# special-symbols

##### Special symbols

`.SD`

, `.BY`

, `.N`

, `.I`

and `.GRP`

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`

.

- Keywords
- data

##### Details

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 are used in `j`

and defined as follows.

`.SD`

is a`data.table`

containing the**S**ubset of`x`

's**D**ata 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.

##### See Also

##### Examples

```
# NOT RUN {
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
X[, DT[.BY, y, on="x"], by=x] # join within each group
# }
```

*Documentation reproduced from package data.table, version 1.12.0, License: MPL-2.0 | file LICENSE*