选择矩阵中满足条件的行 [英] Select rows of a matrix that meet a condition

问题描述

在带有矩阵的 R 中:

In R with a matrix:

     one two three four
 [1,]   1   6    11   16
 [2,]   2   7    12   17
 [3,]   3   8    11   18
 [4,]   4   9    11   19
 [5,]   5  10    15   20

我想提取其行的第三列 = 11 的子矩阵.即:

I want to extract the submatrix whose rows have column three = 11. That is:

      one two three four
 [1,]   1   6    11   16
 [3,]   3   8    11   18
 [4,]   4   9    11   19

我想在不循环的情况下执行此操作.我是 R 的新手，所以这可能很明显，但是文档通常有些简洁.

I want to do this without looping. I am new to R so this is probably very obvious but the documentation is often somewhat terse.

推荐答案

如果您使用 as.data.frame() 将矩阵转换为数据框，这会更容易做到.在这种情况下，前面的答案(使用子集或 m$three)将起作用，否则将不起作用.

This is easier to do if you convert your matrix to a data frame using as.data.frame(). In that case the previous answers (using subset or m$three) will work, otherwise they will not.

要对矩阵执行操作，您可以按名称定义列:

To perform the operation on a matrix, you can define a column by name:

m[m[, "three"] == 11,]

或按编号:

m[m[,3] == 11,]

注意，如果只有一行匹配，结果是一个整数向量，而不是一个矩阵.

Note that if only one row matches, the result is an integer vector, not a matrix.

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