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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