[datatable-help] join example from faq

[Juliet Hannah]

Hi Matthew,

Thanks for your response.

I'm following things right up to:

>  "Advanced: In the X[Y,j] form of grouping, the j expression sees
> variables in X first, then Y. We call this join inherited scope. If the
> variable is not in X or Y then the calling frame is searched, its
> calling frame, and so on in the usual way up to and including the global
> environment."

I'm mainly confused about

X[Y]

In the example Y does not have key. Y's column names are also
different.  So how did data.table
know to use column "V1" to match with X's key. Also, if the sentence
above applies how does X[Y j] relate to this example
in which I have used X[Y].

And yes, without data.table, I would have been in trouble for my
current project. I had
to repeat the aggregation 20 or so times. It would have been ugly.

Thanks,

Juliet