index of duplicates everywhere in a vector in R

Question

I know I can get the index of duplicates using duplicated in R. But I wonder how to get the index of all instances of a duplicate in the entire vector?

As a trivial example, if 1, 7 are duplicates, how to get the indices of theses values all over the vector?

a = c(1, 7, 5, 7, 4, 1)

duplicated(a)

Desired output:

c(T, T, F, T, F, T)

score 3 · Accepted Answer · answered Feb 12 '20 at 22:24

3

You can do:

duplicated(a) | duplicated(a, fromLast = TRUE)

[1]  TRUE  TRUE FALSE  TRUE FALSE  TRUE

answered Feb 12 '20 at 22:24

tmfmnk

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Could you please tell me what the difference it made by adding `fromLast` argument? – Daman deep Sep 30 '21 at 04:41
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@Daman deep `fromLast = TRUE` looks for duplicates from the opposite direction. That is, from the last element of the vector to the first one. – tmfmnk Sep 30 '21 at 14:16

akrun · Answer 2 · 2020-05-08T23:43:02.390

0

An option with table

(table(a) > 1)[as.character(a)]

Or using ave

ave(a, a, FUN = length) > 1

edited May 08 '20 at 23:43

answered May 08 '20 at 22:19

akrun

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index of duplicates everywhere in a vector in R

Desired output:

2 Answers2