Sara Sara - 3 months ago 20
R Question

R: extract matrix from array, using a matrix of indices

I´m coding in R and I have a 3 dimensional array that contains data (ab in the example). Then I have a matrix that contains indices of the 3rd array dimension (idx). This matrix has the same number of rows and columns of the array. I want to use the indices contained in idx to extract data from the array, to get a matrix with same dimension of idx. Please see the example below:

a <- c(1:9)
b <- rev(a)

#array of data
ab <- array(c(a,b), dim = c(3,3,2))
ab
, , 1

[,1] [,2] [,3]
[1,] 1 4 7
[2,] 2 5 8
[3,] 3 6 9

, , 2

[,1] [,2] [,3]
[1,] 9 6 3
[2,] 8 5 2
[3,] 7 4 1

#matrix of indices
idx <- matrix(sample(1:2,9,replace=TRUE), nrow = 3)
idx
[,1] [,2] [,3]
[1,] 2 2 2
[2,] 2 1 1
[3,] 1 1 1

#now I want to get the following matrix:
[,1] [,2] [,3]
[1,] 9 6 3
[2,] 8 5 8
[3,] 3 6 9

#these two don´t do the job
ab[idx]
ab[ , ,idx]


Does anybody know how can I get that?

Many thanks!

Sara

Answer

We need an index for row/column and the third dimension (from 'idx') to extract the elements. We do this by cbinding the row index, column index with the 'idx'.

i1 <- dim(ab)[1]
j1 <- dim(ab)[2]
matrix(ab[cbind(rep(seq_len(i1),  j1),rep(seq_len(j1), each = i1), c(idx))], ncol=3)
#     [,1] [,2] [,3]
#[1,]    9    6    3
#[2,]    8    5    8
#[3,]    3    6    9