{"id":11770,"date":"2022-03-31T12:47:40","date_gmt":"2022-03-31T16:47:40","guid":{"rendered":"https:\/\/blogs.mathworks.com\/simulink\/?p=11770"},"modified":"2022-04-14T17:54:35","modified_gmt":"2022-04-14T21:54:35","slug":"specifying-variable-values-using-mathematical-expressions-generated-code","status":"publish","type":"post","link":"https:\/\/blogs.mathworks.com\/simulink\/2022\/03\/31\/specifying-variable-values-using-mathematical-expressions-generated-code\/","title":{"rendered":"Specifying Variable Values Using Mathematical Expressions: Generated Code"},"content":{"rendered":"
Some time ago, I published a post where I described <\/span>how to specify variable values using mathematical expressions<\/span><\/a> using the <\/span>slexpr<\/span><\/a> function. This triggered a lot of questions about the impact on the code generated from a model using <\/span>Embedded Coder<\/span><\/a>™.<\/span><\/div>
To help answering those questions, I am happy to welcome back guest blogger <\/span>David Balbuena<\/span><\/a>.<\/span><\/div>

Default Behavior<\/span><\/h2>
My first thought when I read Guy's <\/span>recent post<\/span><\/a> on <\/span>slexpr<\/span><\/a> was \"What about code generation?!\"<\/span><\/div>
Today I will show how you can use <\/span>Embedded Coder<\/span><\/a>™ to preserve parameter expressions in the generated code.<\/span><\/div>
Let's take the part of Guy's model that uses the variable <\/span>area<\/span> (I'll explain why I am leaving out the hypotenuse later):<\/span><\/div>
\"area_model.png\"<\/img><\/div>
Where <\/span>area<\/span> is created using this code:<\/span><\/div>
a = Simulink.Parameter(3);<\/span><\/span><\/div><\/div>
b = Simulink.Parameter(4);<\/span><\/span><\/div><\/div>
area = Simulink.Parameter(slexpr(<\/span>\"(a*b)*0.5\"<\/span>));<\/span><\/span><\/div><\/div><\/div>
If you generate code using the default settings of <\/span>Simulink.Parameter<\/span><\/a> objects, the evaluated value of <\/span>area<\/span> gets inlined:<\/span><\/div>
\"nontunable_code.png\"<\/img><\/div>

Specifying Storage Classes to Preserve Expressions<\/span><\/h2>
It is possible for Embedded Coder to preserve the mathematical expression in the generated code by making two changes:<\/span><\/div>
  1. For <\/span>a<\/span> and <\/span>b<\/span> change the <\/span>storage class<\/span><\/a> to <\/span>Define<\/span><\/a><\/li>
  2. For <\/span>area<\/span>, set the storage class to <\/span>ExportedGlobal<\/span><\/a><\/li><\/ol>
    I like to use the <\/span>Code Mappings Editor<\/span><\/a> (ctrl+shift+C) for that, since I can mass-edit code generation settings with ctrl+click:<\/span><\/div>
    \"tempgif.gif\"<\/img><\/div>
    Then in the code, <\/span>area<\/span> will show up as a tunable parameter:<\/span><\/div>
    \"tunable_code.png\"<\/img><\/div>
    And the definition for <\/span>area<\/span> contains the mathematical expression:<\/span><\/div>
    \"area_definition.png\"<\/img><\/div>
    Because I used the <\/span>Define<\/span> storage class for <\/span>a<\/span> and <\/span>b<\/span>, those show up as macros in the model header file:<\/span><\/div>
    \"expression_params_definition.png\"<\/img><\/div>
    These are not the only storage classes you can use. The general rule is:<\/span><\/div>