special-symbols: Special symbols

Description Details See Also Examples

Description

.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.

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.

See Also

data.table, :=, set, datatable-optimize

Examples

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## 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

## End(Not run)

data.table documentation built on Sept. 30, 2018, 5:05 p.m.