Description Details See Also Examples

`.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**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.`.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.

`data.table`

, `:=`

, `set`

, `datatable-optimize`

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 | ```
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 group
``` |

```
x v y a b
1: b 1 1 1 9
2: b 1 3 2 8
3: b 1 6 3 7
4: a 2 1 4 6
5: a 2 3 5 5
6: a 1 6 6 4
7: c 1 1 7 3
8: c 2 3 8 2
9: c 2 6 9 1
x v foo
1: c 8 4
2: b 7 2
x v y a b
1: c 2 6 9 1
[1] 9
x N
1: b 3
2: a 3
3: c 3
x v y
1: b 1 1
2: b 1 3
3: b 1 6
4: a 2 1
5: a 2 3
6: a 1 6
7: c 1 1
8: c 2 3
9: c 2 6
x v y a b
1: b 1 1 1 9
x v y a b
1: b 1 1 1 9
2: a 2 1 4 6
3: c 1 1 7 3
x N v y a b
1: b 3 3 10 6 24
2: a 3 5 10 15 15
3: c 3 5 10 24 6
x V1
1: b 1
2: a 4
3: c 7
rleid N
1: 1 3
2: 2 2
3: 3 2
4: 4 2
rleid y v y a b
1: 1 6 1 1 1 7
2: 2 3 2 1 4 5
3: 3 6 1 1 6 3
4: 4 6 2 3 8 1
x V1
1: c 1
2: c 3
3: c 6
4: b 1
5: b 3
6: b 6
```

Embedding an R snippet on your website

Add the following code to your website.

For more information on customizing the embed code, read Embedding Snippets.