# [R] do.call("[", ...) question

Dimitris Rizopoulos dimitris.rizopoulos at med.kuleuven.ac.be
Wed Sep 8 16:25:53 CEST 2004

```Hi Robin,

you could try the following:

lis <- lapply(seq(1, length(dim(a))+1), function(x,a.,jj.) if(x==1) a.
else jj.[[x-1]] ,a.=a, jj.=jj)
do.call("[",lis)

I hope this helps.

Best,
Dimitris

----
Dimitris Rizopoulos
Ph.D. Student
Biostatistical Centre
School of Public Health
Catholic University of Leuven

Address: Kapucijnenvoer 35, Leuven, Belgium
Tel: +32/16/396887
Fax: +32/16/337015
Web: http://www.med.kuleuven.ac.be/biostat/
http://www.student.kuleuven.ac.be/~m0390867/dimitris.htm/

----- Original Message -----
From: "Robin Hankin" <rksh at soc.soton.ac.uk>
To: <r-help at stat.math.ethz.ch>
Sent: Wednesday, September 08, 2004 4:08 PM
Subject: [R] do.call("[", ...) question

> Hi again everyone
>
> I have an arbitrarily dimensional array "a" and a list "jj" of
length
> length(dim(a)).    The elements of jj are vectors of indexes.
>
> How do I use do.call() to extract a[ jj[[1]], jj[[2]], jj[[3]], ...]
?
>
>
> Toy example follows:
>
> a <- matrix(1:30,5,6)
> jj <- list(5:1,6:1)
>
> I want the following
>
>   a[ jj[[1]],jj[[2]] ]
>
> How do I do this?
>
>
>
> OBAttempts:
>
> do.call("[",list(a,jj))
> do.call("[",c(a,jj))
> do.call("[",list(a,unlist(jj)))
>
>
> Of course, the one that works is
>
> do.call("[",list(a,jj[[1]],jj[[2]]))
>
> but I don't know how long jj is apriori so this won't do.
> --
> Robin Hankin
> Uncertainty Analyst
> Southampton Oceanography Centre
> SO14 3ZH
> tel +44(0)23-8059-7743
> initialDOTsurname at soc.soton.ac.uk (edit in obvious way; spam
precaution)
>
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