Special symbols
.SD, .BY, .N, .I, .GRP, and .NGRP are read-only symbols for use in j. .N can be used in i as well. See the vignettes and examples here and in data.table.
.EACHI is a symbol passed to by; i.e. by=.EACHI.
The bindings of these variables are locked and attempting to assign to them will generate an error. If you wish to manipulate .SD before returning it, take a copy(.SD) first (see FAQ 4.5). Using := in the j of .SD is reserved for future use as a (tortuously) flexible way to update DT by reference by group (even when groups are not contiguous in an ad hoc by).
These symbols used in j are defined as follows.
.SD is a data.table containing the Subset of x's Data for each group, excluding any columns used in by (or keyby).
.BY is a list containing a length 1 vector for each item in by. This can be useful when by is not known in advance. The by variables are also available to j directly by name; useful for example for titles of graphs if j is a plot command, or to branch with if() depending on the value of a group variable.
.N is an integer, length 1, containing the number of rows in the group. This may be useful when the column names are not known in advance and for convenience generally. When grouping by i, .N is the number of rows in x matched to, for each row of i, regardless of whether nomatch is NA or NULL. It is renamed to N (no dot) in the result (otherwise a column called ".N" could conflict with the .N variable, see FAQ 4.6 for more details and example), unless it is explicitly named; e.g., DT[,list(total=.N),by=a].
.I is an integer vector equal to seq_len(nrow(x)). While grouping, it holds for each item in the group, its row location in x. This is useful to subset in j; e.g. DT[, .I[which.max(somecol)], by=grp].
.GRP is an integer, length 1, containing a simple group counter. 1 for the 1st group, 2 for the 2nd, etc.
.NGRP is an integer, length 1, containing the number of groups.
.EACHI is defined as NULL but its value is not used. Its usage is by=.EACHI (or keyby=.EACHI) which invokes grouping-by-each-row-of-i; see data.table's by argument for more details.
DT = data.table(x=rep(c("b","a","c"),each=3), v=c(1,1,1,2,2,1,1,2,2), y=c(1,3,6), a=1:9, b=9:1)
DT
X = data.table(x=c("c","b"), v=8:7, foo=c(4,2))
X
DT[.N] # last row, only special symbol allowed in 'i'
DT[, .N] # total number of rows in DT
DT[, .N, by=x] # number of rows in each group
DT[, .SD, .SDcols=x:y] # select columns 'x' and 'y'
DT[, .SD[1]] # first row of all columns
DT[, .SD[1], by=x] # first row of 'y' and 'v' for each group in 'x'
DT[, c(.N, lapply(.SD, sum)), by=x] # get rows *and* sum columns 'v' and 'y' by group
DT[, .I[1], by=x] # row number in DT corresponding to each group
DT[, .N, by=rleid(v)] # get count of consecutive runs of 'v'
DT[, c(.(y=max(y)), lapply(.SD, min)),
by=rleid(v), .SDcols=v:b] # compute 'j' for each consecutive runs of 'v'
DT[, grp := .GRP, by=x] # add a group counter
DT[, grp_pct := .GRP/.NGRP, by=x] # add a group "progress" counter
X[, DT[.BY, y, on="x"], by=x] # join within each groupPlease choose more modern alternatives, such as Google Chrome or Mozilla Firefox.