## 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. If
you generate code from your model, you must initialize a persistent variable for your MATLAB
functions. For more information, see `persistent`

.

When using MATLAB functions that contain persistent variables in Simulink^{®} models, you should follow these guidelines:

Initialize the 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, several conditions produce an initialization error:

MATLAB Function blocks with persistent variables where the

**Allow direct feedthrough**property is clearedMATLAB Function blocks with persistent variables in models with 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, the function `fcn`

below uses a persistent
variable, `n`

. `fcn`

violates both guidelines. The initial
value of `n`

depends on the input `u`

and the
`return`

statement interrupts the normal control flow of the function.
Consequently, 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

To prevent the error, initialize the persistent variable by setting it to a constant value
and removing the `return`

statement. This modified version of
`fcn`

initializes the persistent variable without producing an
error:

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

### MATLAB Function Block with No Direct Feedthrough

This model contains a MATLAB Function block that uses the first version of `fcn`

, described previously. 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.

Modify the MATLAB Function block code, as shown in the corrected version of `fcn`

. Simulate the model again.

### State Control Block in Synchronous Mode

This model contains a MATLAB Function block that uses the first version of `fcn`

, described previously. 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`

.

Modify the MATLAB Function block code, as shown in the corrected version of `fcn`

. Simulate the model again.

### Stateflow Chart Implementing Moore Semantics

This model contains a Stateflow Chart with a MATLAB function that uses the first version of `fcn`

, described previously. The MATLAB function adds the value (`1`

or `-1`

) determined by the active state 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 chart implements Moore semantics.

Modify the MATLAB function code, as shown in the corrected version of `fcn`

. Simulate the model again.

## See Also

### Blocks

- MATLAB Function | State Control (HDL Coder) | Chart (Stateflow)