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This is very nice to learn about, Denis, but it seems only fair to point out that the result of rbindlist is not a data frame. You can convert it to a data frame easily, but the copy and indexing semantics of data tables are quite different than data tables, which could be a real headache for someone not prepared for those differences. (To learn more, read the data tables vignette.) Tibbles (as

I am an old, long time SAS programmer. I need to produce R code that processes a dataframe in a manner that is equivalent to that produced by using a by statement in SAS and an if first.day statement and a retain statement: I want to take data (olddata) that looks like this ID Day 1 1 1 1 1 2 1 2 1 3 1 3 1 4 1 4 1 5 1 5 2 5 2 5 2 5 2 6 2 6 2 6 3 10 3 10 and make it look like this: (withing each