Thanks Jason.
merge-with seems to be made to support a function like this, I wonder
where is the slowdown coming from? Is apply slow?
I named your version seq-to-multimap2. The timing results are below:
------------
user> (def a (reverse (take 100000 (iterate (fn [x] (rand-int 100))
1))))
#'user/a
user> (time (def b (seq-to-multimap2 a identity)))
"Elapsed time: 443.774747 msecs"
#'user/b
user> (time (def b (seq-to-multimap a identity)))
"Elapsed time: 146.814412 msecs"
#'user/b
------------
user> (def a (reverse (take 100000 (iterate (fn [x] (rand-int 10))
1))))
#'user/a
user> (time (def b (seq-to-multimap a identity)))
"Elapsed time: 70.812128 msecs"
#'user/b
user> (time (def b (seq-to-multimap2 a identity)))
"Elapsed time: 284.089358 msecs"
#'user/b
------------
user> (def a (reverse (take 100000 (iterate (fn [x] (rand-int 10000))
1))))
#'user/a
user> (time (def b (seq-to-multimap2 a identity)))
"Elapsed time: 530.553921 msecs"
#'user/b
user> (time (def b (seq-to-multimap a identity)))
"Elapsed time: 240.03259 msecs"
#'user/b
Regards,
Boris
On Apr 28, 7:28 pm, Jason Wolfe <jawo...@berkeley.edu> wrote:
> (defn seq-to-multimap
> "takes a sequence s of possibly repeating elements
> and converts it to a map, where keys are obtained by applying key-
> fn
> to elements of s and values are sequence of all elements of s with
> the particular key"
> [s key-fn]
> (apply merge-with concat
> (map (fn [x] {(key-fn x) [x]}) s)))
>
> Here's a more concise, but probably significantly slower version ....
>
> -Jason
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