[Rd] duplicates() function

From: Duncan Murdoch <murdoch.duncan_at_gmail.com>
Date: Fri, 08 Apr 2011 10:59:10 -0400

I need a function which is similar to duplicated(), but instead of returning TRUE/FALSE, returns indices of which element was duplicated. That is,

> x <- c(9,7,9,3,7)
> duplicated(x)

> duplicates(x)

[1] NA NA 1 NA 2

(so that I know that element 3 is a duplicate of element 1, and element 5 is a duplicate of element 2, whereas the others were not duplicated according to our definition.)

Is there a simple way to write this function? I have an ugly implementation in R that loops over all the values; it would make more sense to redo it in C, if there isn't a simple implementation I missed.

Duncan Murdoch

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