special-symbols in package(data.table)

===================================================================================

The general form of data.table syntax is:

DT[ i,  j,  by ] # + extra arguments
        |   |   |
        |   |    -------> grouped by what?
        |    -------> what to do?
         ---> on which rows?

===================================================================================

.SD , .SDcols , .N 等

.SD stands for something like "Subset of Data.table". There's no significance to the initial ".",
except that it makes it even more unlikely that there will be a clash with a user-defined column name.（注: .SD相当于对行操作，取数据集的子集）

.SDcols Specifies the columns of x to be included in the special symbol, subset of columns . （注: .SD相当于对列操作，取符合条件的列）

.N可以用来表示行的数量或者最后一行

===================================================================================

.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.

DT[.N]                                 # last row, only special symbol allowed in 'i'
DT[, .N]                               # total number of rows in DT
DT[, .SD, .SDcols=x:y]                 # select columns 'x' and 'y'

===================================================================================