How to Add a New Node to Hash-Map Graph in Clojure?

Question

I have the following array-map created in Clojure.

{:node 7, :children [{:node 8, :children []} {:node 6, :children []} {:node 23, :children {}} {:node 43, :children []}]}

How do i go about adding elements into this, running the following code

(def tree (assoc-in tree [:node] 12))

gives me

{:node 12, :children [{:node 8, :children []} {:node 6, :children []} {:node 10, :children {}} {:node 13, :children []} {:node 28, :children []}]}`

and running

(def tree (assoc-in tree [:node :children] 12))

gives me the following error message. how do i add elements into the children sections on the array-map

Exception in thread "main" java.lang.ClassCastException: java.lang.Long cannot be cast to clojure.lang.Associative,

Answer 1

Let's assign our tree to t:

(def t {:node 7, 
        :children [{:node 8, :children []} 
                   {:node 6, :children []} 
                   {:node 23, :children []} 
                   {:node 43, :children []}]})

To add a new child note:

(defn add-child [tree node]
  (assoc-in tree 
            [:children] 
            (conj (:children tree) node)))

(add-child t :foo)
;; => {:node 7, 
;;     :children [{:node 8, :children []} 
;;                {:node 6, :children []} 
;;                {:node 23, :children []} 
;;                {:node 43, :children []} 
;;                :foo]}

Of course this is not exactly what we want.

(defn make-node [value children] 
  (let [c (into [] children)] 
    {:node value 
     :children c}))

(make-node 5 nil)
;; => {:node 5, :children []}

(make-node 5 [(make-node 3 nil) (make-node 7 nil)])
;; => {:node 5, 
;;     :children [{:node 3, :children []} 
;;                {:node 7, :children []}]}

Building trees is now a matter of combining make-node & add-child.

If you want to work on deep hierarchies, I suggest using a zipper.