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.

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.

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

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

```
;; 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
(add-to-load-path (dirname (current-filename)))
(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))))
```

A huge amount can be learnt from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

- What compromises have been made?
- Are there new concepts here that I could read more about to develop my understanding?

## Community comments

4d47saystidy using the zero? function