Use of `[` with array and resulting class

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

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

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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:


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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




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?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

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R-devel at r-project.org mailing list
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Hello,

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

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