Simulink: Error getting properties from an Extrinsic function

I am running a function in simulink simulation mode in a matlab block that contains factorGraph as an extrinsic function(code below).

[NumNodes, NumFactors] = SLAM();
function [NumNodes, NumFactors] = SLAM()
    coder.extrinsic('factorGraph','addFactor','factorTwoPoseSE2');
    G = factorGraph;
    fctr = factorTwoPoseSE2([1 2], Measurement=[0 0 0]);
    addFactor(G,fctr);
    NumNodes = double(0.0); %#ok<NASGU> 
    NumFactors = double(0.0); %#ok<NASGU> 
    NumNodes = G.NumNodes;
    NumFactors = G.NumFactors;
end

However I can't extract the properties of the factorGraph and get the following error (It works without issue in matlab):

Attempt to extract field 'NumNodes' from 'mxArray'.

Attempt to extract field 'NumFactors' from 'mxArray'.

I found:

https://www.mathworks.com/help/coder/ug/use-matlab-engine-to-execute-a-function-call-in-generated-code.html#bq1h2z9-46

and

https://www.mathworks.com/matlabcentral/answers/122471-attempt-to-extract-field-value-from-mxarray

To be relevent but their advice did not result in success.

Answer 1

[Sanjana]

Hi,

It appears that the issue is related to the class of the output from an extrinsic function ,specifically it is of type ‘mxArray’ i.e MATLAB Array .The only valid operations for an ‘mxArray’ are storing it in a variable, passing it to another extrinsic function, or returning it to MATLAB. To perform any other operation on an ‘mxArray’ value, such as using it in an expression in your code, you must convert the ‘mxArray’ to a known type at run time. To perform this action, assign the ‘mxArray’ to a variable whose type is already defined by a prior assignment. I have seen that, in your code you have preinitialized NumNodes and NumFactors, to the required output type, but here ‘G’ is also of class ‘mxArray’, so you cannot access ’ factorGraph’ Properties using ‘.’ Operation.

But then again, you cannot use ‘ factorGraph’ directly inside the 'MATLAB Function Block' as it doesn’t support code Generation for “factorGraph”. That will be a deadlock, so I would suggest you to find an alternative way to create custom Block Functionality in Simulink.

Please refer to this Documentation, for finding the best way to incorporate MATLAB code into a block in Simulink Model

https://www.mathworks.com/help/releases/R2021a/simulink/ug/comparison-of-custom-block-functionality.html

For Understanding the working of MATALB Function Block, refer

https://www.mathworks.com/help/releases/R2021a/simulink/ug/what-is-a-matlab-function-block.html#mw_661f742e-c0a7-475a-92ff-ebbcde663ece

For further Assistance you can refer,

https://www.mathworks.com/matlabcentral/answers/297092-how-do-i-incorporate-matlab-code-into-simulink

Hope this helps!