[R] which element is duplicated?

[Bert Gunter]

It is not clear to what you want for the general case. Perhaps:

> v <- letters[c(2,2,1,2,1,1)]
> wh <- tapply(seq_along(v),factor(v), '[',1)
> w <- wh[match(v,v[wh])]
> w
b b a b a a
1 1 3 1 3 3
> ## and if you want NA's for the first occurences of unique values
> ## of course:
> w[wh] <- NA
> w
 b  b  a  b  a  a
NA  1 NA  1  3  3

I'd like to see a cleverer solution that vectorizes and avoids the
tapply(), though.

Cheers,
Bert




On Mon, Nov 12, 2018 at 8:33 PM Bert Gunter <bgunter.4567 using gmail.com> wrote:

> > match(v, unique(v))
> [1] 1 2 2 1
>
> Bert Gunter
>
> "The trouble with having an open mind is that people keep coming along and
> sticking things into it."
> -- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )
>
>
> On Mon, Nov 12, 2018 at 5:08 PM Duncan Murdoch <murdoch.duncan using gmail.com>
> wrote:
>
>> The duplicated() function gives TRUE if an item in a vector (or row in a
>> matrix, etc.) is a duplicate of an earlier item.  But what I would like
>> to know is which item does it duplicate?
>>
>> For example,
>>
>> v <- c("a", "b", "b", "a")
>> duplicated(v)
>>
>> returns
>>
>> [1] FALSE FALSE  TRUE  TRUE
>>
>> What I want is a fast way to calculate
>>
>>   [1] NA NA 2 1
>>
>> or (equally useful to me)
>>
>>   [1] 1 2 2 1
>>
>> The result should have the property that if result[i] == j, then v[i] ==
>> v[j], at least for i != j.
>>
>> Does this already exist somewhere, or is it easy to write?
>>
>> Duncan Murdoch
>>
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>

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