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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