Vectors & arrays in Lisp

Reads daily temperatures from a #(...) literal and writes into an array built by make-array.

;; #(...) reads as a simple vector, and AREF indexes it from zero.
(defvar *highs* #(61 64 58 72 70))
(defvar *monday* (aref *highs* 0))
(defvar *thursday* (aref *highs* 3))

;; MAKE-ARRAY builds one at run time; :INITIAL-ELEMENT fills it, and
;; SETF AREF writes a slot. Never write into a #(...) literal.
(defvar *lows* (make-array 5 :initial-element 48))
(setf (aref *lows* 3) 55)

;; LENGTH and ELT accept any sequence, so the same call reads a
;; vector, a list, or a string.
(format t "~a days, monday ~a, thursday ~a/~a~%"
        (length *highs*) *monday* *thursday* (aref *lows* 3))
(format t "~a ~a ~a~%" (elt *highs* 1) (elt '(:rain :sun) 1)
        (elt "cloudy" 0))

How it works

1. (aref *highs* 3) indexes the literal vector from zero.
2. (make-array 5 :initial-element 48) builds a writable vector that (setf (aref *lows* 3) 55) updates.
3. length and elt work across sequences, so (elt '(:rain :sun) 1) and (elt "cloudy" 0) read a list and a string.

Keywords and builtins used here

• aref
• defvar
• elt
• format
• length
• setf

The run, in numbers

Lines

16

Characters to type

662

Tokens

93

Three-star pace

75 tpm

At the three-star pace of 75 tokens a minute, this run takes about 74 seconds.

Type this snippet

Step 3 of 3 in Collections, step 18 of 27 in Language basics.

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