which element is duplicated?

PIKAL Petr 
    on Tue, 13 Nov 2018 08:42:22 +0000 writes:

v <- letters[c(2,2,1,2,1,1)]
vf<-factor(v)
as.numeric(vf)

[1] 2 2 1 2 1 1

Cheers
Petr

-----Original Message-----
From: R-help <r-help-bounces at r-project.org> On Behalf Of Bert Gunter
Sent: Tuesday, November 13, 2018 6:44 AM
To: Duncan Murdoch <murdoch.duncan at gmail.com>
Cc: R-help <R-help at r-project.org>
Subject: Re: [R] which element is duplicated?

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 at 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 at 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