Hey Steven, here's a variation of my first example that, I think, gets
closer to what you're proposing (with maybe-add handled in-line):
(defn to-consolidated-map [parts]
(reduce (fn [h [k v]]
(assoc h k (+ (get h k 0) v)))
{}
parts))
Using assoc instead of update-in allows it to handle nils with a default
value to get. All core lib fns. WDYT?
Also, this:
(defn to-consolidated-map [parts]
(apply merge-with + (map (partial apply hash-map) parts)))
or this variant:
(defn to-consolidated-map [parts]
(->> parts (map (partial apply hash-map)) (apply merge-with +))
each use only core lib fns, which you say you're looking for. I find this
less accidental-complexity-ish than the implementations that use reduce
with a custom (anonymous) fn, which each require handling nil in some
fashion. WDYT?
On Sat, Aug 17, 2013 at 10:42 AM, Steven Degutis <[email protected]>wrote:
> 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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