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# utraz's solution

## to Leap in the Emacs Lisp Track

Published at Mar 02 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)
"Check if given year is a leap year"
(interactive "nWhat year do you want to check? ")
(let ((year (float year)))
(cond ((if (= (/ year 400) (truncate (/ year 400)))
(message "Yes, %d is a leapyear" year)))
((if (= (/ year 100) (truncate (/ year 100)))
(message "No, %d is not a leapyear" year)))
((if (= (/ year 4) (truncate (/ year 4)))
(message "Yes, %d is a leapyear" year)))
((message "No, %d is not a leapyear" year)))))

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

### What can you learn from this solution?

A huge amount can be learned from reading other peopleâ€™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?