[R] matrix -> delete last row -> list

[Marc Schwartz]

> On Jul 3, 2015, at 9:33 AM, Zander, Joscha <joscha.zander at roche.com> wrote:
> 
> Good day R-community,
> 
> i just wondered if it is a bug or a feature...
> 
> When i have a matrix "mat" with one column and i delete the last row with
> 
> mat <- mat[-nrow(mat),] the result is a list.
> 
> So my next call mat[10,] will throw an "wrong dimension" error.
> The proper call must be:
> 
> mat <- as.matrix(mat[-nrow(mat),])
> 
> So is this desired behavior or a bug?
> 
> I use R-version 2.15.3, but reconstructed this behavior in 3.2.0 as well.
> 
> greetings
> 
> -- 
> *Joscha Zander*


See:

  http://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-do-my-matrices-lose-dimensions_003f


mat <- matrix(1:12, ncol = 1)

> str(mat)
 int [1:12, 1] 1 2 3 4 5 6 7 8 9 10 ...

> mat[-nrow(mat), ]
 [1]  1  2  3  4  5  6  7  8  9 10 11

# This is a vector, not a list
> str(mat[-nrow(mat), ])
 int [1:11] 1 2 3 4 5 6 7 8 9 10 …

> mat[-nrow(mat), , drop = FALSE]
      [,1]
 [1,]    1
 [2,]    2
 [3,]    3
 [4,]    4
 [5,]    5
 [6,]    6
 [7,]    7
 [8,]    8
 [9,]    9
[10,]   10
[11,]   11

# This is a matrix
> str(mat[-nrow(mat), , drop = FALSE])
 int [1:11, 1] 1 2 3 4 5 6 7 8 9 10 …


Regards,

Marc Schwartz

P.S. are you restricted in being able to upgrade from a version of R that is two years old?