[datatable-help] Not getting behavior described in FAQ

[Harish]

Hi,

I recently ran across data.table and am experimenting with it.  I have a few questions that I am hoping to get your help with.

*** Question #1 ***
I like the concept of data.tables quite a bit and would like to use it as much as possible.  However, some other packages I want to use (e.g. ggplot2) need data frames as inputs.

Would I take a huge performance hit if I convert to data frames intermittently and use data.tables as much as possible?  Or should I use data.tables only when I have significant querying to do?

*** Question #2 ***
For some reason, FAQ 1.1 and 1.2 do not work for me as described.

I have version 1.4 of the data.table package (downloaded from r-forge today).

Example:
   a <- as.data.table(installed.packages())
   a[,5]
   a[,"Version"]

==> a[,5] does not return 5 as indicated in the FAQ 1.1, but returns the 5th column.
==> a[,"Version"] does not return "Version" as indicated in the FAQ 1.2, but returns the "Version" column.

What is the bug -- the FAQ or the behavior of the code?

*** Question #3 ***

FAQ 1.10 says "although it appears as though x[y] does not return the columns in y, you can actually use the columns from y in the j expression."  However, I don't get this behavior.

Example:
   x <- data.table( a=rep(letters[1:4], each=3), b=rep(letters[10:12], times=4), v=1:12, key='a,b')

   y <- data.table( a=letters[1:3], b=letters[10:12], col3=1:3, key='a,b' )

Then, I expect one of the following to give me col3 from y...
   x[y,"col3", with=FALSE]
   x[y, list(col3)]
...but I don't.

What am I doing wrong?


BTW, this is a great package.

Regards,
Harish