从矩阵列表中获取选定的矩阵列 [英] Getting selected matrix columns from a list of matrices

问题描述

我有一个尺寸相同的矩阵列表，例如:

I have a list of matrices with identical dimensions, for example:

mat.list=rep(list(matrix(rnorm(n=12,mean=1,sd=1), nrow = 3, ncol=4)),3)

我正在寻找一种从列表中的每个矩阵中检索列的有效方法，其中每个矩阵中感兴趣的列索引由向量指定.例如，对于此向量的列索引:

I'm looking for an efficient way to retrieve a column from each matrix in the list where the column index of interest from each matrix is specified by a vector. For example, for this vector of column indices:

idx.vec=c(3,2,3)

我想从矩阵1中获得第3列，从矩阵2中获得第2列，并从矩阵3中获得第3列，作为一个矩阵，这样该矩阵维就是列表中矩阵的行数乘以列表中的矩阵.

I would like to obtain column 3 from matrix 1, column 2 from matrix 2, and column 3 from matrix 3, as a matrix so that this matrix dimensions are the number of rows of the matrices in the list by the number of matrices in the list.

因此，对于此示例，结果将是:

For this example the result would therefore be:

cbind(mat.list[[1]][,3],mat.list[[2]][,2],mat.list[[3]][,3])
           [,1]      [,2]       [,3]
[1,]  1.4852810  1.305448  1.4852810
[2,]  1.8647327 -1.237507  1.8647327
[3,] -0.0416013  2.156055 -0.0416013

推荐答案

一种可能的方法是mapply('[', mat.list, TRUE, idx.vec).诀窍是使用'['进行子设置，并使用TRUE作为选择所有行的参数.运作方式如下:

One possible approach would be mapply('[', mat.list, TRUE, idx.vec). The trick is to use '[' for subsetting and TRUE as an argument to select all the rows. Here is how it works:

'['(matrix(1:4, ncol = 2), TRUE, 2)
# [1] 3 4

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