Question

I have a data.table in R where I want to throw away the first and the last n rows. I want to to apply some filtering before and then truncate the results. I know I can do this this way:

example=data.table(row1=seq(1,1000,1),row2=seq(2, 3000,3))
e2=example[row1%%2==0]
e2[100:(nrow(e2)-100)]

Is there a possiblity of doing this in one line? I thought of something like:

example[row1%%2==0][100:-100]

This of course does not work, but is there a simpler solution which does not require a additional variable?

Answer 1

In this case you know the name of one column (row1) that exists, so using length(<any column>) returns the number of rows within the unnamed temporary data.table:

example=data.table(row1=seq(1,1000,1),row2=seq(2, 3000,3))

e2=example[row1%%2==0]
ans1 = e2[100:(nrow(e2)-100)]

ans2 = example[row1%%2==0][100:(length(row1)-100)]

identical(ans1,ans2)
[1] TRUE