unique.data.table select last row in place of the first

data.table, r

Solution

Create a `data.table` that contains the unique combinations of the key variables then join using `mult = 'last'`

Using `.SD` is convenient, but slow. You could use `.I` instead if you wished.

dtu <- unique(dt)[,key(dt), with = FALSE]
dt[dtu, mult = 'last']

Or

 dt[ dt[,  .I[.N], by = key(dt)]$V1]

Problem

calling `unique` on a keyed `data.table` you'll have unique lines per each group. In case of duplicated lines the first will be taken. When I need the take the last instead ( in general the last temporal transaction) I use `.SD[.N]` ``` library(data.table) library(microbenchmark) dt <- data.table(id=sample(letters, 10000, T), var=rnorm(10000), key="id") microbenchmark(unique(dt), dt[, .SD[.N], by=id]) Unit: microseconds expr min lq median uq max neval unique(dt) 570.882 586.1155 595.8975 608.406 3209.122 100 dt[, .SD[.N], by = id] 6532.739 6637.7745 6694.3820 6776.968 208264.433 100 ``` do you know a faster way to do the same?

Original source