[R] which element is duplicated?

[Bert Gunter]

> 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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