[R] which element is duplicated?

[Bert Gunter]

"I'd like to see a cleverer solution that vectorizes..."

and Herve provided it.


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 9:43 PM Bert Gunter <bgunter.4567 using gmail.com> wrote:

> 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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