[R] Index mapping on arrays

[Robin Hankin]

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.
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--
Robin Hankin
Uncertainty Analyst
National Oceanography Centre, Southampton
European Way, Southampton SO14 3ZH, UK
  tel  023-8059-7743


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