Use of .BY and .EACHI in the data.table package

Question

I am trying to get a better grasp on how some of the special variables in the data.table package work. One of these is the .BY statement. I have

Answer 1

.BY is a named list containing the values of the by variables.

Passing an unnamed list to main will work, however a named list will fail (wholly unrelated to data.table

plot(1, main = list(1))
# works....
plot(1, main = list(s=1))
# Error in title(...) : invalid graphics parameter

There is a recent commit to data.table 1.9.3 which fixed a bug to do with naming in `.BY Closes bug #5415. .BY gets names attribute set properly in april this year.

If you had more than 1 "by" variable, you would want to be able to concatenate some how

perhaps

iris[,plot(Sepal.Length~Sepal.Width,main=do.call(paste,.BY)),by=Species]

will work (unless you have a column called collapse!)

EACHI is completely unrelated to this. Please read the NEWS for data.table 1.9.3 for an understanding of this.