Multiple values in Lisp

Splits a lap time into minutes and seconds, then contrasts floor with truncate.

;; A form can return several values: VALUES packages them up and
;; MULTIPLE-VALUE-BIND takes them apart again.
(defun split-seconds (total)
  "Return TOTAL seconds as whole minutes and leftover seconds."
  (multiple-value-bind (minutes rest) (floor total 60)
    (values minutes rest)))

(multiple-value-bind (minutes seconds) (split-seconds 227)
  (format t "lap time ~d:~2,'0d~%" minutes seconds))

;; FLOOR rounds down and TRUNCATE rounds toward zero, so the two
;; disagree on negative numbers. NTH-VALUE picks out the remainder.
(format t "floor    ~a ~a~%" (floor -7 2) (nth-value 1 (floor -7 2)))
(format t "truncate ~a ~a~%" (truncate -7 2) (nth-value 1 (truncate -7 2)))

How it works

1. split-seconds calls (floor total 60) inside multiple-value-bind and returns both parts with values.
2. The caller unpacks them with another multiple-value-bind and prints ~d:~2,'0d.
3. (floor -7 2) and (truncate -7 2) differ on negatives; (nth-value 1 (floor -7 2)) pulls out the remainder.

Keywords and builtins used here

• defun
• floor
• format
• rest
• truncate
• values

The run, in numbers

Lines

14

Characters to type

670

Tokens

85

Three-star pace

70 tpm

At the three-star pace of 70 tokens a minute, this run takes about 73 seconds.

Type this snippet

Step 3 of 3 in Functions, step 12 of 27 in Language basics.

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