Best way to convert Array{Array{Float64},1} to Array{Float64,2} and vise versa

Question

Consider an array of arrays

julia> a
2-element Array{Array{Float64,1},1}:
 [1.0, 2.0, 3.0]
 [4.0, 5.0, 6.0]

I want to convert a into an Array{Float64,2}

2×3 Array{Float64,2}:
 1.0  2.0  3.0
 4.0  5.0  6.0

like so.

I found out that one solution hcat(a...)'

julia> hcat(a...)'
2×3 Adjoint{Float64,Array{Float64,2}}:
 1.0  2.0  3.0
 4.0  5.0  6.0

Here type is Adjoint{Float64,Array{Float64,2}}. But for my problem, I need only Array{Float64,2}. And after some computations, I need to convert it back into array of arrays. I am wondering, what should be the best way to do this.

Thanks in advance.

Answer 1

You can do:

julia> reduce(vcat, transpose.(a))
2×3 Array{Float64,2}:
 1.0  2.0  3.0
 4.0  5.0  6.0

or e.g. (this does not check if the dimensions of the vectors match)

julia> [v[i] for v in a, i in axes(a[1], 1)]
2×3 Array{Float64,2}:
 1.0  2.0  3.0
 4.0  5.0  6.0

the way back is simpler:

julia> b = reduce(vcat, transpose.(a))
2×3 Array{Float64,2}:
 1.0  2.0  3.0
 4.0  5.0  6.0

julia> copy.(eachrow(b))
2-element Array{Array{Float64,1},1}:
 [1.0, 2.0, 3.0]
 [4.0, 5.0, 6.0]

or

julia> [b[i, :] for i in axes(b, 1)]
2-element Array{Array{Float64,1},1}:
 [1.0, 2.0, 3.0]
 [4.0, 5.0, 6.0]

Answer 2

Are you looking for vcat(a'...) It does what you want.