At first I came up with the same solution as your second one. But I
couldn't help but feel that it wasn't descriptive enough. It felt too
incidental-complexity-ish.
To me, (into {} the-map) seems mostly right. But obviously it would just
squash the values you need. So I figured it should just be modified a bit.
Seeing as how it's really short for reduce conj, I figured conj was the
place to swap out.
Here's what I came up with:
(def the-map [[:a 1] [:b 2] [:a 3]])
(defn add-maybe [& nums]
(->> nums
(remove nil?)
(reduce +)))
(reduce (fn [m [k v]]
(update-in m [k] add-maybe v))
{}
the-map)
;; => {:b 2, :a 4}
But I don't really like the add-maybe function. It seems like I'm probably
missing an opportunity for a core-lib function that I'm forgetting about. I
really just wanted to have access to the original value, so I could do (or
orig-value 0) and then just use + instead of add-maybe.
-Steven
On Fri, Aug 16, 2013 at 11:57 PM, David Chelimsky <dchelim...@gmail.com>wrote:
> I've got a vector of 2-element vectors e.g. [[:a 1] [:b 2]] where the
> first val of any vec might appear in another vec e.g. [[:a 1] [:b 2] [:a
> 3]]. I need a fn that will consolidate this into a hash-map with the vals
> consolidated e.g.
>
> (to-consolidated-map [[:a 1] [:b 2] [:a 3]])
> ; {:a 4 :b 2}
>
> I've got two candidate implementations and I'm curious which you like
> better and why, or if I'm missing a better way:
>
> (defn to-consolidated-map [parts]
> (reduce (fn [h [k v]]
> (if (contains? h k)
> (assoc h k (+ (k h) v))
> (assoc h k v)))
> {} parts))
>
> (defn to-consolidated-map [parts]
> (->> parts
> (group-by first)
> (map (fn [[k v]] [k (->> v (map last) (reduce +))]))))
>
> TIA,
> David
>
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