R devel - May 2008 - Multiple Intersections

Hi all,

I don't know if this is the correct venue for this question, but I am sure
that someone will correct me if I am in the wrong list.

I have been searching throughout R for a function that can find the
intersection of multiple sets of "things". Say for instance, I have a
list
of $n$ character vectors and would like to find the intersection of all $k$
subsets. I don't believe that there is such a function to do this (or am I
wrong?). It is a pretty easy to encode such a function...there was an e-mail
about how a recursive function to intersect an arbitrary number of sets
which is elegant and useful (sorry I forgot the person's name who wrote the
2 line function).

My question is two-fold:

1. If such a function already exists, what is it called?
2. If such a function does not exists, it is worthwhile to encode it (i.e.
can I send my code to someone?).

Cheers,
--tony

	[[alternative HTML version deleted]]

Hi Tony -- off-list, as I think you're looking for something else...

"Tony Chiang" <tchiang at fhcrc.org> writes:
> Hi all,
>
> I don't know if this is the correct venue for this question, but I am
sure
> that someone will correct me if I am in the wrong list.
>
> I have been searching throughout R for a function that can find the
> intersection of multiple sets of "things". Say for instance, I
have a list
> of $n$ character vectors and would like to find the intersection of all $k$
> subsets. I don't believe that there is such a function to do this (or
am I
> wrong?). It is a pretty easy to encode such a function...there was an
e-mail
> about how a recursive function to intersect an arbitrary number of sets
> which is elegant and useful (sorry I forgot the person's name who wrote
the
> 2 line function).
Maybe this is what you were thinking of?

Intersect=function(x, ...) {
    if (length(list(...))>1)
        Intersect(x, Intersect(...))
    else
        intersect(x, ...)
}
> Intersect(letters[1:5], letters[3:6], letters[4:7])
[1] "d" "e"
> l=list(letters[1:5], letters[3:6], letters[4:7])
> do.call("Intersect", l)
[1] "d" "e"

Also
> Reduce(intersect, l, letters)
[1] "d" "e"

This won't be efficient for a large set of 'things', and I'm not
getting how n and k fit in -- 'all k subsets' implies all elements of
n?

Martin
> My question is two-fold:
>
> 1. If such a function already exists, what is it called?
> 2. If such a function does not exists, it is worthwhile to encode it (i.e.
> can I send my code to someone?).
>
> Cheers,
> --tony
>
> 	[[alternative HTML version deleted]]
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
-- 
Martin Morgan
Computational Biology / Fred Hutchinson Cancer Research Center
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