使用行，列索引的矩阵中的索引值 [英] Index values from a matrix using row, col indices

问题描述

我有一个500行×335列的2D矩阵mat和一个120425行的data.frame dat. data.frame dat具有两列I和J，它们是用于索引行的整数，列是mat的列.我想将mat中的值添加到dat的行中.

I have a 2D matrix mat with 500 rows × 335 columns, and a data.frame dat with 120425 rows. The data.frame dat has two columns I and J, which are integers to index the row, column from mat. I would like to add the values from mat to the rows of dat.

这是我的概念上的失败:

Here is my conceptual fail:

> dat$matval <- mat[dat$I, dat$J]
Error: cannot allocate vector of length 1617278737

(我在Win32上使用R 2.13.1).深入研究，我发现我在滥用矩阵索引，因为看来我只得到mat的子矩阵，而不是我所期望的一维值数组，即:

(I am using R 2.13.1 on Win32). Digging a bit deeper, I see that I'm misusing matrix indexing, as it appears that I'm only getting a sub-matrix of mat, and not a single-dimension array of values as I expected, i.e.:

> str(mat[dat$I[1:100], dat$J[1:100]])
 int [1:100, 1:100] 20 1 1 1 20 1 1 1 1 1 ...

我期待的是类似int [1:100] 20 1 1 1 20 1 1 1 1 1 ...的东西.使用行，列的索引获取值来索引2D矩阵的正确方法是什么?

I was expecting something like int [1:100] 20 1 1 1 20 1 1 1 1 1 .... What is the correct way to index a 2D matrix using indices of row, column to get the values?

推荐答案

几乎.需要提供给"["作为两列矩阵:

Almost. Needs to be offered to "[" as a two column matrix:

dat$matval <- mat[ cbind(dat$I, dat$J) ] # should do it.

有一个警告:尽管这也适用于数据帧，但它们首先被强制转换为矩阵类，如果有任何非数字类，则整个矩阵将成为最低分母"类.

There is a caveat: Although this also works for dataframes, they are first coerced to matrix-class and if any are non-numeric, the entire matrix becomes the "lowest denominator" class.

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