r - Find indices of duplicated rows

Question

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Answer 1

Here's an example:

df <- data.frame(a = c(1,2,3,4,1,5,6,4,2,1))

duplicated(df) | duplicated(df, fromLast = TRUE)
#[1]  TRUE  TRUE FALSE  TRUE  TRUE FALSE FALSE  TRUE  TRUE  TRUE

How it works?

The function duplicated(df) determines duplicate elements in the original data. The fromLast = TRUE indicates that "duplication should be considered from the reverse side". The two resulting logical vectors are combined using | since a TRUE in at least one of them indicates a duplicated value.