I implemented something like that. my goal was to avoid the predicate
being called twice, but apart from that it has some overhead.
(defn- apl-compr-
"the apl compress operator: 0 1 1 0 1 / 1 2 3 4 5 -> 2 3 5
implemented in clojure (apl-compr- [false true true false true] '(1
2 3 4 5)) -> (2 3 5)"
[bcoll coll mod]
(lazy-seq
(loop [b (seq bcoll) s (seq coll)]
(when (and b s)
(if (mod (first b))
(cons (first s) (apl-compr- (rest b) (rest s) mod))
(recur (next b) (next s)) )))))
(defn part1
[f s]
(let [m (map f s)]
[(apl-compr- m s identity) (apl-compr- m s not)] ))
There is also a similar function in erlang, partition, but it is eager
and recursive, so not really suited for clojure
On 23 Maj, 21:56, B Smith-Mannschott <[email protected]> wrote:
> On Sun, May 23, 2010 at 21:21, Michael Gardner <[email protected]> wrote:
> > I need to use a predicate to divide one list into two, one containing all
> > values for which the predicate is true, and the other with all remaining
> > values. I can do this easily by filtering twice, once with the predicate
> > and once with its complement, but is there some core or contrib function
> > that will do this more directly and efficiently? I'm not sure what the best
> > way to search for something like this would be.
>
> Yes and No.
>
> Yes: such a function exists: clojure.contrib.seq-utils/separate
>
> No: It's not as efficient as you'd like. It does just what you
> describe in your post: it filters twice.
>
> (defn separate
> "Returns a vector:
> [ (filter f s), (filter (complement f) s) ]"
> [f s]
> [(filter f s) (filter (complement f) s)])
>
> // Ben
>
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