Wednesday, May 04, 2011

Clojure: Get All Nested Map Values (N levels deep)

I recently needed to pull all the values from a nested map. The map I was working with was designed to give easy access to data like so (formatted):
(def my-data {"Jay"
{:name "Jay", :language "clojure", :enjoys true},
{:name "Jay", :language "ruby", :enjoys true}}
{:name "Jon", :language "java", :enjoys true}}} )
user=> (get-in my-data ["Jon" "java"])
{:name "Jon", :language "java", :enjoys true}
user=> (get-in my-data ["Jay" "ruby"])
{:name "Jay", :language "ruby", :enjoys true}
This worked for all of the ways the data was accessed, until someone asked for a list of all people and all languages.

I needed a function that grabbed all the values nested to a point in the map (grabbing only the values instead of the deepest map wouldn't have provided valuable information). I created the following function that should grab all the values up to the nesting level specified.
(defn nth-vals* [a i m]
(if (and (map? m) (> i 0))
(reduce into a (map (fn [v] (nth-vals* a (dec i) v)) (vals m)))
(conj a m)))

(defn nth-vals [i m]
(if (nil? m)
(nth-vals* [] i m)))
The nth-vals function can be used as the following example shows. (assuming the same map for my-data) (formatted)
user=> (nth-vals 2 my-data)
[{:name "Jay", :language "clojure", :enjoys true}
{:name "Jay", :language "ruby", :enjoys true}
{:name "Jon", :language "java", :enjoys true}]
For reference, here's what's returned if we reduce the map all the way to it's values.
user=> (nth-vals 3 my-data)
["Jay" "clojure" true "Jay" "ruby" true "Jon" "java" true]
That list of values may be helpful for someone else, but it wouldn't have solved our current problem.

And, if you're interested, here's what's returned if you ask for 1 level deep of values. (formatted, again)
user=> (nth-vals 1 my-data)
[{"clojure" {:name "Jay", :language "clojure", :enjoys true},
"ruby" {:name "Jay", :language "ruby", :enjoys true}}
{"java" {:name "Jon", :language "java", :enjoys true}}]
I wouldn't at all be surprised if something like this already exists in Clojure core, but I haven't found it yet.

1 comment:

  1. Justin10:44 PM

    'for' can be useful for unpacking nested structures:

    ;; same as (nth-vals 2 my-data)
    (for [[_ vs] my-data [_ vvs] vs] vvs)

    ;; same as (nth-vals 3 my-data)
    (for [[_ vs] my-data [_ vvs] vs [_ vvvs] vvs] vvvs)


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