[datatable-help] select * and getting the full sub data.table/frame

[David Bellot]

Hi,

I've been looking all around the web without a clear answer to this trivial
problem. I'm sure I'm not looking where I should:

in fact, I want to replace my use of ddply from the plyr package by
data.table. One of my main use is to group a big data.frame by a group of
variable and do something on this sub data.frame:

ddply( my_df, my_grouping_var, function (d)   { do something with d } )
----> d is a data.frame again

and it's slow on big data.frame.


However, I don't really understand how to redo the same thing with a
data.table. Basically if "j" in a data.table is equivalent to the select
clause in SQL, then how do I do SELECT * FROM etc...

I want to be able to pass a function like in ddply that will receive not
only a few columns but the full subset that is selected by the "by" clause.

Thanks...
Best,
David
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.r-forge.r-project.org/pipermail/datatable-help/attachments/20130117/06f0cdae/attachment.html>