Wow. Sorry for the awful formatting. Re-pasting it from Google Groups
(instead of email) to fix it:
(def a [[: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))
{}
a)
;; => {:b 2, :a 4}
-Steven
On Saturday, August 17, 2013 3:40:23 AM UTC-5, Steven Degutis wrote:
>
> 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 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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