[R] intersect() without discarding duplicates?

[David Winsemius]

On May 20, 2010, at 5:58 PM, Jonathan wrote:

> Hi all,
>   The ?intersect entry kindly points out that it discards duplicate
> entries.  I'm looking, however, to get the intersection while KEEPING
> duplicate entries, and there are no instructions on how to  
> accomplish this
> using intersect().
>
> Does anybody have any idea how this might be done, or am I going to  
> need to
> program something from scratch (something like ordering the vectors  
> and then
> looping through them)?
>
>
>
> ex:
>
>> a <- c(2,4,2,3)
>> b<-c(6,6,5,2,2,8,4)
>> intersect(a,b)
> [1] 2 4

 > b %in% a
[1] FALSE FALSE FALSE  TRUE  TRUE FALSE  TRUE

# Now use logical indexing on "b"

 > b[b %in% a]
[1] 2 2 4

>
>
>
> I'd hope the answer to be 2 2 4.
>
> Regards,
> Jonathan
>
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>
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David Winsemius, MD
West Hartford, CT