data.table Using .SD and .SDcols for the subset of data
Introduction
The special symbol
.SD is available in j of DT[i,j,by], capturing the Subset of Data for each by group surviving the filter, i. .SDcols is a helper. Type ?`special-symbols` for the official docs.
Remarks
A reminder: DT[where, select|update|do, by] syntax is used to work with columns of a data.table.
- The "where" part is the
iargument - The "select|update|do" part is the
jargument
These two arguments are usually passed by position instead of by name.
