Bug in loadLearnerForCoder from MATLAB coder

Hello,

I've built a classification model (esemble), and saved it using saveLearnerForCoder. However, when compiling my code to C using MATLAB coder, an error is raised. I'm using MATLAB 2020a. This issue appears to have been previously reported here. Please, find detailed information below. I would very appreciate a workaround.

I'm loading the classifier as recommended:

cough_detection_model = loadLearnerForCoder('my_model');  % I tried with both ' and ", just in case

This triggers the following error:

??? The input to coder.const cannot be reduced to a constant: Unsupported value.
Error in ==> loadLearnerForCoder Line: 41 Column: 68

My classifier was created with the following code:

template_tree = templateTree('MaxNumSplits', 700, 'MinLeafSize', 20);         
ensemble_model = fitcensemble(features, classes, ...
    'Learners', template_tree, 'Method','RUSBoost', 'NumLearningCycles', 500, ...
    'LearnRate', 0.1, 'ScoreTransform', 'None');     
saveLearnerForCoder(ensemble_model, 'my_model')

I am able to load the classifier and use it normally. The error only happens when using the codegen.

I made some digging, and created a bypass function which avoids the problems from loadLearnerForCoder. Obviously it would crash somewhere, but I think it gave me some further information. This is the code I managed to create:

matFile = coder.load("my_model");
fromStructFcn = str2func('classreg.learning.classif.CompactClassificationEnsemble.fromStruct');
obj = fromStructFcn(matFile.compactStruct);  % raises error

Which raises the following error:

??? Failed to compute constant value for nontunable property 'NumPredictors'. In code generation, nontunable
properties can only be assigned constant values.

I have tried to overpass this by doing:

matFile.compactStruct.DataSummary.NumPredictors = coder.const(117);

But this doesn't help either.

Can please anyone help me?

[Diogo Tecelão]

Update

By using the following code:

fromStructFcn = str2func('classreg.learning.classif.CompactClassificationEnsemble.fromStruct');
obj = fromStructFcn(coder.const(matFile.compactStruct));

I now get a different error:

??? Code generation does not support MATLAB class 'classreg.learning.coder.classif.CompactClassificationEnsemble'
in this context because the class does not implement toRedirect() and/or fromRedirect() methods.

Answer 1

[Sriram Padavettan]

The problem is 2020A. I think there's a bug in that version. I messed around with this same issue all day yesterday on 2020A, tried 2020B instead, and it worked. Maybe even 2019b or earlier would work.

:)

Answer 2

[Marou AD]

It seems there is a bug in MATLAB 2020a that might be causing the problem. I had a similar issue, and after experimenting, I found that switching to MATLAB 2020b resolved the problem for me. It seems there might be a compatibility or bug fix in the later version. I recommend giving MATLAB 2020b a try , it will resolves your compilation issue.