How to create and bind an empty multidimensional array

Question

I want to create a empty multidimensional array and then bind it to an existing array.

If my array was not empty, I can bind it with the abind package:

library(abind)
c=matrix(0,2,3)
test=array(0,c(2,3,1))
test2=abind(test,c,along=3)
test2 #exactly what I expected
, , 1

     [,1] [,2] [,3]
[1,]    0    0    0
[2,]    0    0    0

, , 2

     [,1] [,2] [,3]
[1,]    0    0    0
[2,]    0    0    0

Now I want to do the same thing except instead of two full arrays, I want one of them to be empty. Here is what would happen if I had characters:

test3=character() #this is empty
test3=c(test3,'hi') #I bind the word hi to it
test3
[1] "hi"

This does not exactly work if I try to use arrays:

empty=array()
abind(empty,test,along=3)
Error in abind(empty, test, along = 3) : 
'X1' does not fit: should have `length(dim())'=3 or 2

So I am assuming that array() is not how you create an empty multidimensional array.

Note the differences between the two commands:

empty=array()
> empty
[1] NA
test3=character()
> test3
character(0)

Solution

You can define an empty array for example in this way:

empty <- array(numeric(),c(2,3,0)) 

Note that you need to set at least one dimension to zero, otherwise the array will contain something by definition. Think for example to a matrix, if you define both dimensions greater than zero, you automatically instantiate a rectangular structure and it cannot be empty, at most it can be filled by NAs.

That being said, abind works like rbind/cbind but in a generalized way.
So, as rbind/cbind add a 1-dimensional structure to a 2-dimensional one, using abind with a 3-dimensional array, you need to add a 2-dimensional structure to the original array, since you want to append a new structure to a chosen dimension.

Here's an example of abind usage starting from a 3-dimensional empty array:

Create an empty array 2 x 3 x 0 :

a <- array(numeric(),c(2,3,0)) 

> a
<2 x 3 x 0 array of double>
     [,1] [,2] [,3]
[1,]
[2,]

Append a matrix (or a 2-dim array if you prefer) to the 3rd dimension of the array obtaining a new array 2 x 3 x 1 :

a <- abind(a, matrix(5,nrow=2,ncol=3), along=3)

> a
, , 1

     [,1] [,2] [,3]
[1,]    5    5    5
[2,]    5    5    5

Append a matrix again (or a 2-dim array if you prefer) to the 3rd dimension of the previous array obtaining a new array 2 x 3 x 2 :

a <- abind(a, matrix(7,nrow=2,ncol=3), along=3)

> a
, , 1

     [,1] [,2] [,3]
[1,]    5    5    5
[2,]    5    5    5

, , 2

     [,1] [,2] [,3]
[1,]    7    7    7
[2,]    7    7    7

and so and so forth...