[datatable-help] changing data.table by-without-by syntax to require a "by"

Thu Apr 25 16:54:36 CEST 2013

Whatever the "right" way to do things is, the key issue is that default behavior should not be changed since existing code will rely on it.  So even though I tend to agree with Eduard, I would strongly advocate against any change in current behavior.  This aside, let me throw my 2 pennies in for the sake of data.table.2:

As for CROSS APPLY, to be honest, my experience with SQL has been primarily with MySQL < 5 so I didn't even know that existed.  As for your specific example a couple of e-mails ago, I believe this works:

X = data.table(a=1:3,b=1:15, key="a")
Y = data.table(a=c(1,2,1), top=c(3,4,2))
X[Y][, head(.SD, top[1]), by=list(a, top)]

Granted, this is somewhat inefficient since we now have the `top` vector replicated for each value of `a` in `X`.  You can probably come up with other examples that are inefficient or just don't work (e.g. `Y = data.table(a=c(1,2,1, 1), top=c(3,4,2,2))`), but the point here isn't whether you should allow CROSS APPLY or not, but what the "correct" syntax for invoking cross apply is.

I would argue that the correct output to:

X[Y, sum(a * top)]

Should be 21, not:

   a V1
1: 1  3
2: 2  8
3: 1  2
While the output above may be convenient to you, it is not intuitive at all.  In fact, it is an advanced caveat to standard behavior ("J is an expression evaluated in the context of X") that isn't straigthforward to circumvent, and would likely bewilder most beginner users of data.table.  I think given the parallels between data.table and SQL, "X[Y, sum(a * top)]" should mean "SELECT sum(X.a * Y.top) FROM X INNER JOIN Y USING(a)", not some more complex expression involving a CROSS APPLY.  Note that if you want a CROSS APPLY in SQL, you have to ask for it (I guess I picked at terrible example here, since the GROUP is implied...).

I think the "correct" way to do the original task would be something along the lines of:

X[Y, head(.SD, i.top), cross.apply=TRUE]

or some such.

That said, data.table is yours.  It is a fantastic tool, and if you want to behave in a manner that simplifies your work rather than matches the intuitions of others, then it is your hard earned right that I fully respect.

Slightly off topic, why aren't the columns from the Y table available in joint inherited scope when not doing a by without by?  I find it odd that:

X[Y, sum(a * top), by=b]

Produces:
Error in `[.data.table`(X, Y, sum(a * top), by = b) : 
  object 'top' not found
Finally, is i.top documented?

S.

From: eduard.antonyan at gmail.com
Date: Thu, 25 Apr 2013 07:45:45 -0500
To: mdowle at mdowle.plus.com
CC: datatable-help at lists.r-forge.r-project.org
Subject: Re: [datatable-help] changing data.table by-without-by syntax to require a "by"

Well, so can .I or .N or .GRP or .BY, yet those are used as special names, which is exactly why I suggested .J.
The problem with using 'missingness' is that it already means smth very different when i is not a join/cross, it means *don't* do a by, thus introducing the whole case thing one has to through in their head every time as in OP (which of course becomes automatic after a while, but it's a cost nonetheless, which is in particular high for new people). So I see absence of 'by' as an already taken and used signal and thus something else has to be used for the new signal of cross apply (it doesn't have to be the specific option I mentioned above). This is exactly why I find optional turning off of this behavior unsatisfactory, and I don't see that as a solution to this at all. 

I think in the x+y context the appropriate analog is - what if that added x and y normally, but when x and y were data.frames it did element by element multiplication instead? Yes that's possible to do, and possible to document, but it's not a good idea, because it takes place of adding them element by element. The recycling behavior doesn't do that - what that does is it says it doesn't really make sense to add them as is, but we can do that after recycling, so let's recycle. It doesn't take the place of another existing way of adding vectors. 

On Apr 25, 2013, at 4:28 AM, Matthew Dowle <mdowle at mdowle.plus.com> wrote:

I see what you're getting at. But .J may be a column name, which is the current meaning of by = single symbol. And why .J?  If not .J, or any single symbol what else instead?  A character value such as by="irows" is taken to mean the "irows" column currently (for consistency with by="colA,colB,colC").  But some signal needs to be passed to by=, then (you're suggesting), to trigger the cross apply by each i row.  Currently, that signal is missingness  (which I like, rely on, and use with join inherited scope).

As I wrote in the S.O. thread,  I'm happy to make it optional (i.e. an option to turn off by-without-by), since there is no downside.   But you've continued to argue for a change to the default, iiuc.

Maybe it helps to consider :

     x+y

Fundamentally in R this depends on what x and y are.  Most of us probably assume (as a first thought) that x and y are vectors and know that this will apply "+" elementwise,  recycling y if necessary.  In R we like and write code like this all the time.   I think of X[Y, j] in the same way: j is the operation (like +) which is applied for each row of Y.   If you need j for the entire set that Y joins to,  then like a FAQ says,  make j missing too and it's X[Y][,j]. But providing a way to make X[Y,j] do the same as X[Y][,j] would be nice and is on the list:  drop=TRUE would do that (as someone mentioned on the S.O. thread).  So maybe the new option would be datatable.drop (but with default FALSE not TRUE).  If you wanted to turn off by-without-by you might set options(datatable.drop=TRUE). Then you can use data.table how you prefer (explicit by) and I can use it how I prefer.

I'm happy to add the argument to [.data.table,  and make its default changeable via a global option in the usual way. 
Matthew

On 25.04.2013 05:16, Eduard Antonyan wrote:

That's really interesting, I can't currently think of another way of doing that as after X[Y] is done the necessary information is lost. 
To retain that functionality and achieve better readability, as in OP, I think smth along the lines of X[Y, head(.SD, i.top), by=.J] would be a good replacement for current syntax.

On Apr 24, 2013, at 6:01 PM, Eduard Antonyan <eduard.antonyan at gmail.com> wrote:

that's an interesting example - I didn't realize current behavior would do that, I'm not at a PC anymore but I'll definitely think about it and report back, as it's not immediately obvious to me

On Wed, Apr 24, 2013 at 5:50 PM, Matthew Dowle <mdowle at mdowle.plus.com> wrote:

i. prefix is just a robust way to reference join inherited columns:   the 'top' column in the i table.   Like table aliases in SQL.

What about this? :

1> X = data.table(a=1:3,b=1:15, key="a")
1> X
 a b
 1: 1 1
 2: 1 4
 3: 1 7
 4: 1 10
 5: 1 13
 6: 2 2
 7: 2 5
 8: 2 8
 9: 2 11
10: 2 14
 11: 3 3
12: 3 6

13: 3 9
14: 3 12
15: 3 15
1> Y = data.table(a=c(1,2,1), top=c(3,4,2))

1> Y
 a top
1: 1 3
2: 2 4
3: 1 2
1> X[Y, head(.SD,i.top)]
 a b
1: 1 1
2: 1 4
3: 1 7
4: 2 2
5: 2 5
6: 2 8
7: 2 11
8: 1 1
9: 1  4
1> 

On 24.04.2013 23:43, Eduard Antonyan wrote:

I assumed they meant create a table :)
that looks cool, what's i.top ? I can get a very similar to yours result by writing:
X[Y][, head(.SD, top[1]), by = a]
and I probably would want the following to produce your result (this might depend a little on what exactly i.top is):
X[Y, head(.SD, i.top), by = a]

On Wed, Apr 24, 2013 at 5:28 PM, Matthew Dowle <mdowle at mdowle.plus.com> wrote:

That sentence on that linked webpage seems incorect English, since table is a noun not a verb.  Should "table" be "join" perhaps?

Anyway, by-without-by is often used with join inherited scope (JIS).  For example, translating their example :

1> X = data.table(a=1:3,b=1:15, key="a")
1> X
    a  b
 1: 1  1
 2: 1  4
 3: 1  7
 4: 1 10
 5: 1 13
 6: 2  2
 7: 2  5
 8: 2  8
 9: 2 11
10: 2 14
11: 3  3
12: 3  6

13: 3  9
14: 3 12
15: 3 15
1> Y = data.table(a=c(1,2), top=c(3,4))
1> Y
   a top
1: 1   3
2: 2   4
1> X[Y, head(.SD,i.top)]
   a  b
1: 1  1
2: 1  4
3: 1  7
4: 2  2
5: 2  5

6: 2  8
7: 2 11
1> 

If there was no by-without-by (analogous to CROSS BY),  then how would that be done?

On 24.04.2013 22:22, Eduard Antonyan wrote:

By that you mean current behavior? You'd get current behavior by explicitly specifying the appropriate "by" (i.e. "by" equal to the key).
Btw, I'm trying to understand SQL CROSS APPLY vs JOIN using http://explainextended.com/2009/07/16/inner-join-vs-cross-apply/, and I can't figure out how by-without-by (or with by-with-by for that matter:) ) helps with e.g. the first example there:

"We table table1 and table2. table1 has a column called rowcount.

For each row from table1 we need to select first rowcount rows from table2, ordered by table2.id"

On Wed, Apr 24, 2013 at 4:01 PM, Matthew Dowle <mdowle at mdowle.plus.com> wrote:

But then what would be analogous to CROSS APPLY in SQL?

 > I'd agree with Eduard, although it's probably too late to change behavior
 > now.  Maybe for data.table.2?  Eduard's proposal seems more closely
 > aligned with SQL behavior as well (SELECT/JOIN, then GROUP, but only if

 > requested).
 >
 > S.
 >
 >> Date: Mon, 22 Apr 2013 08:17:59 -0700
>> From: eduard.antonyan at gmail.com
 >> To: datatable-help at lists.r-forge.r-project.org

>> Subject: Re: [datatable-help] changing data.table by-without-by
 >> syntax       to      require a "by"
 >>
 >> I think you're missing the point Michael. Just because it's possible to

 >> do it
 >> the way it's done now, doesn't mean that's the best way, as I've tried
 >> to
 >> argue in the OP. I don't think you've addressed the issue of unnecessary

 >> complexity pointed out in OP.
 >>
 >>
 >>
 >> --
 >> View this message in context:
 >> http://r.789695.n4.nabble.com/changing-data-table-by-without-by-syntax-to-require-a-by-tp4664770p4664990.html

 >> Sent from the datatable-help mailing list archive at Nabble.com.
 >> _______________________________________________
 >> datatable-help mailing list
>> datatable-help at lists.r-forge.r-project.org

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