Matlab system block dimension: Dimension 1 is fixed on the left-hand side but varies on the right

I am trying to write a "Matlab system" to use as a block in simulink and get the following error:

Caused by: Simulink detected an error 'Dimension 1 is fixed on the left-hand side but varies on the right ([30 x 1] ~= [:? x 1]).'.

It works when I "Simulate using: Interpreted Execution". But, it doesnt work when I "Simulate using: Code generation".

I thought I had defined the sizes of everything and can't figure out how to solve the problem. Thank you very much for your help!

classdef implicit_adaptor < matlab.System
    % implicit_adaptor should take in z and e, and output de/du that can be
    % used for an adaptive filter for C2 (feedforward control)

    % States: A (N x N) and old_z (N x 1)
    % Outputs: de_dz (N x 1) and error (1 x 1)

    properties
        % Mu Step size
        Mu = 0.005;

        % N  length of z
        N = 30;

        % Sample time
        dt = 1/500;
    end

    properties (Nontunable)

    end

    properties (DiscreteState)
        A;
        old_z;
    end

    methods (Access = protected)

        % Initialise states
        function setupImpl(obj)
            obj.A = zeros(obj.N);
            obj.old_z = zeros(obj.N, 1);
        end

        % Set up output data
        function [dataout1, dataout2] = getOutputDataTypeImpl(~)
            dataout1 = 'double';
            dataout2 = 'double';
        end
        function [cplxout1, cplxout2] = isOutputComplexImpl(~)
            cplxout1 = false;
            cplxout2 = false;
        end

        function [size_de_dz, size_error] = getOutputSizeImpl(obj)
            size_de_dz = [obj.N 1];
            size_error = [1 1];
        end
        function varargout = isOutputFixedSizeImpl(obj)
            % Get outputs fixed size.
            varargout = cell(1, getNumOutputs(obj));
            for i = 1:getNumOutputs(obj)
                varargout{i} = true;
            end
        end
        function [sz,dt,cp] = getDiscreteStateSpecificationImpl(obj, name)
            if strcmp(name,'A')
                sz = [obj.N obj.N];
                dt = 'double';
                cp = false;
            elseif strcmp(name,'old_z')
                sz = [obj.N 1];
                dt = 'double';
                cp = false;
            else
                error(['Error: Incorrect State Name: ', name.']);
            end
        end

        % Code that executes each step
        function [de_dz, de_dt_error] = stepImpl(obj, z, de_dt)
            % Find dz/dt
            dz = zeros(obj.N, 1) + z - obj.old_z;
            obj.old_z = z;

            % Find output of filter
            de_dz = obj.A * z;

            % Find filter error
            de_dt_error = dot(de_dz, dz)/obj.dt - de_dt;

            % Adapt the filter

        end

        function resetImpl(obj)
            obj.A = zeros(obj.N);
            obj.old_z = zeros(obj.N,1);
        end

    end

    % Block icon and dialog customizations
    methods (Static, Access = protected)
        function header = getHeaderImpl
            header = matlab.system.display.Header(...
                'lmsSysObj', ...
                'Title', 'Implicit Adaptor');
        end

        function groups = getPropertyGroupsImpl
            upperGroup = matlab.system.display.SectionGroup(...
                'Title','General',...
                'PropertyList',{'Mu'});

            lowerGroup = matlab.system.display.SectionGroup(...
                'Title','Coefficients', ...
                'PropertyList',{'Length of z','N'});

            bottomGroup = matlab.system.display.SectionGroup(...
                'Title','Time step', ...
                'PropertyList',{'Controller time step (s)','dt'});

            groups = [upperGroup,lowerGroup, bottomGroup];
        end
    end

    methods (Access = protected)
        function icon = getIconImpl(~)
            icon = sprintf('Implicit Adaptor\n for bio-inpired controller');
        end
        function [in1name, in2name] = getInputNamesImpl(~)
            in1name = 'Z Vector';
            in2name = 'de/dt';
        end
        function [out1name, out2name] = getOutputNamesImpl(~)
            out1name = 'de/dz';
            out2name = 'de/dt error';
        end
    end
end

Answer 1

[Brandon Stevens]

You are seeing this error when simulating via Code Generation because of how you defined the output variables. In the "getOutputSizeImpl" method you are using a variable not explicitly defined as 'Nontunable' with 'N'. The attribute of your system properties should be "Nontunable" otherwise when you run the Simulation in Code Generation mode, MATLAB Compiler assumes that those variables are tunable and thus assumes it is variable size.

For example, I would change your definition of Mu, N, and dt to be in "properties (Nontunable)" instead.

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