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 not seen a lot of examples of people using it, but the documentation implies that is is useful in plotting.
For example, the following code seems to work quite well (showing a plot for each of the species and assigning the right title to each plot) in data.table 1.9.3
:
iris <- data.table(iris)
iris[,plot(Sepal.Length ~ Sepal.Width, main = unlist(.BY)), by = Species]
While this code does not work as intended by me:
iris[ , plot(Sepal.Length ~ Sepal.Width, main = .BY), by = Species]
Why are these two different? From the comments, it does not seem to be an issue in data.table 1.9.2
. In what other ways might it be useful to use the .BY
statement? How is this different compared to the .EACHI
statement?
.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.
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