使用row，col指示的矩阵的索引值 [英] Index values from a matrix using row, col indicies

问题描述

这可能很容易解决。我有一个2D矩阵 mat 有500行×335列，data.frame dat 有120425行。 data.frame dat 有两列 I 和 J ，它是从 mat 索引行，列的整数。我想将 mat 中的值添加到 dat 的行中。

This is probably simple to solve. 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

的向量

（我在Win32上使用R 2.13.1）。深入挖掘，我发现我滥用矩阵索引，因为看起来我只得到 mat 的子矩阵，而不是单维我预期的值数组，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 ...

我期待类似 int [1：100] 20 1 1 1 20 1 1 1 1 1 ... 。使用行，列索引来索引2D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

几乎。需要提供[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.

有一点需要注意：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，则整个矩阵成为最低分母类。

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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