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

[Matthew Dowle]

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 [11]> 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/ [8],
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 [9]" 
>>> 
>>> On Wed, Apr 24, 2013 at 4:01 PM,
Matthew Dowle <mdowle at mdowle.plus.com [10]> 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 [1]
>>>> >> To:
datatable-help at lists.r-forge.r-project.org [2]
>>>> 
>>>>>> 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
[3]
>>>> >> Sent from the datatable-help mailing list archive at
Nabble.com.
>>>> >> _______________________________________________
>>>>
>> datatable-help mailing list >>
datatable-help at lists.r-forge.r-project.org [4]
>>>> 
>>>>>>
https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help
[5]
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>>>> >
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[6]
>>>> >
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[7]

 

Links:
------
[1] mailto:eduard.antonyan at gmail.com
[2]
mailto:datatable-help at lists.r-forge.r-project.org
[3]
http://r.789695.n4.nabble.com/changing-data-table-by-without-by-syntax-to-require-a-by-tp4664770p4664990.html
[4]
mailto:datatable-help at lists.r-forge.r-project.org
[5]
https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help
[6]
mailto:datatable-help at lists.r-forge.r-project.org
[7]
https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help
[8]
http://explainextended.com/2009/07/16/inner-join-vs-cross-apply/
[9]
http://table2.id
[10] mailto:mdowle at mdowle.plus.com
[11]
mailto:mdowle at mdowle.plus.com
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