How to write an array of structs?

[matthijscox]

I noticed that I can read normal arrays of structs and cell arrays of structs with MAT.jl, but I cannot write arrays of structs. At least not by default. Would love to know how to do that.

MATLAB input data

Here's an example:

s = struct();
s.array = [struct('x', [1,2], 'y', [3,4]), struct('x', [5,6], 'y', [7,8])];
s.cell = {struct('x', [1,2], 'y', [3,4]), struct('x', [5,6], 'y', [7,8])};
save('struct_test.mat', 's')

This is the input, note that we have a 1x2struct array

>> s

s = 

  struct with fields:

    array: [1×2 struct]
     cell: {[1×1 struct]  [1×1 struct]}

>> s.array

ans =

[1x2 struct, 608 bytes]
x: [1, 2]
y: [3, 4]

Julia conversion

Let's read and write in Julia without editing:

using MAT
vars = matread("struct_test.mat")
matwrite("julia_struct_test.mat", vars)

Note that inside Julia, we already have something unexpected. I expected a struct/dict array for the MATLAB struct array, but instead we get arrays of arrays inside the "x" and "y" keys, e.g. see "x" => Any[[1.0 2.0] [5.0 6.0]].
In the cell array case we do have an array of dicts.

julia> vars["s"]
Dict{String, Any} with 2 entries:
  "array" => Dict{String, Any}("x"=>Any[[1.0 2.0] [5.0 6.0]], "y"=>Any[[3.0 4.0] [7.0 8.0]])
  "cell"  => Any[Dict{String, Any}("x"=>[1.0 2.0], "y"=>[3.0 4.0]) Dict{String, Any}("x"=>[5.0 6.0], "y"=>[…

julia> vars["s"]["array"]
Dict{String, Any} with 2 entries:
  "x" => Any[[1.0 2.0] [5.0 6.0]]
  "y" => Any[[3.0 4.0] [7.0 8.0]]

julia> vars["s"]["cell"]
1×2 Matrix{Any}:
 Dict{String, Any}("x"=>[1.0 2.0], "y"=>[3.0 4.0])  Dict{String, Any}("x"=>[5.0 6.0], "y"=>[7.0 8.0])

MATLAB loading

Now loading this data back in MATLAB, we see that we get a [1x1 struct] while originally we had a [1x2 struct]. Now we have cell arrays inside the fields (similar to what we found while loaded in Julia above).
The cell array is unchanged.

>> load("julia_struct_test.mat")

>> s

s = 

  struct with fields:

    array: [1×1 struct]
     cell: {[1×1 struct]  [1×1 struct]}

>> s.array

ans = 

  struct with fields:

    x: {[1 2]  [5 6]}
    y: {[3 4]  [7 8]}

Final question

So my main question is: how can I get back the [1x2 struct] in the s.array field? Is that even possible with MAT.jl?

[matthijscox]

I notice in the code that m_write and m_writearray only work for AbstractArray{<:HDF5BitsOrBool}, so only for HDF5.BitsTypes or Booleans. Otherwise m_write defaults to cell arrays.

So this feature I want is simply missing.

Ignoring backwards compatibility, I suppose if we want to write arrays of structs, we'd have to define another m_writearray for some special type, where we know for sure that each struct will have the same fields. Maybe for an array of named tuples?

[matthijscox]

fixed in 0.11