[datatable-help] Select from second key but not first

[Matthew Dowle]

Hi,

Those are single keys of multiple columns. Given dt keyed by (y,z), Chris
wanted to binary search on z, without changing the key, and without
copying the whole table just to put a different key on it, IIUC.

The suggestion for consistency of return type of single name j was
unrelated to secondary keys per se, it just jogged my memory.

> Hi Matt,
>
> I though multiple key indexing was already supported in data.table(). For
> example:
>
> dt1 <- data.table(x=1:100, y=1:2, z=1:4, key="y, z")
> dt2 <- data.table(a=100:1, b=2:1, c=4:1, key="b, c")
>
> dt2[dt1]
> #       b c  a x
> #  [1,] 1 1 97 1
> #  [2,] 1 1 93 1
> #  [3,] 1 1 89 1
> #  [4,] 1 1 85 1
> #  [5,] 1 1 81 1
> #  [6,] 1 1 77 1
> #  [7,] 1 1 73 1
> #  [8,] 1 1 69 1
> #  [9,] 1 1 65 1
> # [10,] 1 1 61 1
> # First 10 rows of 2500 printed.
>
> dt1[J(c(1, 1))]
> #    y  x z
> #    1  1 1
> #    1  5 1
> #    1  9 1
> #    1 13 1
> #    1 17 1
> #    1 21 1
> #    1 25 1
> # First 7 rows printed.
>
> dt1[c(1, 1)]
> #      x y z
> # [1,] 1 1 1
> # [2,] 1 1 1
>
> That seems to work as expected. I don't get why you and Chris are
> suggesting
> a fix is necessary. Could you maybe clarify for the rest of us?
>
> Many thanks, --Mel.
>
>
>
> -----Original Message-----
> From: datatable-help-bounces at r-forge.wu-wien.ac.at
> [mailto:datatable-help-bounces at r-forge.wu-wien.ac.at] On Behalf Of Matthew
> Dowle
> Sent: Tuesday, July 12, 2011 8:54 PM
> To: Chris Neff
> Cc: datatable-help at lists.r-forge.r-project.org
> Subject: Re: [datatable-help] Select from second key but not first
>
>
> Hi Chris,
>
> Welcome.
>
> That's a 'secondary key'. FR#1007 is to build in secondary keys :
> https://r-forge.r-project.org/tracker/index.php?func=detail&aid=1007&group_i
> d=240&atid=978
>
> In the meantime you can do a 'manual' secondary key :
>
>   idx = dt[,list(z,y,i=1:nrow(dt))]
>   setkey(idx,z,y)
>   dt[idx[J(3),i]$i]
>
> Btw, that [,i]$i is ugly and I'm coming around to the idea of making
> that more consistent, as requested by a previous poster (can't find it
> now).
>
> I'm thinking it should work like this :
>
>     DT[c("a","b"),j]
>     # always returns vector, for consistency, even though 2 groups are
> joined to via the mult="all" default and the correspondence might be
> lost. Then mult="first" and mult="last" would return vector too,
> consistent with mult="all"
>
>     DT[c("a","b"),list(j)]
>     # list() needed to retain the group columns and return a data.table
> rather than a vector. Same type (i.e. data.table) returned for all
> values of mult.
>
> Would that be better?  Throwing that out to all.
>
> Btw, posting from googlegroups does work then, that's good. This thread
> should be mirrored in all places; it shouldn't matter where you post
> from or to, but any probs please let me know as it's the first time.
>
> Matthew
>
>
> On Tue, 2011-07-12 at 16:21 -0700, Chris Neff wrote:
>> Hi all,
>>
>>
>> I'm really new to data.table and something really simple has me
>> stumped.
>>
>>
>> Lets say I have the following (but much bigger so timing matters)
>>
>>
>> dt = data.table(x=1:100,y=1:2,z=1:4,key="y,z")
>>
>>
>> From the documentation, I understand that
>>
>>
>> dt[J(1,3)]
>>
>>
>> is significantly faster
>>
>>
>> dt[y==1 & z==3]
>>
>>
>>
>>
>> and I could do
>>
>>
>> dt[J(1)]
>>
>>
>> instead of
>>
>>
>> dt[y==1]
>>
>>
>> but is there any way to do
>>
>>
>> dt[z==3]
>>
>>
>> faster?  I want to do something like
>>
>>
>> df[J( ,3)]
>>
>>
>> but I know that doesn't make sense. Is it because z is not the primary
>> key that I can't seem to figure out how to use J to do this? Since it
>> isn't sorted on z anyway I doubt I can get a speed up right?
>> _______________________________________________
>> datatable-help mailing list
>> datatable-help at lists.r-forge.r-project.org
>>
> https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help
>
>
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