<div dir="ltr"><div><div><div><div>Arun,<br><br></div>Yes, DT1[DT2, y, .JOIN = FALSE] would do the same as DT1[DT2][, y] does currently.<br></div>No, DT1[DT2, y, .JOIN=FALSE], will NOT do a by-without-by, which is literally a 'by' by each of the rows of DT2 that are in the join (thus each.i! - the operation 'y' will be performed for each of the rows of 'i' and then combined and returned). There is no efficiency issue here that I can see, but Matthew can correct me on this. As far as I understand the efficiency comes into play when e.g. the rows of 'i' are unique, and after the join you'd like to do a 'by' by those, then DT1[DT2][, j, by = key(DT1)] would be less efficient since the 'by' could've already been done while joining.<br>
<br></div>DT1[DT2, .JOIN=FALSE] would be equivalent to both current and future DT1[DT2] - in this expression there is no by-without-by happening in either case.<br><br></div><div>The purpose of this is NOT for j just being a column or an expression that gets evaluated into a signal column. It applies to any j. The extra 'by-without-by' column is currently output independently of how many columns you output in your j-expression, the behavior is very similar as to when you specify a by=., except that the 'by' happens by a very special expression, that only exists when joining two data-tables and that generally doesn't exist before or after the join.<br>
</div><div><br></div>Hope this answers your questions.<br><div class="gmail_extra"><br><br><div class="gmail_quote">On Tue, Apr 30, 2013 at 8:48 AM, Arunkumar Srinivasan <span dir="ltr"><<a href="mailto:aragorn168b@gmail.com" target="_blank">aragorn168b@gmail.com</a>></span> wrote:<br>
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Eduard, thanks for your reply. But somethings are unclear to me still. I'll try to explain them below.
</div><div><br></div><div>First I prefer .JOIN (or cross.apply) just because `each.i` seems general (that it is applicable to *every* i operation, which as of now seems untrue). .JOIN is specific to data.table type for `i`.</div>
<div><br></div><div>From what I understand from your reply, if (.JOIN = FALSE), then,</div><div><br></div><div> DT1[DT2, y, .JOIN = FALSE] <=> DT1[DT2][, y]</div><div><br></div><div>Is this right? It's a bit confusing because I think you're okay with "by-without-by" and I got the impression from Sadao that he finds the syntax of "by-without-by" unaccessible/advanced for basic users. So, just to clarify, here the DT1[DT2, y, .JOIN=FALSE] will still do the "by-without-by" and then result in a "vector", right? </div>
<div><br></div><div>Matthew explains in the current documentation that DT1[DT2][, y] would "join" all columns of DT1 and DT2 and then subset. I assume the implementation underneath is *not* DT1[DT2][, y] rather the result is an efficient equivalence. Then, that of course seems alright to me.</div>
<div><br></div><div>If what I've told so far is right, then the syntax `DT1[DT2, .JOIN=FALSE]` doesn't make sense/has no purpose to me. At least I can't think of any at the moment. </div><div><br></div><div>To conclude, IMHO, if the purpose of `.JOIN` is to provide the same as DT1[i, j] for DT1[DT2, j] (j being a column or an expression that results in getting evaluated as a scalar for every group in the current by-without-by syntax), then, I find this is covered in `drop = TRUE/FALSE`. Correct me if I am wrong. But, one could do: `DT1[DT2, j, drop=TRUE]` instead of `DT1[DT2, j, .JOIN=FALSE]` and DT1[i, j, drop=FALSE] instead of DT1[i, list(x,y)].</div>
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<div><div>If you/anyone believes it's wrong, I'd be all ears to clarify as to what's the purpose of `drop` then (and also how it *doesn't* suit here as compared to .JOIN).</div><div><br></div>
<div>Arun</div><div><br></div></div><div><div>
<p style="color:#a0a0a8">On Tuesday, April 30, 2013 at 2:54 PM, Eduard Antonyan wrote:</p>
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<span><div><div><div>Arun,</div><div><br></div><div>If the new boolean is false, the result would be the same as without it and would be equal to current behavior of d[i][, j]. If it's true, it will only have an effect if i is a join (I think each.i= fits slightly better for this description than .join=) - this will replicate current underlying behavior. If you think the cross-apply is something that could work not just for i being a data-table but other things as well, then it would make perfect sense to implement that action too when the bool is true.</div>
<div><br>On Apr 30, 2013, at 2:58 AM, Arunkumar Srinivasan <<a href="mailto:aragorn168b@gmail.com" target="_blank">aragorn168b@gmail.com</a>> wrote:<br><br></div><blockquote type="cite"><div>
<div>(The earlier message was too long and was rejected.)
</div><div><div>So, from the discussion so far, I see that Matthew is nice enough to implement `.JOIN` or `cross.apply`. I've a couple of questions. Suppose,</div><div><br></div><div> DT1 <- data.table(x=c(1,1,2,3,3), y=1:5, z=6:10)</div>
<div> setkey(DT1, "x")</div><div> DT2 <- data.table(x=1)</div><div> DT1[DT2, y, .JOIN=TRUE] # I guess the syntax is something like this. I expect here the same output as current DT1[DT2, y]</div><div>
<br></div><div>The above syntax seems "okay". But my first question is what is `.JOIN=FALSE` supposed to do under these two circumstances? Suppose, </div><div><br></div><div> DT1 <- data.table(x=c(1,1,2,3,3), y=1:5, z=6:10)</div>
<div> setkey(DT1, "x")</div><div> DT2 <- data.table(x=c(1,2,1), w=c(11:13))</div><div> # what's the output supposed to be for?</div><div> DT1[DT2, y, .JOIN=FALSE]</div><div> DT1[DT2, .JOIN = FALSE]</div>
<div><br></div><div>Depending on this I'd have to think about `drop = TRUE/FALSE`. Also, how does it work with `subset`? </div><div><br></div><div> DT1[x %in% c(1,2,1), y, .JOIN=TRUE] # .JOIN is ignored?</div><div>
<span style="white-space:pre-wrap"> </span></div><div>Is this supposed to also do a "cross-apply" on the logical subset? I guess not. So, .JOIN is an "extra" parameter that comes into play *only* when `i` is a `data.table`? </div>
<div><br></div><div>I'd love to have some replies to these questions for me to take a stance on `.JOIN`. Thank you.</div><div><br></div><div>Best,</div><div>Arun.</div></div><div><br></div>
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