# [plt-scheme] Is there a set data-type?

 From: Robby Findler (robby at eecs.northwestern.edu) Date: Fri Feb 5 11:05:29 EST 2010 Previous message: [plt-scheme] Is there a set data-type? Next message: [plt-scheme] Is there a set data-type? Messages sorted by: [date] [thread] [subject] [author]

```On Fri, Feb 5, 2010 at 10:03 AM, Paulo J. Matos <pocmatos at gmail.com> wrote:
> Robby Findler wrote:
>>
>> On Fri, Feb 5, 2010 at 9:54 AM, Paulo J. Matos <pocmatos at gmail.com> wrote:
>>>
>>> I guess the characteristic function approach involves using lambdas to
>>> represent a set... :-/ I am worried with efficiency but I will give it a
>>> try.
>>
>> FWIW, there are some sets that are far more efficiently represented
>> using functions than other data structures if you can write the
>> function directly (eg, (lambda (x) (< 0 x 100000)) is far better than
>> checking membership in a 100000 element list), but when you pile up a
>> series of unions and intersections and things like that you end up
>> getting something like linear work in the original construction of the
>> set. If you want to do better maybe the right thing is to considering
>> developing a little PL and interpreting it (with special purpose
>> optimizations in there for the things you expect to happen
>> frequently).
>>
>> Just a thought, of course. :)
>>
>
> Thanks for the thoughts on performance.
> Another thing I will have to think about is computing the cardinality of the
> set which might return +inf.0 of course. I am not even sure this
> representation allows the computation of cardinality in a finite amount of
> time.

You could use objects that have to implement various methods, member?,
cardinality, union, enumerate (to debug; for infinite sets it would
just stop after a while).

Robby

```

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