[datatable-help] Is there a way to merge-assign on something other than the key?

[Frank Erickson]

Thanks, Steve. I was thinking that that approach spends too much memory or
computational time by making a copy (while I was thinking of editing dt1
in-place). If I'm wrong, maybe that's what I ought to do. If not, I look
forward to hearing your other thoughts.
--Frank


On Thu, Aug 15, 2013 at 11:23 AM, Steve Lianoglou
<lianoglou.steve at gene.com>wrote:

> Hi Frank,
>
> I don't have time to get into the details of your entire question
> right now, but:
>
> On Thu, Aug 15, 2013 at 8:49 AM, Frank Erickson <FErickson at psu.edu> wrote:
> > Hi,
> >
> > I really like the DT1[DT2,z:=...] idiom. Unfortunately, the value of a
> > merge() on other columns is a new data.table, so modifying DT1, like
> > merge(DT1,DT2,by=...)[,z:=...], is not possible. Or is there actually a
> way
> > to do this that I am missing?
>
> I just wanted to mention that unless I am misunderstanding what you
> want, this is entirely possible, and the way you suggest it might work
> is actually the way to do it.
>
> Consider:
>
> R> dt1 <- data.table(a=sample(letters[1:2], 5, rep=T), b=runif(5), key='a')
> R> dt2 <- data.table(a=c('a', 'b'), c=rnorm(5), key='a')
>
> R> dt1
>    a          b
> 1: a 0.02517147
> 2: a 0.85459776
> 3: a 0.67472168
> 4: a 0.89684769
> 5: b 0.11619613
>
> R> dt2
>    a           c
> 1: a -0.07817539
> 2: b -1.28897689
>
> R> out <- merge(dt1, dt2)[, d := b + c]
>    a          b           c           d
> 1: a 0.02517147 -0.07817539 -0.05300392
> 2: a 0.85459776 -0.07817539  0.77642237
> 3: a 0.67472168 -0.07817539  0.59654629
> 4: a 0.89684769 -0.07817539  0.81867230
> 5: b 0.11619613 -1.28897689 -1.17278075
>
> Will come back later to look through the rest of your question if
> still necessary.
>
> HTH,
> -steve
>
> --
> Steve Lianoglou
> Computational Biologist
> Bioinformatics and Computational Biology
> Genentech
>
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