All,
I am not understanding a few basic things. I am looking at pg 5 of the faq.
X = data.table(grp=c("a","a","b","b","b","c","c"), foo=1:7)
setkey(X,grp)
Y = data.table(c("b","c"), bar=c(4,2))
X[Y]
The faq says X[Y] is a join looking up X's rows using Y.
Does this mean data.table looks up X's key using Y?
Y has two columns. How does it know to use the first column in this
example? Y's key has not been set.
Hope my question is not too obvious. :)
Thanks,
Juliet
P.S. Thanks for the wonderful package. I had to do some aggregations
the other day and my other solutions were
running for hours, but data.table finished in a couple of minutes!
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