Initialize nested arrays

General Usage
1

I need to initialize a Matrix, containing Matrices, the latter containing a Vector or Matrices.

I did something like this:

sizeMat11 = 2
sizeMat12 = 3
sizeMat21 = 5
sizeMat22 = 6
sizeVec31 = 9
struct SomeStruct 
    x::Float64 
end
function SomeFunction(l::Int64 , n::Int64 ,m::Int64)::Tuple{Array{Float64,2},Array{Float64,2},Vector{Float64}}
    a = zeros(l,l)
    b = zeros(m,m)
    c = rand(n)
return a,b,c
end

v = Array{Array{Vector{AbstractMatrix{SomeStruct}}, 2} , 2}(undef , sizeMat11 ,  sizeMat12)
for imat11 = 1:sizeMat11
   for imat12 = 1:sizeMat12
      for imat21 = 1:sizeMat21
          for imat22 = 1:sizeMat22
              a,b,c = SomeFunction(10,15,9)
              v[imat11,imat12][imat21,imat22] = [a,b,c]
           end
      end
   end
end

I get an undef error.
I consulted isbits type undef error post, but so far no success.

2

protip, if you don’t care about flow control like continue and break, you can write nested loops really easily with:

for i=1:I, j=1:J, k=1:K
    # blah
end

the j loop is the most tightly nested, and the i loop is the outer loop.

anyway, I’m too lazy to check if this has the dimensions you’re looking for, but you might consider this:

v = [[SomeFunction(10,15,9) for imat21=1:sizeMat21, imat22=1:sizeMat22] for imat12=1:sizeMat12, imat21=1:sizeMat21]

you can also make a 4-d array, in case that makes things easier in any way:

v = [SomeFunction(10,15,9) for imat21=1:sizeMat21, imat22=1:sizeMat22, imat12=1:sizeMat12, imat21=1:sizeMat21]

list comprehensions have the nice feature of not needing to think through entirely ahead of time the proper allocation structure.

3

Yeah, this won’t work. Here’s a simpler example:

julia> a = Vector{Vector{Int}}(undef, 2)
2-element Vector{Vector{Int64}}:
 #undef
 #undef

julia> a[1][1] = 3
ERROR: UndefRefError: access to undefined reference

You haven’t allocated any inner vector that you can write to, the outer vector just contains two undef references. You have to pre-allocate the innermost arrays, before you can start accessing and writing to them. Or you can write a[1] = [1,2,3], but you cannot index into an undef, only replace it with a value.

The easiest way is normally to use comprehensions, like @uniment said, then you will also get correct element types. You should definitely avoid having an AbstractMatrix eltype in there, that will harm performance.

1 Like
4

I would like also to point out that if the sizes of the matrices/vectors are constant, then there is no need to make them nested. You can just make higher dimensional one.

3 Likes
5

Thank you, it works :slight_smile:
Thanks to everyone

1 Like