Hello,
I recently discovered a possible inconsistency with usage of an object of
class array.
Consider the following example:
## Setup
a <- array(1:6, dim = c(1, 3, 2))
a
, , 1
[,1] [,2] [,3]
[1,] 1 2 3
, , 2
[,1] [,2] [,3]
[1,] 4 5 6
class(a)
[1] "array"
dim(a)
[1] 1 3 2
## Now use `[`
a[1,,]
[,1] [,2]
[1,] 1 4
[2,] 2 5
[3,] 3 6
class(a[1,,])
[1] "matrix" "array"
dim(a[1,,])
[1] 3 2
Up until this point, it makes sense to me. Now, let's consider when dim c(1,
6, 1). This is where I have a little trouble understanding the
behavior.
## Array with dim = c(1, any_number_here, 1)
b <- array(1:6, dim = c(1, 6, 1))
b
, , 1
[,1] [,2] [,3] [,4] [,5] [,6]
[1,] 1 2 3 4 5 6
class(b)
[1] "array"
dim(b)
[1] 1 6 1
## The problem
b[1,,]
[1] 1 2 3 4 5 6
dim(b[1,,])
NULL
class(b[1,,])
[1] "integer"
I would have expected:
b[1,,] ## produced the output with matrix(1:6, ncol = 1)
[,1]
[1,] 1
[2,] 2
[3,] 3
[4,] 4
[5,] 5
[6,] 6
class(b[1,,])
[1] "matrix" "array"
dim(b[1,,])
[1] 3 1
Is this a bug? If not, any help understanding this behaviour would be much
appreciated.
Thanks,
Joseph
[[alternative HTML version deleted]]
This is due to `[` dropping dimensions by default. In your first example, think of a[1, , ] as having dimension c(1, 3, 2), but, because drop = TRUE, all dimensions of extent 1 (the first dimension) are dropped and the result has dimension c(3, 2). In your second example, b[1, , ] would have dimension c(1, 6, 1), but now both the first and third dimensions are dropped, resulting in a vector with no dimensions. We can use the drop function to explicitly see why b[1, ,] doesn't result in a matrix.> drop(matrix(1:6, 6, 1))[1] 1 2 3 4 5 6 Steve On Fri, 29 Sept 2023 at 23:28, Joseph Wood <jwood000 at gmail.com> wrote:> > Hello, > > I recently discovered a possible inconsistency with usage of an object of > class array. > > Consider the following example: > > ## Setup > > a <- array(1:6, dim = c(1, 3, 2)) > a > , , 1 > > [,1] [,2] [,3] > [1,] 1 2 3 > > , , 2 > > [,1] [,2] [,3] > [1,] 4 5 6 > > class(a) > [1] "array" > > dim(a) > [1] 1 3 2 > > ## Now use `[` > a[1,,] > [,1] [,2] > [1,] 1 4 > [2,] 2 5 > [3,] 3 6 > > class(a[1,,]) > [1] "matrix" "array" > > dim(a[1,,]) > [1] 3 2 > > Up until this point, it makes sense to me. Now, let's consider when dim > c(1, 6, 1). This is where I have a little trouble understanding the > behavior. > > ## Array with dim = c(1, any_number_here, 1) > > b <- array(1:6, dim = c(1, 6, 1)) > b > , , 1 > > [,1] [,2] [,3] [,4] [,5] [,6] > [1,] 1 2 3 4 5 6 > > class(b) > [1] "array" > > dim(b) > [1] 1 6 1 > > ## The problem > > b[1,,] > [1] 1 2 3 4 5 6 > > dim(b[1,,]) > NULL > > class(b[1,,]) > [1] "integer" > > I would have expected: > > b[1,,] ## produced the output with matrix(1:6, ncol = 1) > [,1] > [1,] 1 > [2,] 2 > [3,] 3 > [4,] 4 > [5,] 5 > [6,] 6 > > class(b[1,,]) > [1] "matrix" "array" > > dim(b[1,,]) > [1] 3 1 > > Is this a bug? If not, any help understanding this behaviour would be much > appreciated. > > Thanks, > Joseph > > [[alternative HTML version deleted]] > > ______________________________________________ > R-devel at r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-devel
Hi Joseph This behaviour is as expected (and as documented by ?`[`) because the default of drop=TRUE coerces the result to the lowest possible dimension, which in your case is a vector rather than array. Using for example: b[1,,,drop=FALSE] ... results in an output array with the same number of dimensions (i.e. length of dim()) as the input array. I always use drop=FALSE when programming in situations where the dimensions of the input array aren't known (and forgetting to do so has several times bitten me when I later come to pass in an array like yours). I am not aware of a way to make the extract operator behave as you apparently expected (drop the dimension corresponding to the supplied length-1 index but leaving all others even if they are also length 1). If you want this behaviour then you would be best to create a wrapper function, or perhaps even a derived class, but that is not a topic for r-devel (rather r-help). Best wishes, Matt ?On 30/09/2023, 05:28, "R-devel on behalf of Joseph Wood" <r-devel-bounces at r-project.org <mailto:r-devel-bounces at r-project.org> on behalf of jwood000 at gmail.com <mailto:jwood000 at gmail.com>> wrote: [You don't often get email from jwood000 at gmail.com <mailto:jwood000 at gmail.com>. Learn why this is important at https://aka.ms/LearnAboutSenderIdentification <https://aka.ms/LearnAboutSenderIdentification> ] Hello, I recently discovered a possible inconsistency with usage of an object of class array. Consider the following example: ## Setup a <- array(1:6, dim = c(1, 3, 2)) a , , 1 [,1] [,2] [,3] [1,] 1 2 3 , , 2 [,1] [,2] [,3] [1,] 4 5 6 class(a) [1] "array" dim(a) [1] 1 3 2 ## Now use `[` a[1,,] [,1] [,2] [1,] 1 4 [2,] 2 5 [3,] 3 6 class(a[1,,]) [1] "matrix" "array" dim(a[1,,]) [1] 3 2 Up until this point, it makes sense to me. Now, let's consider when dim c(1, 6, 1). This is where I have a little trouble understanding the behavior. ## Array with dim = c(1, any_number_here, 1) b <- array(1:6, dim = c(1, 6, 1)) b , , 1 [,1] [,2] [,3] [,4] [,5] [,6] [1,] 1 2 3 4 5 6 class(b) [1] "array" dim(b) [1] 1 6 1 ## The problem b[1,,] [1] 1 2 3 4 5 6 dim(b[1,,]) NULL class(b[1,,]) [1] "integer" I would have expected: b[1,,] ## produced the output with matrix(1:6, ncol = 1) [,1] [1,] 1 [2,] 2 [3,] 3 [4,] 4 [5,] 5 [6,] 6 class(b[1,,]) [1] "matrix" "array" dim(b[1,,]) [1] 3 1 Is this a bug? If not, any help understanding this behaviour would be much appreciated. Thanks, Joseph [[alternative HTML version deleted]] ______________________________________________ R-devel at r-project.org <mailto:R-devel at r-project.org> mailing list https://stat.ethz.ch/mailman/listinfo/r-devel <https://stat.ethz.ch/mailman/listinfo/r-devel>
?s 17:33 de 29/09/2023, Joseph Wood escreveu:> Hello, > > I recently discovered a possible inconsistency with usage of an object of > class array. > > Consider the following example: > > ## Setup > > a <- array(1:6, dim = c(1, 3, 2)) > a > , , 1 > > [,1] [,2] [,3] > [1,] 1 2 3 > > , , 2 > > [,1] [,2] [,3] > [1,] 4 5 6 > > class(a) > [1] "array" > > dim(a) > [1] 1 3 2 > > ## Now use `[` > a[1,,] > [,1] [,2] > [1,] 1 4 > [2,] 2 5 > [3,] 3 6 > > class(a[1,,]) > [1] "matrix" "array" > > dim(a[1,,]) > [1] 3 2 > > Up until this point, it makes sense to me. Now, let's consider when dim > c(1, 6, 1). This is where I have a little trouble understanding the > behavior. > > ## Array with dim = c(1, any_number_here, 1) > > b <- array(1:6, dim = c(1, 6, 1)) > b > , , 1 > > [,1] [,2] [,3] [,4] [,5] [,6] > [1,] 1 2 3 4 5 6 > > class(b) > [1] "array" > > dim(b) > [1] 1 6 1 > > ## The problem > > b[1,,] > [1] 1 2 3 4 5 6 > > dim(b[1,,]) > NULL > > class(b[1,,]) > [1] "integer" > > I would have expected: > > b[1,,] ## produced the output with matrix(1:6, ncol = 1) > [,1] > [1,] 1 > [2,] 2 > [3,] 3 > [4,] 4 > [5,] 5 > [6,] 6 > > class(b[1,,]) > [1] "matrix" "array" > > dim(b[1,,]) > [1] 3 1 > > Is this a bug? If not, any help understanding this behaviour would be much > appreciated. > > Thanks, > Joseph > > [[alternative HTML version deleted]] > > ______________________________________________ > R-devel at r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-develHello, You are using the default behavior for objects of class "array" (and "matrix"), which is drop = TRUE. See ?`[`. You probably want the second example below. b <- array(1:6, dim = c(1, 6, 1)) # the default is drop = TRUE b[1,,] #> [1] 1 2 3 4 5 6 # keep the dim attribute b[1, , , drop = FALSE] #> , , 1 #> #> [,1] [,2] [,3] [,4] [,5] [,6] #> [1,] 1 2 3 4 5 6 Hope this helps, Rui Barradas -- Este e-mail foi analisado pelo software antiv?rus AVG para verificar a presen?a de v?rus. www.avg.com