I prefer ~ and/or NJ() over -. The not-join operation is different from the subsetting operation usually associated with -.
I don't know what characters are available for this sort of thing, but @x, @(x,y) seems natural enough as syntax for a getter. On Mon, Jun 10, 2013 at 9:35 AM, Matthew Dowle <[email protected]>wrote: > > Hm, another good point. We need ~ for formulae, although I can't > imagine a formula in i (only in j). But in both i and j we might want to > get(x). > > I thought about ^ i.e. X[^Y] in the spirit of regular expression syntax, > but ^ doesn't parse with a RHS only. Needs to be parsable as a prefix. > > - maybe then? Consistent with - meaning in R. I don't think I actually > had a specific use in mind for - and +, to reserve them for, but at the > time it just seemed a shame to use up one of -/+ without defining the > other. If - does a not join, then, might + be more like merge() (i.e. > returning the union of the rows in x and i by join). I think I had > something like that in mind, but hadn't thought it through. > > Some might say it should be a new argument e.g. notjoin=TRUE, but my > thinking there is readability, since we often have many lines in i, j and > by in that order, and if the "notjoin=TRUE" followed afterwards it would be > far away from the i argument to which it applies. If we incorporate > merge() into X[Y] using X[+Y] then it might avoid adding yet more > parameters, too. > > > > On 10.06.2013 15:02, Gabor Grothendieck wrote: > >> The problem with ~ is that it is using up a special character (of >> which there are only a few) for a case that does not occur much. >> >> I can think of other things that ~ might be better used for. For >> example, perhaps ~ x could mean get(x). One aspect of data.table that >> tends to be difficult is when you don't know the variable name ahead >> of time and this woiuld give a way to specify it concisely. >> >> On Mon, Jun 10, 2013 at 5:21 AM, Arunkumar Srinivasan >> <[email protected]> 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<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<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 >>> >>> >>> >>> >>> >>> >>> >>> ______________________________**_________________ >>> datatable-help mailing list >>> [email protected].**r-project.org<[email protected]> >>> >>> https://lists.r-forge.r-**project.org/cgi-bin/mailman/** >>> listinfo/datatable-help<https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help> >>> >> > ______________________________**_________________ > datatable-help mailing list > [email protected].**r-project.org<[email protected]> > https://lists.r-forge.r-**project.org/cgi-bin/mailman/** > listinfo/datatable-help<https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help> >
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