How to Use Persistent Variable in MATLAB: Syntax and Examples

In MATLAB, use the persistent keyword inside a function to declare a variable that keeps its value between calls. Unlike regular variables, persistent variables do not reset every time the function runs, allowing you to store data across multiple calls.

Syntax

The persistent keyword declares a variable inside a function that retains its value between calls. You write persistent varName at the start of the function. If the variable is empty, you can initialize it once. This keeps the variable's value until MATLAB clears it or the function is cleared.

matlab

function count = examplePersistent()
    persistent counter
    if isempty(counter)
        counter = 0; % Initialize once
    end
    counter = counter + 1;
    count = counter;
end

Example

This example shows a function that counts how many times it has been called using a persistent variable. Each call increases the count by one and returns it.

matlab

function count = callCounter()
    persistent counter
    if isempty(counter)
        counter = 0; % Initialize only once
    end
    counter = counter + 1;
    count = counter;
end

% Test calls
count1 = callCounter();
count2 = callCounter();
count3 = callCounter();

fprintf('Call 1: %d\nCall 2: %d\nCall 3: %d\n', count1, count2, count3);

Output

Call 1: 1
Call 2: 2
Call 3: 3

Common Pitfalls

• Forgetting to check isempty before initializing causes the variable to reset every call.
• Persistent variables are local to the function and cannot be accessed outside.
• They keep values until MATLAB or the function is cleared, so unexpected values may persist if not reset manually.

matlab

function count = wrongPersistent()
    persistent counter
    counter = 0; % Wrong: resets every call
    counter = counter + 1;
    count = counter;
end

% Correct way:
function count = correctPersistent()
    persistent counter
    if isempty(counter)
        counter = 0; % Initialize once
    end
    counter = counter + 1;
    count = counter;
end

Quick Reference

Persistent Variable Tips:

• Declare with persistent varName inside a function.
• Initialize only if isempty(varName) is true.
• Use to store data between function calls without global variables.
• Cleared when MATLAB or function is cleared.

Key Takeaways

Use persistent inside functions to keep variable values between calls.

Always check isempty before initializing persistent variables to avoid resetting.

Persistent variables are local to the function and not accessible outside.

They retain values until MATLAB or the function is cleared.

Use persistent variables to maintain state without using global variables.