unique(Species) is of length 3, where as the 2nd entry c(1.5, 2) is of length 2.
J in J(.) is replaced with list(.) internally (using lazy evaluation),
following which it’s converted to a data.table using as.data.table(list(.)).
And here your list is:
list(c("setosa", "versicolor", "virginica") , c(1.5, 2.0)) which results in the
warning because it has to recycle to convert it to a data.table.
In the example you’ve linked, J(.) and CJ(.) will return the same result
(because there’s just one value in 2nd column). So, the results don’t change.
But the general expression is to use CJ(.) along with nomatch=0L, as you’ve
done.
Those two expressions are equivalent, yes.
Arun
From: G See [email protected]
Reply: G See [email protected]
Date: June 15, 2014 at 5:45:11 PM
To: [email protected]
[email protected]
Subject: [datatable-help] subsetting by second key
Hi,
I want to subset a data.table using only its second key, which is
demonstrated here
http://stackoverflow.com/questions/15597685/subsetting-data-table-by-2nd-column-only-of-a-2-column-key-using-binary-search/15597713#15597713
However, I need to subset with more than one value in the secondary key
Is this warning expected? What exactly is it telling me?
library(data.table)
DT <- data.table(iris, key="Species,Petal.Width")
DT[J(unique(Species), c(1.5, 2.0)), nomatch=0L]
# Sepal.Length Sepal.Width Petal.Length Petal.Width Species
#1: 6.0 2.2 5.0 1.5 virginica
#2: 6.3 2.8 5.1 1.5 virginica
#Warning message:
#In as.data.table.list(i) :
# Item 2 is of size 2 but maximum size is 3 (recycled leaving a
remainder of 1 items)
It looks like I can get what I want with either of these; can you
confirm that both of these will always return the same result?
DT[Petal.Width %in% c(1.5, 2.0)] # vector scan
DT[CJ(unique(Species), c(1.5, 2.0)), nomatch=0L]
Thanks,
Garrett
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