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

[Bacou, Melanie]

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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