[datatable-help] extracting columns dynamically

[Paul Johnson]

I think this is an example of the "drop gotcha", masked by data table.  See
http://pj.freefaculty.org/blog/?p=274

basically, R defaults to "demote" one column matrices to vectors, we avoid
that by additional argument drop = FALSE.  However, layering data.table on
top of that confuses the situation somewhat.

On Thu, Feb 12, 2015 at 3:27 PM, Ben Tupper <btupper at bigelow.org> wrote:

> 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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> datatable-help at lists.r-forge.r-project.org
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>



-- 
Paul E. Johnson
Professor, Political Science      Assoc. Director
1541 Lilac Lane, Room 504      Center for Research Methods
University of Kansas                 University of Kansas
http://pj.freefaculty.org               http://quant.ku.edu
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