.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 can be used when grouping by i, when grouping by by, keyed by, and _ad hoc_ by
Does that mean that the daughter data.tables is held in memory for the next operation?