.SD looks useful but I do not really know what I am doing with it. What does it stand for? Why is there a preceding period (full stop). What is happening when I use it? I read: .SD is a data.table containing the subset of x‘s data for each group, excluding the group column(s). It … Read more
I’m having a little trouble understanding the pass-by-reference properties of data.table. Some operations seem to ‘break’ the reference, and I’d like to understand exactly what’s happening. On creating a data.table from another data.table (via <-, then updating the new table by :=, the original table is also altered. This is expected, as per: ?data.table::copy and … Read more
To get rid of a column named “foo” in a data.frame, I can do: df <- df[-grep(‘foo’, colnames(df))] However, once df is converted to a data.table object, there is no way to just remove a column. Example: df <- data.frame(id = 1:100, foo = rnorm(100)) df2 <- df[-grep(‘foo’, colnames(df))] # works df3 <- data.table(df) df3[-grep(‘foo’, … Read more
Overview I’m relatively familiar with data.table, not so much with dplyr. I’ve read through some dplyr vignettes and examples that have popped up on SO, and so far my conclusions are that: data.table and dplyr are comparable in speed, except when there are many (i.e. >10-100K) groups, and in some other circumstances (see benchmarks below) … Read more
