Hello,
I would like to extract a column of a data.table, but I get unexpected (to me)
results when I specify a column dynamically.
DT <- data.table(ID = c("b","b","b","a","a","c"), a = 1:6, b = 7:12, c = 13:18)
thisone = "a"
str(DT[,a])
# int [1:6] 1 2 3 4 5 6
str(DT[,"a", with = FALSE])
# Classes ‘data.table’ and 'data.frame': 6 obs. of 1 variable:
# $ a: int 1 2 3 4 5 6
# - attr(*, ".internal.selfref")=<externalptr>
str(DT[, thisone, with = FALSE])
# Classes ‘data.table’ and 'data.frame': 6 obs. of 1 variable:
# $ a: int 1 2 3 4 5 6
# - attr(*, ".internal.selfref")=<externalptr>
I can't noodle out from the help why the latter two don't produce a vector as
the first one does. I'm looking at this online resource
http://www.rdocumentation.org/packages/data.table/functions/data.table and it
doesn't seem like the description of with points to having two different
results.
"with By default with=TRUE and j is evaluated within the frame of x;
column names can be used as variables. When with=FALSE, j is a vector of names
or positions to select, similar to a data.frame. with=FALSE is often useful in
data.table to select columns dynamically."
How should I extract a single column dynamically to retrieve a vector?
Cheers and thanks,
Ben
> sessionInfo()
R version 3.1.0 (2014-04-10)
Platform: x86_64-apple-darwin13.1.0 (64-bit)
locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8
attached base packages:
[1] stats graphics grDevices utils datasets methods base
other attached packages:
[1] data.table_1.9.5 devtools_1.6.1
loaded via a namespace (and not attached):
[1] chron_2.3-45 evaluate_0.5.5 formatR_1.0 httr_0.5 knitr_1.7
RCurl_1.95-4.1 stringr_0.6.2
[8] tools_3.1.0
Ben Tupper
Bigelow Laboratory for Ocean Sciences
60 Bigelow Drive, P.O. Box 380
East Boothbay, Maine 04544
http://www.bigelow.org
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