.NGRP are read-only symbols for use in
.N can be used in
i as well. See the vignettes and examples here and in
.EACHI is a symbol passed to
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
.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 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
.N is the number of rows in
x matched to, for each row of
i, regardless of whether
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.,
.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
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
keyby=.EACHI) which invokes grouping-by-each-row-of-i; see
by argument for more details.
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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] # first row of all columns DT[, .SD, 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, 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 group
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