mapcar & reduce in Lisp

Adds tax to a list of cart prices with mapcar, then totals them with reduce.

;; MAPCAR applies a function to every element and collects the answers.
(defvar *cart* '(2.50 3.75 1.20 4.05))

(defun with-tax (price)
  "Return PRICE plus eight percent sales tax."
  (* price 1.08))

;; The function can be a named one or a lambda written in place.
(defvar *taxed* (mapcar #'with-tax *cart*))
(defvar *cents* (mapcar (lambda (price) (round (* 100 price))) *cart*))

;; REDUCE folds a list into a single value; :INITIAL-VALUE seeds the
;; fold, so an empty cart still adds up to zero.
(defvar *total* (reduce #'+ *taxed* :initial-value 0))

(format t "cents ~a~%" *cents*)
(format t "total ~,2f~%" *total*)

How it works

1. (mapcar #'with-tax *cart*) passes the named function defined just above it.
2. (mapcar (lambda (price) (round (* 100 price))) *cart*) uses an inline lambda instead.
3. (reduce #'+ *taxed* :initial-value 0) folds the list to one number and covers the empty case.

Keywords and builtins used here

• defun
• defvar
• format
• lambda
• mapcar
• reduce
• round

The run, in numbers

Lines

17

Characters to type

619

Tokens

84

Three-star pace

70 tpm

At the three-star pace of 70 tokens a minute, this run takes about 72 seconds.

Type this snippet

Step 1 of 3 in Higher-order, step 13 of 27 in Language basics.

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