r中具有数组的子集矩阵 [英] Subset matrix with arrays in r

问题描述

这可能是相当基本的，但我还没有找到简单的解决方案.

It is probably fairly basic but I have not found an easy solution.

假设我有一个三维矩阵:

Assume I have a three-dimensional matrix:

m <- array(seq_len(18),dim=c(3,3,2))

，我想用索引数组对矩阵进行子集处理:

and I would like to subset the matrix with the arrays of indexes:

idxrows <- c(1,2,3)
idxcols <- c(1,1,2)

获取位置(1,1)，(2,1)和(3,2)的数组，即:

obtaining the arrays in position (1,1),(2,1) and (3,2), that is:

       [,1] [,2] [,3]
[1,]    1    5    9
[2,]   10   14   18

我尝试了m[idxrows,idxcols,]，但是没有任何运气.

I have tried m[idxrows,idxcols,] but without any luck.

反正有做吗(显然没有使用for循环)?

Is there anyway to do it (without obviously using a for loop)?

推荐答案

不确定 extract 语法是否内置任何简单的方法，但是您可以使用mapply解决此问题:

Not sure if there is any easy built in extract syntax, but you can work around this with mapply:

mapply(function(i, j) m[i,j,], idxrows, idxcols)

#     [,1] [,2] [,3]
#[1,]    1    2    6
#[2,]   10   11   15

---

或更复杂的是，创建一个索引矩阵，其列与原始数组的尺寸匹配:

---

Or slightly more convoluted, create a index matrix whose columns match the dimensions of the original array:

thirdDim <- dim(m)[3]
index <- cbind(rep(idxrows, each = thirdDim), rep(idxcols, each = thirdDim), 1:thirdDim)
matrix(m[index], nrow = thirdDim)

#     [,1] [,2] [,3]
#[1,]    1    2    6
#[2,]   10   11   15

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