Index mapping on arrays
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 ...
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