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I can select a few columns from a data.frame:

> z[c("events","users")]
     events  users
1  26246016 201816
2    942767 158793
3  29211295 137205
4  30797086 124314

but not from a data.table:

> best[c("events","users")]
Starting binary search ...Error in `[.data.table`(best, c("events", "users")) : 
  typeof x.pixel_id (integer) != typeof i.V1 (character)
Calls: [ -> [.data.table

What do I do? Is there a better way than to turn the data.table back into a data.frame?

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sds
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    column subsetting should be done in `j`, not in `i`. `DT[, c("x", "y"), with=FALSE]`. Have a look at the 4th slide [here](http://datatable.r-forge.r-project.org/CologneR_2013.pdf) – Arun Jan 16 '14 at 18:35
  • @Arun: thanks - please turn the comment into an answer so that I can accept it. – sds Jan 16 '14 at 18:42
  • sds, have done. Feel free to turn my comments into answers, if appropriate, for the future. – Arun Jan 16 '14 at 18:58
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    @sds Please also read the [data.table FAQ](http://datatable.r-forge.r-project.org/datatable-faq.pdf). The first few FAQs cover this. – Matt Dowle Jan 17 '14 at 01:37
  • I will repeat something I have said many times before. Implicity is a bitch. Just like sometimes `mydf[, 1, drop = FALSE` comes mighty handy. – Roman Luštrik Jan 17 '14 at 10:46

2 Answers2

6

Given that you're looking for a data.table back you should use list rather than c in the j part of the call. 

z[, list(events,users)]    # first comma is important

Note that you don't need the quotes around the column names.

Matt Dowle
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Matt Weller
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5

Column subsetting should be done in j, not in i. Do instead:

DT[, c("x", "y")]

Check this presentation (slide 4) to get an idea of how to read a data.table syntax (more like SQL). That'll help convince you that it makes more sense for providing columns in j - equivalent of SELECT in SQL.

MichaelChirico
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Arun
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