fixed fields; alists
;; suit longer tables and keys that are not keywords.
(defvar *book* (list :title "Dune" :year 1965 :pages 412))
(defvar *year* (getf *book* :year))
(defvar *edition* (getf *book* :edition "first"))

(format t "206 is ~a, 999 is ~a~%" *city* *missing*)
(format t "~a (~a), ~a edition~%" (getf *book* :title) *year* *edition*)

How it works

1. (assoc 206 *area-codes*) returns the matching cons, so cdr gets the city and a miss returns nil.
2. (list :title "Dune" :year 1965 :pages 412) is a flat plist that (getf *book* :year) reads.
3. (getf *book* :edition "first") takes a default, and the comment contrasts fixed records with longer tables.

Keywords and builtins used here

• assoc
• cdr
• defvar
• format
• getf
• list

The run, in numbers

Lines

17

Characters to type

752

Tokens

99

Three-star pace

70 tpm

At the three-star pace of 70 tokens a minute, this run takes about 85 seconds.

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Step 2 of 3 in Collections, step 17 of 27 in Language basics.

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