Dear R-community, I have some trouble with index mappings for arrays. If, for example, I have the array R> A <- array(1:9, c(3,3,2)) and two index mappings both of same size R> x <- c(2, 3) R> y <- c(1, 2) Now I want to access the elements (A[1, x[i], y[i]])_i of A, i.e. A[1, x[1], y[1]] = A[1, 2, 1] and A[1, x[2], y[2]] = A[1, 3, 2]. If I use R> A[1, x, y] I would get every combinations of indices of all elements of x and y i.e. A[1, x[1], y[1]], A[1, x[1], y[2]], A[1, x[2], y[1]] and A[1, x[2], y[2]]. But how can I access the elements (A[1, x[i], y[i]])_i. My arrays dimensions are actually large in the second component (for example the dimension might be 10*10000*10) so I'm looking for a method avoiding loops. The question is probably trivial for you, but I just could not figure it out. So sorry for bugging you and many thanks in advance for any help. Best wishes, Demi Anderson. -- "Feel free" - 10 GB Mailbox, 100 FreeSMS/Monat ...
probably you want something like the following:
A[cbind(rep(1, length(x)), x, y)]
I hope it helps.
Best,
Dimitris
----
Dimitris Rizopoulos
Ph.D. Student
Biostatistical Centre
School of Public Health
Catholic University of Leuven
Address: Kapucijnenvoer 35, Leuven, Belgium
Tel: +32/(0)16/336899
Fax: +32/(0)16/337015
Web: http://med.kuleuven.be/biostat/
http://www.student.kuleuven.be/~m0390867/dimitris.htm
----- Original Message -----
From: "Demi Anderson" <demi.anderson at gmx.net>
To: <r-help at stat.math.ethz.ch>
Sent: Thursday, February 01, 2007 2:34 PM
Subject: [R] Index mapping on arrays
> Dear R-community,
>
> I have some trouble with index mappings for arrays. If, for example,
> I have the array
>
> R> A <- array(1:9, c(3,3,2))
>
> and two index mappings both of same size
>
> R> x <- c(2, 3)
> R> y <- c(1, 2)
>
> Now I want to access the elements (A[1, x[i], y[i]])_i of A, i.e.
> A[1, x[1], y[1]] = A[1, 2, 1] and A[1, x[2], y[2]] = A[1, 3, 2].
>
> If I use
>
> R> A[1, x, y]
>
> I would get every combinations of indices of all elements of x and y
> i.e. A[1, x[1], y[1]], A[1, x[1], y[2]], A[1, x[2], y[1]] and A[1,
> x[2], y[2]]. But how can I access the elements (A[1, x[i], y[i]])_i.
> My arrays dimensions are actually large in the second
> component (for example the dimension might be 10*10000*10) so I'm
> looking for a method avoiding loops.
>
> The question is probably trivial for you, but I just could not
> figure it out. So sorry for bugging you and many thanks in advance
> for any help.
>
> Best wishes, Demi Anderson.
> --
> "Feel free" - 10 GB Mailbox, 100 FreeSMS/Monat ...
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
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> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm
Hello Demi The trick for array indexing on an array A, where length(dim(A))==n, is to use an n-column matrix M to extract the elements by rows of M. If I understand correctly, the following should help: > A <- array(1:18,c(3,3,2)) > x <- 2:3 > y <- 1:2 > A[cbind(x,y,1)] [1] 2 6 You may find the following useful too: > A[as.matrix(cbind(1,expand.grid(x,y)))] [1] 4 7 13 16 > best rksh On 1 Feb 2007, at 13:34, Demi Anderson wrote:> Dear R-community, > > I have some trouble with index mappings for arrays. If, for example, > I have the array > > R> A <- array(1:9, c(3,3,2)) > > and two index mappings both of same size > > R> x <- c(2, 3) > R> y <- c(1, 2) > > Now I want to access the elements (A[1, x[i], y[i]])_i of A, i.e. > A[1, x[1], y[1]] = A[1, 2, 1] and A[1, x[2], y[2]] = A[1, 3, 2]. > > If I use > > R> A[1, x, y] > > I would get every combinations of indices of all elements of x and y > i.e. A[1, x[1], y[1]], A[1, x[1], y[2]], A[1, x[2], y[1]] and A[1, > x[2], y[2]]. But how can I access the elements (A[1, x[i], y[i]])_i. > My arrays dimensions are actually large in the second > component (for example the dimension might be 10*10000*10) so I'm > looking for a method avoiding loops. > > The question is probably trivial for you, but I just could not > figure it out. So sorry for bugging you and many thanks in advance > for any help. > > Best wishes, Demi Anderson. > -- > "Feel free" - 10 GB Mailbox, 100 FreeSMS/Monat ... > > ______________________________________________ > R-help at stat.math.ethz.ch mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting- > guide.html > and provide commented, minimal, self-contained, reproducible code.-- Robin Hankin Uncertainty Analyst National Oceanography Centre, Southampton European Way, Southampton SO14 3ZH, UK tel 023-8059-7743 -- This e-mail (and any attachments) is confidential and intend...{{dropped}}