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)
[1] FALSE FALSE TRUE FALSE TRUE
> 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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