Recursion

Fixme work in progress

Recursion is used greatly in Clojure to iterate through data and as anything can be treated as data in Clojure you can understand why.

The constructs available in Clojure for recursion include

  • loop and recur
  • Named function that calls itself
  • map, reduce, filter, remove, etc.
  • for

Recursively calling the same function

Lets iterate though a collection using recursion by writing a function that calls itself

(defn recursively-use-a-collection [collection]
  (println (first collection))
  (if (empty? collection)
    (print-str "no more values to process")
    (recursively-use-a-collection  (rest collection))))

(recursively-use-a-collection [1 2 3])

Lets take this recursive approach to create a function that can tell us the length of a collection (list or vector)

We define a function that takes a collection of an argument. The collection is tested to see if it is empty and if so a zero value is returned. If the collection is not empty, then we

(defn length [collection]
  (if (empty? collection)
    0
    (+ 1 (length (rest collection)))))
;; => #'clojure-through-code.01-basics/length

If we call the length function with an empty collection, then the empty? condition will return true and the if expression will evaluate the first expression, 0, returning 0.

(length [])
;; => 0

If we call the length function with a collection containing 3 values, then the empty? function will return false and the if function will evaluate the second expression.

The second expression starts with a simple counter, using the + function and the value one

(length [0 1 2])
;; => 3
(+ 1 (length [1 2]))
(+ 1 (+ 1 (length [2])))
(+ 1 (+ 1 (+ 1 (length []))))
(+ 1 (+ 1 (+ 1 0)))

(length (range 24))
;; => 24

(defn length [collection] (kk))

Further recursion examples

Other functions to consider

  • every
  • accumulating / accumulative
  • keep

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