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Create Custom Functionality Using MATLAB Function Block

This example shows how to create a model that uses the MATLAB Function block to calculate the mean and standard deviation for a vector of values.

Create Model

  1. Create a new Simulink® model and insert a MATLAB Function block from the User-Defined Functions library.

  2. Add a Constant block and set its value to vector [2 3 4 5]. Add two Display blocks to the model. Connect these blocks as shown in the diagram.

  3. Save the model as call_stats_block1.

Program the MATLAB Function Block

Program the block to calculate the mean and standard deviation for a vector of values:

  1. Double-click the MATLAB Function block. A default function signature appears in the MATLAB Function Block Editor. Write any code inside the defined function signatures.

  2. Edit the function header line:

    function [mean,stdev] = stats(vals)

    From this code, you define a function called stats, which calculates a statistical mean and standard deviation for the values in the vector vals. The function header declares vals as an argument to the stats function, with mean and stdev as return values.

  3. In the MATLAB Function Block Editor, enter a line space after the function header and add the following code:

    % Calculates a statistical mean and a standard
    % deviation for the values in vals.
    len = length(vals);
    mean = avg(vals,len);
    stdev = sqrt(sum(((vals-avg(vals,len)).^2))/len);
    function mean = avg(array,size)
    mean = sum(array)/size;
  4. Save the model as call_stats_block2.

Build the Function and Check for Errors

After programming the block in a Simulink model, you can build the function and test for errors. Building your MATLAB Function block requires a supported compiler. MATLAB® automatically selects one as the default compiler. If you have multiple MATLAB-supported compilers installed on your system, you can change the default compiler using the mex -setup command. See Change Default Compiler.

Supported Compilers for Simulation and Code Generation Builds

View a list of compilers for building models containing MATLAB Function blocks simulation and code generation.

  1. Navigate to the Supported and Compatible Compilers page and select your platform.

  2. Scroll to the table under Simulink Product Family.

  3. To check the table for models that contain MATLAB Function blocks for simulation, find the compilers checked in the column titled Simulink For Model Referencing, Accelerator mode, Rapid Accelerator mode, and MATLAB Function blocks.

    To check the table for models that contain MATLAB Function blocks and generate code, find the compilers checked in the column titled Simulink Coder™ .

Supported Compilers for Code Generation

To generate code for models that contain MATLAB Function blocks, you can use any of the C compilers supported by Simulink software for code generation with Simulink Coder. For a list of these compilers:

  1. Navigate to the Supported and Compatible Compilers Web page.

  2. Select your platform.

  3. In the table for Simulink and related products, find the compilers checked in the column titled Simulink Coder.

Locate and Fix Errors

If errors occur during the build process, the Diagnostics Viewer window lists the errors with links to the offending code.

The following exercise shows the way to locate and fix an error in a MATLAB Function block.

  1. In the stats function, change the local function avg to a fictitious local function aug and then compile again to see the following messages in window. The Diagnostics Viewer window displays each detected error with a shaded red line.

  2. Investigate the error titled Undefined function or variable 'aug'. In the diagnostic message for the selected error, click the blue link after the function name to display the offending code. The offending line appears highlighted in the MATLAB Function Block Editor.

  3. The message also links to a report about compile-time type information for variables and expressions in your MATLAB functions. This information helps you diagnose error messages and understand type propagation rules. For more information about the report, see MATLAB Function Reports. To see the report, click the highlighted blue link in the line called Launch diagnostic report

  4. Correct the error by changing aug back to avg and recompile.

Define Inputs and Outputs

By default, function inputs and outputs inherit their data type and size from the signals attached to their ports. Examine input and output data for the MATLAB Function block to verify that it inherits the correct type and size.

  1. Double-click the MATLAB Function block stats.

  2. In the MATLAB Function Block Editor, select Edit Data. The Ports and Data Manager opens to help you define arguments for MATLAB Function blocks.

    The left pane displays the argument vals and the return values mean and stdev that you have already created for the MATLAB Function block. Observe that vals is assigned a Scope of Input, which is short for Input from Simulink. mean and stdev are assigned the Scope of Output, which is short for Output to Simulink.

  3. In the left pane of the Ports and Data Manager, click anywhere in the row for vals to highlight it.

    The right pane displays the Data properties dialog box for vals. By default, the class, size, units, and complexity of input and output arguments are inherited from the signals attached to each input or output port. Inheritance is specified by setting Size to -1, Complexity to Inherited, and Type to Inherit: Same as Simulink.

    The actual inherited values for size and type are set during model compilation, and are reported in the Compiled Type and Compiled Size columns of the left pane.

    You can specify the type of input or output argument in the Type field of the Data properties dialog box, for example, double. You can also specify the size of an input or output argument by entering an expression in the Size field. For example, you can enter [2 3] in the Size field to specify vals as a 2-by-3 matrix. See Type Function Arguments and Size Function Arguments for more information on the expressions that you can enter for type and size.


    The default first index for any arrays that you add to a MATLAB Function block function is 1, just as it would be in MATLAB.

Create a MATLAB Function Object and Query Properties

You can create an object for the MATLAB Function block in your model, and modify the properties that belong to this model. To query the properties in call_stats_block2 model you just created, create a configuration object.

myconfig = get_param('call_stats_block2/MATLAB Function', 'MATLABFunctionConfiguration')
myconfig = 

  MATLABFunctionConfiguration with properties:

                         Path: 'call_stats_block2/MATLAB Function'
               FunctionScript: 'function [mean,stdev] = stats(vals)↵↵len = length(vals);↵mean = avg(vals,len);↵stdev = sqrt(sum(((vals-avg(vals,len)).^2))/len);↵plot(vals,'-+');↵↵function mean = avg(array,size)↵mean = sum(array)/size;↵'
                 UpdateMethod: Inherited
                   SampleTime: '-1'
                  Description: ''
                 DocumentLink: ''
        SupportVariableSizing: 1
       AllowDirectFeedthrough: 1
    SaturateOnIntegerOverflow: 1
                    TreatAsFi: FixedPoint
                   FimathMode: SameAsMATLAB
                       Fimath: 'fimath('RoundingMethod','Nearest','OverflowAction','Saturate','ProductMode','FullPrecision','SumMode','FillPrecision')'
To change any of the properties in your configuration object, use the dot notation with your object name. For example, to change the description to the MATLAB Function block in this model:
myconfig.Description = 'This model outputs the mean and standard deviation values of an array'
To learn more about the properties you can modify in your MATLAB Function configuration object, see MATLABFunctionConfiguration.

Define Local Variables for Code Generation

To generate code from the MATLAB algorithm in a MATLAB Function block, you must explicitly assign the class, size, and complexity of local variables before using them in operations or returning them as outputs (see Data Definition for Code Generation). In the example function stats, the local variable len is defined before being used to calculate mean and standard deviation:

len = length(vals);

Once you assign properties to a variable, you cannot redefine its class, size, or complexity elsewhere in the function body with some exceptions (see Reassignment of Variable Properties).

Generate Code for the MATLAB Function Block

  1. Open the call_stats_block2 model that you saved at the end of Program the MATLAB Function Block.

  2. Double-click stats block.

  3. Select Build Model > Build to compile and build the example model.

    If you get an error related to the Variable-step solver, from Configuration Parameters > Solver, change the solver type to a Fixed-step solver and rerun the build. To learn more about the differences between fixed-step and variable-step solvers, see Fixed-Step Versus Variable-Step Solvers.

    If no errors occur, the Diagnostics Viewer window displays a message indicating success. Otherwise, this window helps you locate errors, as described in Locate and Fix Errors.

Add Code to a MATLAB Function Block Programmatically

This example shows how to programmatically add a MATLAB Function block to a model and populate the block with MATLAB code. If you already have MATLAB code and do not want to add it to a MATLAB Function block manually, this workflow can be convenient.

  1. Create and save a model called myModel.

  2. Create a MATLAB function with the following code and save it in myAdd.m.

    function c = myAdd(a, b)
    c = a + b;

  3. Write a MATLAB script that adds a MATLAB Function block to myModel and populates it with the contents of myAdd.m.

    % Add a MATLAB Function block to a model and populate the block with MATLAB
    % code.
    % Copyright 2018 The Mathworks, Inc.
    libraryBlockPath = 'simulink/User-Defined Functions/MATLAB Function';
    newBlockPath = 'myModel/myBlockName';
    % Add a MATLAB Function to the model
    add_block(libraryBlockPath, newBlockPath);
    % In memory, open models and their parts are represented by a hierarchy of
    % objects. The root object is slroot. This line of the script returns the
    % object that represents the new MATLAB Function block:
    blockHandle = find(slroot, '-isa', 'Stateflow.EMChart', 'Path', newBlockPath);
    % The Script property of the object contains the contents of the block,
    % represented as a character vector. This line of the script loads the
    % contents of the file myAdd.m into the Script property:
    blockHandle.Script = fileread('myAdd.m');
    % Alternatively, you can specify the code directly in a character vector.
    % For example: 
    % blockHandle.Script = 'function c = fcn (a, b)';

  4. Run the script and observe the new MATLAB Function block in myModel.

  5. To see the code that you added to the block, double-click the myBlockName block.

See Also


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