Building lists in Lisp

Growing a playlist with push, then straightening it out with reverse and append.

;; push grows a list at the front, so the additions come out backward
(defparameter *playlist* (list "aja" "deacon blues"))
(push "peg" *playlist*)
(push "home at last" *playlist*)

;; reverse undoes that order and append joins on a closing track
(defparameter *encore* (append (reverse *playlist*) (list "josie")))

(defun announce (tracks)
  (format t "~a tracks, opening with ~a~%" (length tracks) (first tracks))
  (dolist (track tracks)
    (format t "  - ~a~%" track)))

(announce *encore*)

How it works

1. Each push adds to the front of *playlist*, so the tracks stack up in reverse.
2. (append (reverse *playlist*) (list "josie")) restores the order and adds a closing track.
3. announce reports length and first, then walks the whole list with dolist.

Keywords and builtins used here

• append
• defparameter
• defun
• dolist
• first
• format
• length
• list
• push
• reverse

The run, in numbers

Lines

14

Characters to type

488

Tokens

73

Three-star pace

75 tpm

At the three-star pace of 75 tokens a minute, this run takes about 58 seconds.

Type this snippet

Step 2 of 3 in Lists & conses, step 5 of 27 in Language basics.

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Building lists in other languages

• Haskell