Keep column name when filtering matrix columns

Question

I have a matrix, like the one generated with this code:

> m = matrix(data=c(1:50), nrow= 10, ncol = 5);
> colnames(m) = letters[1:5];

If I filter the columns, and the result have more than one column, the new matrix keeps the names. For example:

> m[, colnames(m) != "a"];

       b  c  d  e
 [1,] 11 21 31 41
 [2,] 12 22 32 42
 [3,] 13 23 33 43
 [4,] 14 24 34 44
 [5,] 15 25 35 45
 [6,] 16 26 36 46
 [7,] 17 27 37 47
 [8,] 18 28 38 48
 [9,] 19 29 39 49
[10,] 20 30 40 50

Notice that here, the class is still matrix:

> class(m[, colnames(m) != "a"]);
[1] "matrix"

But, when the filter lets only one column, the result is a vector, (integer vector in this case) and the column name, is lost.

> m[, colnames(m) == "a"]
[1]  1  2  3  4  5  6  7  8  9 10

> class(m[, colnames(m) == "a"]);
[1] "integer"

The name of the column is very important.

I would like to keep both, matrix structure (a one column matrix) and the column's name.

But, the column's name is more important.

I already know how to solve this by the long way (by keeping track of every case). I'm wondering if there is an elegant, enlightening solution.

Answer 1

You need to set drop = FALSE. This is good practice for programatic use

drop

For matrices and arrays. If TRUE the result is coerced to the lowest possible dimension (see the examples)

m[,'a',drop=FALSE]

This will retain the names as well.

Answer 2

You can also use subset:

m.a = subset(m, select = colnames(m) == "a")