Julia: Flattening array of array/tuples

Question

In Julia vec reshapes multidimensional arrays into one-dimension arrays. However it doesn't work for arrays of arrays or arrays of tuples. A part from using array comprehension, is there another way to flatten arrays of arrays/tuples? Or arrays of arrays/tuples of arrays/tuples? Or ...

Answer 1

Iterators.flatten(x) creates a generator that iterates over each element of x. It can handle some of the cases you describe, eg

julia> collect(Iterators.flatten([(1,2,3),[4,5],6]))
6-element Array{Any,1}:
 1
 2
 3
 4
 5
 6

If you have arrays of arrays of arrays and tuples, you should probably reconsider your data structure because it doesn't sound type stable. However, you can use multiple calls to flatten, eg

julia> collect(Iterators.flatten([(1,2,[3,3,3,3]),[4,5],6]))
6-element Array{Any,1}:
 1            
 2            
  [3, 3, 3, 3]
 4            
 5            
 6            

julia> collect(Iterators.flatten(Iterators.flatten([(1,2,[3,3,3,3]),[4,5],6])))
9-element Array{Any,1}:
 1
 2
 3
 3
 3
 3
 4
 5
 6

Note how all of my example return an Array{Any,1}. That is a bad sign for performance, because it means the compiler could not determine a single concrete type for the elements of the output array. I chose these example because the way I read your question it sounded like you may have type unstable containers already.

Answer 2

In order to flatten an array of arrays, you can simply use vcat() like this:

julia> A = [[1,2,3],[4,5], [6,7]]
Vector{Int64}[3]
    Int64[3]
    Int64[2]
    Int64[2]
julia> flat = vcat(A...)
Int64[7]
    1
    2
    3
    4
    5
    6
    7

Answer 3

The simplest way is to apply the ellipsis ... twice.

A = [[1,2,3],[4,5], [6,7]]
flat = [(A...)...]
println(flat)

The output would be [1, 2, 3, 4, 5, 6, 7].