[datatable-help] indexing with nomatch=0

[Arunkumar Srinivasan]

Gabor, 
Both points I agree with. It brings enough clarity and consistency to the syntax.
Does this mean that you don't mind X[Y] not having all functionalities of `merge`? Because this takes care of the confusion of `nomatch` but still does not do all merges, iiuc.

Arun


On Saturday, May 4, 2013 at 1:26 PM, Gabor Grothendieck wrote:

> The proposal at this point would be:
> 
> 1. nomatch= would be replaced by all.i= such that
> X[Y,,nomatch=NA] is the same as X[Y,,all.i=TRUE]
> X[Y,,nomatch=0] is the same as X[Y,,all.i=FALSE]
> nomatch= would be deprecated and ultimately removed.
> 
> Note that #1 is simple to implement as it only involves changing names
> and values of arguments and does not really change any behavior;
> however, its easier to think about because X[Y,,all.i=Z] now has the
> same behavior as merge(X, Y, all.y=Z) and so can be quickly understood
> by anyone who knows merge in R. In contrast nomatch= did not even
> have the same meaning as in match() since match matches the first
> occurrence whereas with mult="all", the default, matching in
> data.table matches all occurrences. Note that the default of merge's
> all.y= is all.y=FALSE but the default of all.i= is all.i=TRUE in order
> that the default behave as indices do. Also note that this solves the
> problem that nomatch= can only be 0 or NA since a logical can only
> have two non-NA values anyways.
> 
> 2. If Y were a numeric index vector then all.i= will have the same
> effect as if Y were a data.table with Y as its column and is merged
> with the row numbers of X. e.g. X[1:4,,all.i=FALSE] would be the
> same as X[1:3] if X only had 3 rows since 4 does not match a row
> number of X and is dropped because all.i=FALSE. If Y were a numeric
> vector with negative values it would be converted to one with positive
> values in such a way as to have the established meaning and then the
> same strategy is applied. If Y were logical then its recycled giving
> YY and the same strategy is applied to which(YY). This description is
> intended to be conceptual and the actual internal mechanism could be
> different.
> 
> Thus #2 allows one to think of **all** i indexing as merging rather
> than as multiple separate concepts (which I believe is consistent with
> the original intention of data.table).
> 
> 
> 
> 
> 
> 
> On Fri, May 3, 2013 at 8:02 PM, Eduard Antonyan
> <eduard.antonyan at gmail.com (mailto:eduard.antonyan at gmail.com)> wrote:
> > I think I like this proposal - maybe you should write up a few examples of
> > what current behavior is, vs the proposed behavior.
> > 
> > 
> > On Fri, May 3, 2013 at 6:54 PM, Gabor Grothendieck <ggrothendieck at gmail.com (mailto:ggrothendieck at gmail.com)>
> > wrote:
> > > 
> > > data.table is supposed to generalize indexing and although not
> > > explicitly stated the generalization seems to be that indexing is
> > > merging with the row numbers so there is indeed merging going on and
> > > that merging should respect nomatch= for consistency.
> > > 
> > > On Fri, May 3, 2013 at 6:54 PM, Eduard Antonyan
> > > <eduard.antonyan at gmail.com (mailto:eduard.antonyan at gmail.com)> wrote:
> > > > There is no join'ing happening here, thus nomatch=0 has no effect.
> > > > 
> > > > 
> > > > On Fri, May 3, 2013 at 5:52 PM, Gabor Grothendieck
> > > > <ggrothendieck at gmail.com (mailto:ggrothendieck at gmail.com)>
> > > > wrote:
> > > > > 
> > > > > The definition of DT was left out by mistake. It should be:
> > > > > 
> > > > > DT <- data.table(a=letters[1:3])
> > > > > 
> > > > > 
> > > > > On Fri, May 3, 2013 at 6:50 PM, Gabor Grothendieck
> > > > > <ggrothendieck at gmail.com (mailto:ggrothendieck at gmail.com)> wrote:
> > > > > > Consider this example:
> > > > > > 
> > > > > > > DT[1:4,,nomatch=0]
> > > > > > a
> > > > > > 1: a
> > > > > > 2: b
> > > > > > 3: c
> > > > > > 4: NA
> > > > > > 
> > > > > > Should it not return only the first 3 rows? It seems to be ignoring
> > > > > > the nomatch=0.
> > > > > > 
> > > > > > --
> > > > > > Statistics & Software Consulting
> > > > > > GKX Group, GKX Associates Inc.
> > > > > > tel: 1-877-GKX-GROUP
> > > > > > email: ggrothendieck at gmail.com (http://gmail.com)
> > > > > > 
> > > > > 
> > > > > 
> > > > > 
> > > > > 
> > > > > --
> > > > > Statistics & Software Consulting
> > > > > GKX Group, GKX Associates Inc.
> > > > > tel: 1-877-GKX-GROUP
> > > > > email: ggrothendieck at gmail.com (http://gmail.com)
> > > > > _______________________________________________
> > > > > datatable-help mailing list
> > > > > datatable-help at lists.r-forge.r-project.org (mailto:datatable-help at lists.r-forge.r-project.org)
> > > > > 
> > > > > 
> > > > > https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help
> > > 
> > > 
> > > 
> > > --
> > > Statistics & Software Consulting
> > > GKX Group, GKX Associates Inc.
> > > tel: 1-877-GKX-GROUP
> > > email: ggrothendieck at gmail.com (http://gmail.com)
> > > 
> > 
> > 
> 
> 
> 
> 
> -- 
> Statistics & Software Consulting
> GKX Group, GKX Associates Inc.
> tel: 1-877-GKX-GROUP
> email: ggrothendieck at gmail.com (http://gmail.com)
> _______________________________________________
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