[datatable-help] Follow-up on subsetting data.table with NAs
Arunkumar Srinivasan
aragorn168b at gmail.com
Mon Jun 10 15:38:27 CEST 2013
Frank,
You're right about my final point. I can't recollect why I wrote that now. I guess the `!` function will be restored automatically.
With my second example, all I wanted to establish was that there was another reason to change `!` from performing the action of a "Not Join" because `DT[!x]` is a perfectly valid syntax (for those who have worked with data.frames and have shifted to data.table) which will not perform the intended action as it'll be a Not Join. In addition, `DT[!x]` gives an error when "x" column has NA. This was meant to be an additional argument for not having `!` for Not Join. But this has caused more confusion. Let's forget about my examples :).
To conclude, "~" or "NJ" makes sense than `!` for "Not join" and of course the function of `!` will be automatically restored to "not" (also preferably with a na.rm = TRUE/FALSE. This is what I intended to say from the original discussion. Sorry for any confusion.
Arun
On Monday, June 10, 2013 at 3:20 PM, Frank Erickson wrote:
> +1 to using ~ for the not-join/join on complement/complement then join. Having some logical-looking i's lead to subsetting and others to not-joins can (for me) lead to mistakes that I'm not likely to catch until much later, if at all.
>
> I'm not sure I follow Arun's second example. If the syntax is changed so that ~ works as ! does now, then presumably !x will be reverted to having only a logical interpretation -- coercing x to logical and taking the subset where x == 0 -- which is the behavior you want. So why is it a separate issue? The remaining difference from data.frames would be that DF[!x] would show NA rows, if any, while DT[!x] would not.
>
> --Frank
>
>
> On Mon, Jun 10, 2013 at 4:21 AM, Arunkumar Srinivasan <aragorn168b at gmail.com (mailto:aragorn168b at gmail.com)> wrote:
> > Matthew,
> >
> > > How about ~ instead of ! ? I ruled out - previously to leave + and - available for future use. NJ() may be possible too.
> > Both "NJ()" and "~" are okay for me.
> >
> > > That result makes perfect sense to me. I don't think of !(x==.) being the same as x!=. ! is simply a prefix. It's all the rows that aren't returned if the ! prefix wasn't there.
> > > >
> > >
> > >
> >
> >
> >
> > I understand that `DT[!(x)]` does what `data.table` is designed to do currently. What I failed to mention was that if one were to consider implementing `!(x==.)` as the same as `x != .` then this behaviour has to be changed. Let's forget this point for a moment.
> >
> > > That needs to be fixed. But we're getting quite theoretical here and far away from common use cases. Why would we ever have row numbers of the table, as a column of the table itself and want to select the rows by number not mentioned in that column?
> > >
> >
> >
> > Probably I did not choose a good example. Suppose that I've a data.table and I want to get all rows where "x == 0". Let's say:
> >
> > set.seed(45)
> > DT <- data.table( x = sample(c(0,5,10,15), 10, replace=TRUE), y = sample(15))
> >
> > DF <- as.data.frame(DT)
> >
> >
> >
> > To get all rows where x == 0, it could be done with DT[x == 0]. But it makes sense, at least in the context of data.frames, to do equivalently,
> >
> > DF[!(DF$x), ] (or) DF[DF$x == 0, ]
> >
> > All I want to say is, I expect `DT[!(x)]` should give the same result as `DT[x == 0]` (even though I fully understand it's not the intended behaviour of data.table), as it's more intuitive and less confusing.
> >
> > So, changing `!` to `~` or `NJ` is one half of the issue for me. The other is to replace the actual function of `!` in all contexts. I hope I came across with what I wanted to say, better this time.
> >
> > Best,
> >
> > Arun
> >
> >
> >
> >
> > On Monday, June 10, 2013 at 10:52 AM, Matthew Dowle wrote:
> >
> > >
> > > Hi,
> > > How about ~ instead of ! ? I ruled out - previously to leave + and - available for future use. NJ() may be possible too.
> > > Matthew
> > >
> > > On 10.06.2013 09:35, Arunkumar Srinivasan wrote:
> > > > Hi Matthew,
> > > > My view (from the last reply) more or less reflects mnel's comments here: http://stackoverflow.com/questions/16239153/dtx-and-dtx-treat-na-in-x-inconsistently#comment23317096_16240143
> > > > Pasted here for convenience:
> > > > data.table is mimicing subset in its handling of NA values in logical i arguments. -- the only issue is the ! prefix signifying a not-join, not the way one might expect. Perhaps the not join prefix could have been NJ not ! to avoid this confusion -- this might be another discussion to have on the mailing list -- (I think it is a discussion worth having)
> > > >
> > > > Arun
> > > >
> > > > On Monday, June 10, 2013 at 10:28 AM, Arunkumar Srinivasan wrote:
> > > >
> > > > > > Hm, good point. Is data.table consistent with SQL already, for both == and !=, and so no change needed?
> > > > > >
> > > > > >
> > > > >
> > > > > Yes, I believe it's already consistent with SQL. However, the current interpretation of NA (documentation) being treated as FALSE is not needed / untrue, imho (Please see below).
> > > > >
> > > > > > And it was correct for Frank to be mistaken.
> > > > > >
> > > > > >
> > > > >
> > > > > Yes, it seems like he was mistaken.
> > > > > > Maybe just some more documentation and examples needed then.
> > > > > >
> > > > > >
> > > > >
> > > > > It'd be much more appropriate if the documentation reflects the role of subsetting in data.table mimicking "subset" function (in order to be in line with SQL) by dropping NA evaluated logicals. From a couple of posts before, where I pasted the code where NAs are replaced to FALSE were not necessary as `irows <- which(i)` makes clear that `which` is being used to get indices and then subset, this fits perfectly well with the interpretation of NA in data.table.
> > > > > > Are you happy that DT[!(x==.)] and DT[x!=.] do treat NA inconsistently? :
> > > > > > http://stackoverflow.com/questions/16239153/dtx-and-dtx-treat-na-in-x-inconsistently
> > > > > >
> > > > > >
> > > > >
> > > > > Ha, I like the idea behind the use of () in evaluating expressions. It's another nice layer towards simplicity in data.table. But I still think there should not be an inconsistency in equivalent logical operations to provide different results. If !(x== .) and x != . are indeed different, then I'd suppose replacing `!` with a more appropriate name as it's much easier to get confused otherwise.
> > > > > In essence, either !(x == .) must evaluate to (x != .) if the underlying meaning of these are the same, or the `!` in `!(x==.)` must be replaced to something that's more appropriate for what it's supposed to be. Personally, I prefer the former. It would greatly tighten the structure and consistency.
> > > > > > "na.rm = TRUE/FALSE" sounds good to me. I'd only considered nomatch before in the context of joins, not logical subsets.
> > > > > >
> > > > > >
> > > > >
> > > > > Yes, I find this option would give more control in evaluating expressions with ease in `i`, by providing both "subset" (default) and the typical data.frame subsetting (na.rm = FALSE).
> > > > > Best regards,
> > > > >
> > > > > Arun
> > > > >
> > > > >
> > > > >
> > > >
> > > >
> > >
> > >
> > >
> > >
> > >
> > >
> >
> >
> >
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