Clojure: Implementing the assoc-in function

Question

Chapter 5, Exercise 3 in Clojure for the Brave and True requires:

Implement the assoc-in function. Hint: use the assoc function and define its parameters as [m [k & ks] v].

Although I have found this solution (see lines 39-54), I wondered if there was a different way of doing it. When working on the previous exercise, I found this very clear answer by jbm on implementing the comp function to be very helpful.

I've been trying to reduce a partial assoc over a conjoined list of keys and apply the returned function to the final value:

(defn my-part-assoc [m k]
  (partial assoc m k))

((reduce my-part-assoc {} [:one :two :three]) "val")

Needless to say, this doesn't work. I am new to Clojure and functional programming and fear my very basic understanding of reduce is leading me down the wrong path. Please can someone provide a more concise answer?

Ooberdan · Accepted Answer · 2016-12-11 07:37:58Z

4

Shortly after posting, I found this, which gets the following definition from the Clojure GitHub repo:

(defn assoc-in
  ;; metadata elided
  [m [k & ks] v]
  (if ks
    (assoc m k (assoc-in (get m k) ks v))
    (assoc m k v)))

answered Dec 11, 2016 at 7:37

Ooberdan

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2 Comments

Ooberdan Over a year ago

I'm not sure if this constitutes a duplicate. Although I found my answer on Stack Overflow, I've left my question active as it's quite specific and may help others in same position.

leetwinski Over a year ago

notice, that unlike the core version, yours would fail with this: (assoc-in {} [:a :b :c] 1)...

jackdbd · Accepted Answer · 2022-07-16 07:37:40Z

1

I think Ooberdan's idea works in case of:

(defn my-as-in 
  "assoc val in nested map with nested key seq"                                
  [mp [ky & kysq] val]                                                          
  (if kysq                                                                     
    (assoc mp ky (my-as-in (get mp ky) kysq val))                                  
    (assoc mp ky val)))   

(my-as-in {} [:is :this :hello] 1)

gives {:is {:this {:hello 1}}} same as assoc-in...

Seems neat and idiomatic Clojure and it showed me the way after I got lost in reduce or multi arity type solutions.

edited Jul 16, 2022 at 7:37

jackdbd

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answered Jul 9, 2022 at 20:43

StanTheBear

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Comments

Rich Wood · Accepted Answer · 2021-02-23 18:36:08Z

0

Here is a solution that doesn't use "assoc-in" which seems to be a requirement:

(defn my-assoc-in
  [m [k & ks] v]
  (if (= (count ks) 0)
    (assoc m k v)
    (let [ordered-ks (reverse ks)
          first-m (get-in m (butlast (cons k ks)))]
      (assoc m k (reduce
                  (fn [curr-m next-k] (assoc {} next-k curr-m))
                  (assoc first-m (first ordered-ks) v)
                  (rest ordered-ks))))))```

edited Feb 23, 2021 at 18:36

answered Feb 23, 2021 at 17:01

Rich Wood

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1 Comment

tastyminerals Over a year ago

This is needlessly complicated and definitely not something the author of the book expects a beginner to achieve :(

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