[datatable-help] can one name a collection of columns by specifying just the first and the last column

[Bacou, Melanie]

Everyone,

The |varA...varZ| construct is borrowed from STATA syntax. Probably a 
reason why it got into subset() in the first place, though definitely 
not very R-like. In fact I’ve never come across this construct in R 
before and had no idea it was actually working either!

I’m not sure |dt[, .SD, .SDcols=list(varA...varZ)]| is less typing, less 
prone to error, or more readable than |dt[, .SD, 
.SDcols=names(dt)[1:24]| and using indices is also more flexible (what 
about if we want more complex sequences). I can see one use case for 
this syntax though if |dt| might change over time but variables always 
come in known sequences.

Not sure we should really encourage it — but agreed with Arun, if it’s 
in base::subset() then no reason why not.

—Mel.

On 2/10/2015 1:50 PM, Arunkumar Srinivasan wrote:

    I had the same reaction when I found out ‘subset’ already did this :-).


        I’ve the same impression that it’s a bit odd, even though some
        people prefer it..

    Arun

    On 10 Feb 2015 at 19:39:29, Chris Neff (caneff at gmail.com) wrote:

        Wow, didn’t realize that worked! So there is precedent then. It
        just looks funny to me, but you are right it is easily avoided.
        I just didn’t want to see more divergence from subset and
        data.frame logic, but since this already works with subset
        that’s fine.
        On Tue Feb 10 2015 at 1:34:03 PM Arunkumar Srinivasan
        aragorn168b at gmail.com <http://mailto:aragorn168b@gmail.com> wrote:

        |Chris,
        But what’s the problem? You can simply not use it?
        It’s not that uncommon. `base::subset()` does this.
        -- 
        Arun

        On 10 Feb 2015 at 19:31:43, Chris Neff (caneff at gmail.com) wrote:
        |

            |I don't like this idea. It adds extra that it doesn't need to.  Doing it with column numbers is more straightforward, and if all you have is names you can get numbers by doing match() or whatever and then getting the sequence with seq(). Having a sequence of column names is odd.
            On Tue Feb 10 2015 at 1:28:25 PM Arunkumar Srinivasan <aragorn168b at gmail.com> wrote:

                 Farrel,
                 It could be useful. Please file an issue on the github project page. Thanks.
                 --
                 Arun

                 On 10 Feb 2015 at 01:08:46, Farrel Buchinsky (fjbuch at gmail.com) wrote:
            |

                |     So lets say one has a data.table with the following columns
                     first.name, last.name, height, weight, shoe.size, eye.color, hair.length, appendage.size, ear.length
                     If one wanted to just include weight through hair.length one would have to go something such as this
                     dt[,list(weight, shoe.size, eye.color, hair.length)]
                     Is there a way to do something along the lines of
                     dt[,list(weight...hair.length)]
                     If so, can you direct me to the documentation? If not can you build it? Is it difficult? Some data.tables have many columns.
                     Thanking you in anticipation.
                     Farrel
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-- 
Melanie BACOU
International Food Policy Research Institute
Snr. Program Manager, HarvestChoice
Work +1(202)862-5699
E-mail m.bacou at cgiar.org
Visit www.harvestchoice.org
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