Finding ALL duplicate rows, including “elements with smaller subscripts”

Question

R's duplicated returns a vector showing whether each element of a vector or data frame is a duplicate of an element with a smaller subscript. So if rows 3, 4, and 5 of a 5-row data frame are the same, duplicated will give me the vector

FALSE, FALSE, FALSE, TRUE, TRUE

But in this case, I actually want to get

FALSE, FALSE, TRUE, TRUE, TRUE

that is, I want to know whether a row is duplicated by a row with a larger subscript too.

Answer 1

To find the duplicate rows including elements with smaller subscripts, you can use the fromLast argument of the duplicated function i.e., call the duplicated function twice, once with fromLast=FALSE and once with fromLast=TRUE and take the rows where either are TRUE.

For example:

v <- c("a", "b", "f","f","f") 

 v[duplicated(v) | duplicated(v, fromLast=TRUE)]

[1] "f" "f" "f"