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

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.
>* Robby
*