[datatable-help] changing data.table by-without-by syntax to require a "by"
Eduard Antonyan
eduard.antonyan at gmail.com
Thu Apr 25 01:01:42 CEST 2013
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.
>>> >> _______________________________________________
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>>> >> datatable-help at lists.r-forge.r-project.org
>>> >>
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>>> >
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>>>
>>
>>
>
>
>
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