Etudes for ClojureScript

Chapter 3. Lists, Vectors, and Higher-Order Functions

In this chapter, you will work with lists and vectors, along with the map, filter, and reduce functions. All of these take functions as one of their arguments, and are thus higher-order functions.

Étude 3-1: Move the Zeros

This is a quick warm-up étude. Given a list of integers that have zeros interspersed throughout, move all the zeros to the end. Name the function move-zeros; it accepts a list as an argument and returns a new list with the zeros at the end. I saw the problem at this page, solved in Java, and wondered if I could do it in ClojureScript. Answer: yes, I could. And so can you. Hint: filter is useful. After I solved it, I realized just how much my thinking about functional programming had changed the way I look at imperative code. You may have the same experience:

move-zeros.core=> (move-zeros [1 0 0 2 0 3 0 4 5 0 6])
(1 2 3 4 5 6 0 0 0 0 0)

See a suggested solution: “Solution 3-1”.

Étude 3-2: More List Manipulation

Write a function named ordinal-day that takes a day, month, and year as its three arguments and returns the ordinal (Julian) day of the year. Bonus points if you return zero for invalid dates such as 29-02-2015 or 40-40-2015. Don’t worry about handling dates before the year 1584 correctly.

You will need to know if a year is a leap year or not. I’ll give you that one for free:

(defn leap-year? "Return true if given year is a leap year; false otherwise" [year] (or (and (= 0 (rem year 4)) (not= ...