[datatable-help] using J() to select for a value that is in something other than the first key

[Farrel Buchinsky]

Oy gevalt <http://en.wiktionary.org/wiki/oy_gevalt>!.Am I correct to
believe that the technique is rearranging the data.table so that J can
accept the input as pertaining to a secondary key? That seems as if it is
too much work for me and my computer. I will rather stick to the vector
scan methods for now.


Farrel



On Thu, Jan 19, 2012 at 00:23, Steve Lianoglou <
mailinglist.honeypot at gmail.com> wrote:

> Hi,
>
> Just to redo the example Matthew is pointing to, I'll put it here.
>
> Note that the current answer is that there is no "neat" way to do it
> just, and it takes some intermediary steps for you to get what you
> want. Here this amounts to creating the `idx` table w/ the keys
> swapped and an `i` column.
>
> There is a feature request to make it neater:
>
> https://r-forge.r-project.org/tracker/index.php?func=detail&aid=1007&group_id=240&atid=978
>
> here's an example:
>
> R> dt <- data.table(a=c('a','a','a','a','b','b','b','b'),
> b=c('a','b','a','b','a','b','b','a'),c=1:8,key=c('a','b'))
> R> dt
>     a b c
> [1,] a a 1
> [2,] a a 3
> [3,] a b 2
> [4,] a b 4
> [5,] b a 5
> [6,] b a 8
> [7,] b b 6
> [8,] b b 7
>
> R> idx <- dt[,list(b, a, i=1:nrow(dt))]
> R> key(idx) <- c('b','a')
>
> Now let's grab all rows from `dt` where the secondary key == 'b'
>
> R> dt[ idx[J("b"), i]$i ]
>     a b c
> [1,] a b 2
> [2,] a b 4
> [3,] b b 6
> [4,] b b 7
>
> HTH,
> -steve
>
> On Wed, Jan 18, 2012 at 11:42 PM, Matthew Dowle <mdowle at mdowle.plus.com>
> wrote:
> > I find Nabble is better for searching the archive. It seems to be
> > optimised for discussion list search.  Link on homepage, 'secondary' into
> > search box, and e.g. thread " Select from second key but not first".
> >
> >> Dear Matthew
> >>
> >> Thank you for trying to guide me. Unfortunately I was not able to really
> >> comprehend what I was reading
> >>
> >> I entered the following into google search "site:
> >> lists.r-forge.r-project.org/pipermail/datatable-help/ secondary"
> >>
> >> I got many hits and I could see a discussion but could not fathom how it
> >> related to my question.
> >>
> >>
> >> Would love some help from you or someone else. I want to be able to
> select
> >> by a criterion in the second key and accept all possible values in the
> >> first key.
> >> Farrel Buchinsky
> >> Google Voice Tel: (412) 567-7870
> >>
> >>
> >>
> >> On Wed, Jan 18, 2012 at 22:40, Matthew Dowle <mdowle at mdowle.plus.com>
> >> wrote:
> >>
> >>> Hi. Try searching datatable-help for 'secondary'.
> >>>
> >>> > In setkey it is possible to have multiple columns be a key. There is
> >>> > however an order to it. To select all the rows by a particular value
> >>> in
> >>> > the
> >>> > first key is easy
> >>> > DT[J("a value from the first key")]
> >>> >
> >>> > It is even quite easy to select by an AND criteria that incorporates
> >>> the
> >>> > second key
> >>> > DT[J("a value from the first key","a value from the second key")]
> >>> >
> >>> > But what happens if one wanted to select by only one criteria being a
> >>> a
> >>> > value that must be present in the second key? I was hoping to get
> >>> lucky
> >>> by
> >>> > just leaving it blank. But it did not see to work.
> >>> >
> >>> > DT[J(,"a value from the second key")]
> >>> >
> >>> > I would love to know what you do.
> >>> >
> >>> >
> >>> > Farrel
> >>> > _______________________________________________
> >>> > datatable-help mailing list
> >>> > datatable-help at lists.r-forge.r-project.org
> >>> >
> >>>
> https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help
> >>>
> >>>
> >>>
> >>>
> >>>
> >>
> >
> >
> >
> >
> >
> >
> > _______________________________________________
> > datatable-help mailing list
> > datatable-help at lists.r-forge.r-project.org
> >
> https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help
>
>
>
> --
> Steve Lianoglou
> Graduate Student: Computational Systems Biology
>  | Memorial Sloan-Kettering Cancer Center
>  | Weill Medical College of Cornell University
> Contact Info: http://cbio.mskcc.org/~lianos/contact
>
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