[R] which element is duplicated?

[Pages, Herve]

Hi,

On 11/12/18 17:08, Duncan Murdoch 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?

I generally use match() for that:

 > v <- c("a", "b", "b", "a")

 > match(v, v)

[1] 1 2 2 1

H.

>
> Duncan Murdoch
>
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-- 
Hervé Pagès

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