99 Clojure Problems – 4: Find the Number of Elements of a List

Example:

(deftest p04-test
  (is (= 3 (my-count (list 1 2 3)))))

Solution

There is the built-in count that solves this problem.

I implemented two possible solutions for this problem:

Source 1. This is again a variation on the theme of using the loop/recur construct (that I also used in the previous problems) with an additional parameter binding aka accumulator to keep track of the state—in this case the number of elements encountered so far.

Source 2 I'm not entirely sure if this was really my own idea or if I saw it or something similar somewhere—I should have made a note. This solution uses reduce, a standard higher-order function found in most functional languages. Clojure's implementation allows for a val argument that we use to accumulate the count by incrementing by one for each element in the collection passed as the third argument.

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