Initialize Persistent Variables in MATLAB Functions

A persistent variable is a local variable in a MATLAB^® function that retains its value in memory between calls to the function. See persistent. You can initialize persistent variables in MATLAB Function blocks and MATLAB functions in Stateflow^® charts. However, some coding practices can produce errors when you use persistent variables.

Avoid Using coder.opaque with Persistent Variables

If you use a variable declared by the coder.opaque (MATLAB Coder) function as a persistent variable, simulating the model generates an error. For example, this function uses a persistent variable, pp, that is first declared by using coder.opaque. When you simulate the function in a model, this code produces an error.

function y = fcn(u)
%#codegen
persistent pp;
if isempty(pp)
    pp = eml.opaque("int","0");
end
q = 0;
q = eml.ceval("(int)",pp);
q = q + 1;

pp = eml.ceval("(int)",q);
y = 0;
y = eml.ceval("(int)",pp);

Use Best Practices to Avoid Errors

To avoid initialization errors caused by persistent variables, follow these best practices:

• Initialize persistent variables in functions only by accessing constants.

• Ensure the control flow of the function does not depend on whether the initialization occurs.

If you do not follow these guidelines, these conditions produce an initialization error:

• MATLAB Function blocks with persistent variables where the Allow direct feedthrough property is cleared

• MATLAB Function blocks with persistent variables in models that contain State Control blocks where State control is set to Synchronous

• Stateflow charts that implement Moore machine semantics and that use MATLAB functions with persistent variables

For example, this version of fcn uses a persistent variable, n. The initial value of n depends on the input u, and the return statement interrupts the normal control flow of the function. This code produces an error when used in a model that has one of the conditions described above.

function y = fcn(u)
    persistent n
        
    if isempty(n)
        n = u;
        y = 1;
        return
    end
    
    y = n;
    n = n + u; 
end

MATLAB Function Block with No Direct Feedthrough

This example model contains a MATLAB Function block that does not follow the best practices for using persistent variables in MATLAB functions. The MATLAB Function block input is a square wave, which is provided by a Sign and Sine Wave block. The MATLAB Function block adds the value of u to the persistent variable n at each time step.

Simulate the model. The simulation returns an error because:

• The initial value of the persistent variable n depends on the input u.

• The return statement interrupts the normal control flow of the function.

• The Allow direct feedthrough property of the MATLAB Function block is cleared.

In the MATLAB Function block, initialize the persistent variable by setting it to a constant value and removing the return statement. Redefine the function by using this code.

function y = fcn(u)
    persistent n

     if isempty(n)
         n = 1;
     end

     y = n;
     n = n + u;
 end

Simulate the model again.

State Control Block in Synchronous Mode

This example model contains a MATLAB Function block that does not follow the best practices for using persistent variables in MATLAB functions. The MATLAB Function block input is a square wave, which is provided by a Sign and Sine Wave block. The MATLAB Function block adds the value of u to the persistent variable n at each time step. The model contains a State Control block where State control is set to Synchronous.

Simulate the model. The simulation returns an error because:

• The initial value of the persistent variable n depends on the input u.

• The return statement interrupts the normal control flow of the function.

• The model contains a State Control block where State control is set to Synchronous.

In the MATLAB Function block, initialize the persistent variable by setting it to a constant value and removing the return statement. Redefine the function by using this code.

function y = fcn(u)
    persistent n

     if isempty(n)
         n = 1;
     end

     y = n;
     n = n + u;
 end

Simulate the model again.

Stateflow Chart That Implements Moore Semantics

This example model contains a Stateflow chart with a MATLAB function that does not follow the best practices for using persistent variables.

The MATLAB function adds 1 or -1 depending on the active state, to the persistent variable, n.

Simulate the model. The simulation returns an error because:

• The initial value of the persistent variable n depends on the input u.

• The return statement interrupts the normal control flow of the function.

• The chart implements Moore semantics.

In the MATLAB function, initialize the persistent variable by setting it to a constant value and removing the return statement. Redefine the function by using this code.

function y = fcn(u)
    persistent n

     if isempty(n)
         n = 1;
     end

     y = n;
     n = n + u;
 end

Simulate the model again.