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
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