ðŸŽ‰ Exercism Research is now launched. Help Exercism, help science and have some fun at research.exercism.io ðŸŽ‰

## 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:

(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)
"return t if leap year, nil otherwise"
(setq is_leap nil)
(if (eq (mod year 4) 0)
(setq is_leap t)
)
(if (eq (mod year 100) 0)
(setq is_leap nil)
)

(if (and (eq (mod year 4) 0)
(not is_leap)
(eq (mod year 400) 0))
(setq is_leap t))
is_leap
)

(defun leap-year-p (year)
"Steal from python"
(and (eq (mod year 4) 0)
(or (not (eq (mod year 100) 0))
(eq (mod year 400) 0)))
)
(leap-year-p 1999)

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