Why do I get the error "Attempt to extract field 'x' from 'double'." when I use MATLAB Coder in MATLAB R2022b?

I have the following simplified function:

function out = fnName() 
s = [ ];
s.x = 10; % This is the line where I get the error. 
end

It runs fine in MATLAB but when I use MATLAB Coder to generate code for it, I get the following error:

Attempt to extract field 'x' from 'double'.

Why do I get this error and how do I resolve it?

Answer 1

[MathWorks Support Team]

In the context of code generation, a variable's datatype cannot be changed once it is assigned to a value. In the function you provided,

s = [];

creates an empty variable with the default data type of “double” in MATLAB. You can check its data type by running:

class(s)

in MATLAB Command Window.

From the perspective of MATLAB Coder, you tried to change the type of "s" from "double" to "struct". While in MATLAB, you can assign a variable with a value of different data type, it generally causes issues during code generation.

A workaround for this issue is to remove the first assignment to "s", so that it is never a "double" to begin with. The function becomes

function out = fnName()
s.x = 10;
end

Now, MATLAB Coder knows that "s" is intended to be a struct with field "x".