Find Value of Specific Key in Nested Map

Question

In Clojure, how can I find the value of a key that may be deep in a nested map structure? For example:

(def m {:a {:b \"b\"
            :c \"c\"
            :d {         


        

Answer 1

Clojure offers tree-seq to do a depth-first traversal of any value. This will simplify the logic needed to find your nested key:

(defn find-nested
  [m k]
  (->> (tree-seq map? vals m)
       (filter map?)
       (some k)))

(find-nested {:a {:b {:c 1}, :d 2}} :c)
;; => 1

Also, finding all matches becomes a matter of replacing some with keep:

(defn find-all-nested
  [m k]
  (->> (tree-seq map? vals m)
       (filter map?)
       (keep k)))

(find-all-nested {:a {:b {:c 1}, :c 2}} :c)
;; => [2 1]

Note that maps with nil values might require some special treatment.

---

Update: If you look at the code above, you can see that k can actually be a function which offers a lot more possibilities:

• to find a string key:

  (find-nested m #(get % "k"))
  

• to find multiple keys:

  (find-nested m #(some % [:a :b]))
  

• to find only positive values in maps of integers:

  (find-nested m #(when (some-> % :k pos?) (:k %)))