Yesterday I need a similar function (interleave, but don't stop when
the shortest seq ends), so I wrote it. I subsequently refactored the
program and didn't need it anymore, but here it is anyway. It could
probably be written more succinctly, but I followed the implementation
of core/interleave.
(defn interleave-all
"Like interleave, but stops when the longest seq is done, instead of
the shortest."
([c1 c2]
(lazy-seq
(let [s1 (seq c1)
s2 (seq c2)]
(cond
(and s1 s2) ; there are elements left in both
(cons (first s1) (cons (first s2)
(interleave-all (rest s1) (rest s2))))
s1 ; s2 is done
s1
s2 ; s1 is done
s2))))
([c1 c2 & colls]
(lazy-seq
(let [ss (filter identity
(map seq
(conj colls c2 c1)))]
(concat (map first ss)
(apply interleave-all (map rest ss)))))))
Usage (commas are mine):
user=> (interleave-all [1 2 3 4 5] '[a b c d e f g h i])
(1 a, 2 b, 3 c, 4 d, 5 e, f, g, h, i)
user=> (interleave-all [1 2 3 4 5] '[a b c d e f g h i] '[A B])
(1 a A, 2 b B, 3 c 4, d 5, e, f, g, h, i)
It is still lazy:
user=> (take 50 (interleave-all [1 2 3 4 5] '[a b c d e f g h i] '[A B
C D E F G H I J K L] (iterate inc 0)))
(1 a A 0, 2 b B 1, 3 c C 2, 4 d D 3, 5 e E 4, f F 5, g G 6, h H 7, i I
8, J 9, K 10, L 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23)
Alex
On 2009-11-19 20:13:28 -0500 John Harrop <[email protected]> wrote:
> On Thu, Nov 19, 2009 at 7:51 PM, Alex Osborne <[email protected]> wrote:
>
>> John Harrop wrote:
>>> This is just (sort (concat [1 2 3 4 5 6 7] [3 2 7])) though.
>>>
>>>
>>> I think he also wants the original order of the first input coll to
>>> be
>>> preserved, though. Sort wouldn't do that.
>>
>> Hmmm.. that's a pretty weird set of requirements. Usually a
>> multiset/bag is unordered. What happens if same elements aren't
>> grouped
>> together in the first input coll it, it just arbitrarily picks one of
>> their positions and moves them all together there? :-)
>>
>> If you're going to go to the trouble of making special multiset
>> functions you may as well create a new multiset collection type that
>> does the "right thing" when you conj to it and such. It is also
>> means
>> you can store things in a way that allows efficient implementation of
>> the operations, instead of just hoping the library user passes data
>> in
>> the right order or using slow but safe n^2 or n log n algorithms
>> everywhere.
>
>
> I don't disagree. We'll have to wait for the original poster to
> clarify his
> requirements.
>
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