Please go through the FAQ. This is outlined in the data.table FAQ 2.17 - smaller syntax differences between data.frame and data.table.
DT[, col] returns a vector because there’s no other way to return a vector and users wanted a way to return a vector. DT[, “col”, with=FALSE] returns a data.table because data.table doesn’t use the ‘drop’ argument. Plus you can always do DT[[“col”]] to subset a column from data.frame/data.tables. ‘drop’ has no other purpose, and it’s default value IMHO is a mistake. I’m not sure about plans to implement it in data.table (I’m not for it). Even if it were, you’d have to do: DT[, “col”, with=FALSE, drop=FALSE] which seems quite bad in comparison to DT[[“col”]]. -- Arun On 12 Feb 2015 at 22:28:08, Ben Tupper ([email protected]) 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 _______________________________________________ datatable-help mailing list [email protected] https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help
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