How to create struct from fieldnames and values cell arrays for code generation ?

Hi,

I am trying to make a "code generation friendly" function:

[output_struct] = declareStruct(list_fields, list_values)

that creates a structure knowing its fieldnames and their associated values.

The goal is to use this function in Simulink, through a Matlab system block, simulated using code generation.

None of the approches described in How to create a struct from a cell array of fieldnames and a cell array of values? - MATLAB Answers - MATLAB Central (mathworks.com) work since code generation does not support cell2struct or transpose for cell arrays.

Is there a way to create a struct this way in MATLAB R2021b, or is it only possible to explicitely declare my struct in a hardcoded fashion ?

Thanks !

Guillaume.

[Stephen23]

"code generation does not support ... transpose for cell arrays."

Why do you need transpose?

[Guillaume Dupré]

The values of my fields do not have the same size.

Here is an example of what I would like to do:

list_fields = {'a'; 'b'; 'c'; 'd'};  % the function fieldnames() returns a column vector in my case
list_values = {1; [2,3,4]; 5; 6};

args = [list_fields'; list_values'];
output_struct = struct(args{:});

which returns:

struct('a', 1, ...
       'b', [2 3 4], ...
       'c', 5, ...
       'd', 6);

However, I cannot create the args variable in a code generation context.

This is needed here to respect the syntax struct(field1, value1, field2, value2, etc...).

[Stephen23]

"I cannot create the args variable in a code generation context."

Perhaps one of these would work:

• RESHAPE()
• create two new cells arrays with the required size, transfer the content:

https://www.mathworks.com/help/coder/ug/cell-array-restrictions-for-code-generation.html

Answer 1

[Guillaume Dupré]

Hi all,

Based on Stephen23 and Matt J advices, I managed to produce the following solution:

list_fields = {'a'; 'b'; 'c'; 'd'};  % the function fieldnames() returns a column vector in my case
list_values = {1; [2,3,4]; 5; 6};

% For loop to cast list_fields and list_values in another cell without
% transpose or concatenate
num_fields = length(list_fields);
args = cell(2,num_fields);
for i = 1:num_fields
    args{1,i} = list_fields{i};
    args{2,i} = list_values{i};
end
output_struct = struct(args{:})

which is basically a "code generation friendly" version of the initial answer https://fr.mathworks.com/matlabcentral/answers/59144-how-to-create-a-struct-from-a-cell-array-of-fieldnames-and-a-cell-array-of-values.

Thank you for your help !

Answer 2

[Matt J]

Does it support transpose for normal arrays? If so, then you might be able to do,

list_fields = {'a'; 'b'; 'c'; 'd'};  % the function fieldnames() returns a column vector in my case
list_values = {1; [2,3,4]; 5; 6};

args = [list_fields, list_values];

idx=reshape(1:numel(args),size(args))';

output_struct = struct(args{idx(:)})

output_struct = struct with fields:
    a: 1
    b: [2 3 4]
    c: 5
    d: 6

[Guillaume Dupré]

Thank you for your answer.

I tried it but unfortunately code generation does not support concatenation of cell arrays.

I will try to combine it with the 2nd solution of Stephen23 to see if this works.