Functioning of permutedims in Julia unclear

Question

What does permutedims() do when called upon a multidimensional array?

From its name, it is evident it has something to do with the dimentions of the array. However, when running the code below, the output is unexpected and not clear.

A = Array{Int64}(undef, 100,100,100)
B = permutedims(A, (1,2,3))
println(A == B)

Output:

`true`

So does it create a copy of the original array? and what is the use of the tuple passed?

Answer 1

The docs of Julia sometimes do not have a complete explanation on a given topic.

permutedims(A::AbstractArray, perm)

perms is a tuple specifying the new order for the dimensions of A, where 1 corresponds to the first dimension (rows), 2 corresponds to the second dimension (columns), 3 corresponds to pages, and so on i.e. this function will return a copy of the array with its dimensions according to the specified perms.

What happened in the code in the question is that by passing the tuple (1,2,3), we were telling Julia that place the first dim of A in the place of the first dim of B and the second in the place of second and so on. This basically created a copy of the array A.

USE CASE EXAMPLE

A = ones(10,20,30) # Creates an array full of 1 of the size (10,20,30)
B = permutedims(A, (3,1,2)) 

println(A == B) 
println(size(B))

OUTPUT

false
(30, 10, 20)