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# behaghel's solution

## to Leap in the Emacs Lisp Track

Published at Mar 07 2021 · 2 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)
"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``````

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

Solution Author
commented 69 days ago

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?

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

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