使用 row, col 索引从矩阵中索引值 [英] Index values from a matrix using row, col indices

问题描述

我有一个 500 行 × 335 列的二维矩阵 mat 和一个 120425 行的 data.frame dat.data.frame dat 有两列 I 和 J，它们是用于索引行的整数，列来自 mat.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.

这是我的概念失败:

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

(我在 Win32 上使用 R 2.13.1).再深入一点，我发现我在滥用矩阵索引，因为看起来我只得到了 mat 的子矩阵，而不是我预期的单维值数组，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 ...

我期待像 int [1:100] 20 1 1 1 20 1 1 1 1 1 ... 这样的东西.使用行、列索引来索引二维矩阵以获取值的正确方法是什么?

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

差不多.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:虽然这也适用于数据帧，但它们首先被强制转换为矩阵类，如果有任何非数字矩阵，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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