# 4d47's solution

## to Leap in the Scheme Track

Published at Jul 13 2018 · 1 comment
Instructions
Test suite
Solution

Given a year, report if it is a leap year.

The tricky thing here is that a leap year in the Gregorian calendar occurs:

``````on every year that is evenly divisible by 4
except every year that is evenly divisible by 100
unless the year is also evenly divisible by 400
``````

For example, 1997 is not a leap year, but 1996 is. 1900 is not a leap year, but 2000 is.

If your language provides a method in the standard library that does this look-up, pretend it doesn't exist and implement it yourself.

## Notes

Though our exercise adopts some very simple rules, there is more to learn!

For a delightful, four minute explanation of the whole leap year phenomenon, go watch this youtube video.

## Source

JavaRanch Cattle Drive, exercise 3 http://www.javaranch.com/leap.jsp

## Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

### leap-test.scm

``````;; Load SRFI-64 lightweight testing specification
(use-modules (srfi srfi-64))

;; Suppress log file output. To write logs, comment out the following line:
(module-define! (resolve-module '(srfi srfi-64)) 'test-log-to-file #f)

;; Require module
(use-modules (leap-year))

(test-begin "leap-year")

(test-eqv "vanilla-leap-year"
#t
(leap-year? 1996))

(test-eqv "any-old-year"
#f
(leap-year? 1997))

(test-eqv "non-leap-even-year"
#f
(leap-year? 1998))

(test-eqv "century"
#f
(leap-year? 1900))

(test-eqv "exceptional-century"
#t
(leap-year? 2400))

(test-end "leap-year")``````
``````(define-module (leap-year)
#:export (leap-year?))

(define (divisible? a b)
(zero? (modulo a b)))

(define (leap-year? y)
(and
(divisible? y 4)
(or
(not (divisible? y 100))
(divisible? y 400))))``````