[datatable-help] datatable-help Digest, Vol 28, Issue 2

[Yike Lu]

On 6/7/2012 12:46 PM, Juliet Hannah wrote:
> Hi Yike,
>
> In Matthew's email, he writes:
>
> So in other words, when i has no key it's the first column of i to the
> first column of x's key, the 2nd column of i to the 2nd column of x's key,
> etc.
Right, I figured that was the part which was supposed to answer the 
question, however the language is ambiguous in Matt's reply, which is 
why I asked for clarification. If what you're saying is the case, then 
what the docs should say is something along the lines of...

"If i is not keyed, then data.table performs a join on a positional 
match between the columns of i and the keys of X. In other words, a join 
is performed on (column 1 of i, column 1 of key of X, then column 2 of 
i, column 2 of key of X, etc...)"

Don't have to use my wording, but the original was ambiguous to me.

Cheers,
Yike
>
>
> I think this answers my original question. In the example (which is
> from the FAQ), i has no key. So it seems
> the  *default* is to match the first column of i to x's key.
>
> That's how data.table "knows". It uses the first column as default.
>
> At least that's the way I understand things now.  Matthew, please
> correct if I've misunderstood.
>
> Regards,
>
> Juliet
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