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Matthew,
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<div><div><blockquote type="cite"><p>How about ~ instead of ! ? I ruled out - previously to leave + and - available for future use. NJ() may be possible too.</p></blockquote></div><div><p>Both "NJ()" and "~" are okay for me.</p><p></p><blockquote type="cite"><div>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.</div><blockquote type="cite"><div></div></blockquote></blockquote><p></p><p>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.</p><p></p><blockquote type="cite"><div>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?</div></blockquote><div>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:</div><div><br></div><div>set.seed(45)</div><div>DT <- data.table( x = sample(c(0,5,10,15), 10, replace=TRUE), y = sample(15)) </div><div><div><p></p><div>DF <- as.data.frame(DT)</div><p></p></div></div><div>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,</div><div><br></div><div>DF[!(DF$x), ] (or) DF[DF$x == 0, ]</div><div><br></div><div>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. </div><div><br></div><div>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.</div><div><br></div><div>Best,</div><div><br></div><div>Arun</div><p></p></div><div><br></div></div>
<p style="color: #A0A0A8;">On Monday, June 10, 2013 at 10:52 AM, Matthew Dowle wrote:</p>
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<p>Hi,</p>
<p>How about ~ instead of ! ? I ruled out - previously to leave + and - available for future use. NJ() may be possible too.</p>
<p>Matthew</p>
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<p>On 10.06.2013 09:35, Arunkumar Srinivasan wrote:</p><blockquote type="cite"><div><!-- html ignored --><!-- head ignored --><!-- meta ignored -->
<div>Hi Matthew,</div>
<div>My view (from the last reply) more or less reflects mnel's comments here: <a href="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</a> </div>
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<div>Pasted here for convenience:</div>
<div><span style="margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; background-color: #fafafa; color: #444444; font-family: Arial,; line-height: 17px; text-align: left;"><code style="margin: 0px; padding: 1px 5px; border: 0px; vertical-align: baseline; background-color: #eeeeee; font-family: Consolas, Menlo, Monaco,;">data.table</code> is mimicing <code style="margin: 0px; padding: 1px 5px; border: 0px; vertical-align: baseline; background-color: #eeeeee; font-family: Consolas, Menlo, Monaco,;">subset</code> in its handling of <code style="margin: 0px; padding: 1px 5px; border: 0px; vertical-align: baseline; background-color: #eeeeee; font-family: Consolas, Menlo, Monaco,;">NA</code> values in logical <code style="margin: 0px; padding: 1px 5px; border: 0px; vertical-align: baseline; background-color: #eeeeee; font-family: Consolas, Menlo, Monaco,;">i</code> arguments. -- the only issue is the <code style="margin: 0px; padding: 1px 5px; border: 0px; vertical-align: baseline; background-color: #eeeeee; font-family: Consolas, Menlo, Monaco,;">!</code> prefix signifying a not-join, not the way one might expect. Perhaps the not join prefix could have been <code style="margin: 0px; padding: 1px 5px; border: 0px; vertical-align: baseline; background-color: #eeeeee; font-family: Consolas, Menlo, Monaco,;">NJ</code> not <code style="margin: 0px; padding: 1px 5px; border: 0px; vertical-align: baseline; background-color: #eeeeee; font-family: Consolas, Menlo, Monaco,;">!</code> to avoid this confusion -- this might be another discussion to have on the mailing list -- (I think it is a discussion worth having)</span><span style="color: #444444; font-family: Arial,; line-height: 17px; text-align: left; background-color: #fafafa;"> </span></div>
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<div>Arun</div>
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<p style="color: #a0a0a8;">On Monday, June 10, 2013 at 10:28 AM, Arunkumar Srinivasan wrote:</p><blockquote style="border-left-style: solid; border-width: 1px; margin-left: 0px; padding-left: 10px;">
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<p>Hm, good point. Is data.table consistent with SQL already, for both == and !=, and so no change needed? </p>
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</div></blockquote><div>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).</div>
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<p>And it was correct for Frank to be mistaken. </p>
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</div></blockquote><div>Yes, it seems like he was mistaken.</div><blockquote type="cite"><div>
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<p>Maybe just some more documentation and examples needed then.</p>
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</div></blockquote><div>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. </div><blockquote type="cite"><div>
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<p>Are you happy that DT[!(x==.)] and DT[x!=.] do treat NA inconsistently? :</p>
<p><a href="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</a></p>
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</div></blockquote><div> 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. </div>
<div>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.</div><blockquote type="cite"><div>
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<p>"na.rm = TRUE/FALSE" sounds good to me. I'd only considered nomatch before in the context of joins, not logical subsets.</p>
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</div></blockquote><div>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).</div>
<div>Best regards,</div>
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<div>Arun</div>
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