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I want to write a function for a zipper that removes all right siblings of a node while staying at the same location.

(defn remove-all-rights-1 [loc] (if (zip/right loc) (recur (zip/remove (zip/right loc))) loc))

The problem here is that remove returns the location that would have preceded the current node in DFS.

Therefore the following example...

(-> (clojure.zip/vector-zip [1 [[2] 3]]) (zip/down) (zip/right) (zip/down) (remove-all-rights-1) (zip/replace :x) (zip/root))

...gives [1 [[:x]]] instead of [1 [:x]] because zip/remove jumped to the bottom leaf instead of just going back left.

How should I remove the right siblings without also changing location in the tree? Thanks in advance!

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3 Answers 3

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Generalizing akond's answer gave the following solution:

(defn remove-all-rights "Removes all right siblings. Stays at original location." [loc] (let [parent-loc (zip/up loc) |lefts| (inc (count (zip/lefts loc)))] (->> (zip/make-node loc (zip/node parent-loc) (take |lefts| (zip/children parent-loc))) (zip/replace parent-loc) (zip/down) (zip/rightmost))))

The main idea is to construct a copy of the parent node where the collection of children does not contain the right siblings.

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(letfn [(kill-right [loc] (let [lost (zip/rights loc) parent (-> loc zip/up zip/node) node (into (empty parent) (take (- (count parent) (count lost)) parent))] (-> loc zip/up (zip/replace node) zip/down zip/rightmost)))] (-> (clojure.zip/vector-zip [1 [[2] 3]]) zip/down zip/right zip/down kill-right (zip/replace :x) zip/root))
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This can be easily accomplished with the Tupelo Forest library:

(dotest (with-forest (new-forest) (let [edn-orig [1 [[2] 3]] root-hid (add-tree (edn->tree edn-orig)) hid (find-hid root-hid [::tf/list ::tf/list]) subtree-edn-orig (-> hid hid->tree tree->edn) >> (kids-update hid butlast) subtree-edn-final (-> hid hid->tree tree->edn) edn-final (-> root-hid hid->tree tree->edn)] (is= subtree-edn-orig [[2] 3]) (is= subtree-edn-final [[2]]) (is= edn-final [1 [[2]]] ))))

The tree created has nodes with :tag values of :tupelo.forest/list at the first & second levels:

 (is= (hid->bush root-hid) [{:tag :tupelo.forest/list, :tupelo.forest/index nil} [#:tupelo.forest{:value 1, :index 0}] [{:tag :tupelo.forest/list, :tupelo.forest/index 1} [{:tag :tupelo.forest/list, :tupelo.forest/index 0} [#:tupelo.forest{:value 2, :index 0}]]]] )

An HID is a pointer to a tree node, so root-hid points to the root node of the tree and hid points to the subtree [[2] 3]. After we remove the right-most node 3, hid points to the subtree [[2]].

For the child nodes (kids), we use the butlast function to remove the right-most, then convert the data from a forest/tree format back into EDN.

See the README here, and the API docs here. There are also many live code examples here. Also see the Clojure Conj video.

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