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Avatar of behaghel

behaghel's solution

to Leap in the Emacs Lisp Track

Published at Mar 07 2021 · 2 comments
Test suite

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

Submitting Incomplete Solutions

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


;;; 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)
  "Assert if YEAR is a leap year."
  (and (= 0 (% year 4))
       (or (= 0 (% year 400))
        (not (= 0 (% year 100))))))

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

Community comments

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Avatar of asadali

Where did you find that you could use = and % instead of eq and mod? I couldn't find that during my googling.

Avatar of behaghel

Hi asadali,

Emacs may be the one topic I recommend not googling about as a first move :) Its self-documentation system is really worth investing on. Particularly if you're trying to become proficient with a subject like Elisp.

  1. Go into the Info mode with C-h i
  2. Then m and start typing Elisp then select that entry
  3. Then m again with 'Numbers'

In this chapter, you have a section on comparisons. That's where I learnt about '='. And two sections later, it talks about arithmetic operations and that's where you can find about '%'.

I found familiarising myself with the first topics under the main Elisp node extremely valuable. I am not saying learning in depth, just knowing they are there and flashing the right section up on demand when I need it.

I hope this helps.

What can you learn from this solution?

A huge amount can be learned 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 you could read more about to improve your understanding?