Another implementation:
(defn groups-by [f coll]
(apply merge-with into
(map (fn [k] (zipmap (f k) (repeat [k])))
coll)))
Or with ->>:
(defn groups-by [f coll]
(->> coll
(map (fn [k] (zipmap (f k) (repeat [k]))))
(apply merge-with into)))
Could use #(zipmap (f %) (repeat [%])) in map, of course.
In any case,
(groups-by #(range 1 (inc %)) [1 2 3])
returns
{3 [3], 2 [2 3], 1 [1 2 3]}
Cheers,
Michał
On 8 July 2013 21:52, Colin Yates <colin.ya...@gmail.com> wrote:
> Perfect, and bonus points for spotting my stupid "not slept in days"
> muppetry. The results I want are as you infer: {1 [1 2 3] 2 [1 2] 3 [3]}.
>
> Now all I have to do is understand your code - time for more coffee I think
> (not implying your code is difficult, rather the difficultly is a function
> of my ignorance).
>
>
>
> On 8 July 2013 20:46, Ben Wolfson <wolf...@gmail.com> wrote:
>>
>> You could do something like this, which is just a generalization of
>> group-by to the multiple value case (except that group-by actually uses
>> transients):
>>
>> user> (defn groups-by [f coll]
>> (reduce (fn [acc x]
>> (let [ks (f x)]
>> (reduce (fn [acc' k] (update-in acc' [k] #(conj (or
>> % []) x))) acc ks)))
>> {} coll))
>> #'user/groups-by
>> user> (groups-by #(range 1 (inc %)) [1 2 3])
>> {3 [3], 2 [2 3], 1 [1 2 3]}
>>
>> (This isn't the return value you say you want, but it seems to be the
>> right return value, unless I've misunderstood the problem: 1, 2, and 3 all
>> include 1, but only 3 includes 3.)
>>
>> You can define regular group-by in terms of this function as (defn
>> group-by [f coll] (groups-by (comp vector f) coll)).
>>
>>
>>
>> On Mon, Jul 8, 2013 at 12:25 PM, Colin Yates <colin.ya...@gmail.com>
>> wrote:
>>>
>>> Hi,
>>>
>>> I have a sequence of items and want to group them into categories, the
>>> value of which is a function of the item. This sounds exactly what group-by
>>> is after.
>>>
>>> The kicker is that the function could return multiple values. Imagine
>>> each item was a date range and I wanted to group them by the number of days
>>> in that date range.
>>>
>>> For example, (group-by #(range 1 (inc %)) [1 2 3] => {(1) [1] (1 2) [2]
>>> (1 2 3) [3]}. I want {1 [1] 2 [1 2] 3 [1 2 3]}.
>>>
>>> Any ideas?
>>>
>>> Thanks!
>>>
>>> Col
>>>
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>>
>>
>>
>>
>> --
>> Ben Wolfson
>> "Human kind has used its intelligence to vary the flavour of drinks, which
>> may be sweet, aromatic, fermented or spirit-based. ... Family and social
>> life also offer numerous other occasions to consume drinks for pleasure."
>> [Larousse, "Drink" entry]
>>
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