R:使用索引矩阵从数组中提取矩阵 [英] R: extract matrix from array, using a matrix of indices

问题描述

我用R编码，我有一个三维数组，其中包含数据(本例中为ab).然后，我有一个矩阵，其中包含第3个数组维数(idx)的索引.该矩阵具有相同数量的行和列的数组.我想使用idx中包含的索引从数组中提取数据，以获得与idx尺寸相同的矩阵.请参见下面的示例:

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]

有人知道我怎么得到吗?

Does anybody know how can I get that?

非常感谢！

萨拉

推荐答案

我们需要行/列的索引和第三个维度(来自"idx")以提取元素.我们通过cbind使用'idx'来对行索引，列索引进行操作.

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

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