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# mrksmrtn's solution

## to Leap in the Emacs Lisp Track

Published at Apr 06 2021 · 0 comments
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.el

;;; leap-test.el --- Tests for Leap exercise (exercism)

;;; Commentary:

;;; Code:
(load-file "leap.el")

(ert-deftest vanilla-leap-year ()
(should (leap-year-p 1996)))

(ert-deftest any-old-year ()
(should-not (leap-year-p 1997)))

(ert-deftest non-leap-even-year ()
(should-not (leap-year-p 1997)))

(ert-deftest century ()
(should-not (leap-year-p 1900)))

(ert-deftest exceptional-century ()
(should (leap-year-p 2000)))

(provide 'leap-test)
;;; leap-test.el ends here
;;; leap.el --- Leap exercise (exercism)

;;; Commentary:

;;; Code:

(defun leap-year-p (year)
"Returns t if year is a leap year and nil otherwise"
(or
(and (equal (mod year 4) 0)
(not (equal (mod year 100) 0)))
(equal (mod year 400) 0)))

(provide 'leap-year-p)
;;; leap.el ends here

## Community comments

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### mrksmrtn's Reflection

A good exercise in logic.