How to Use Global Variables in MATLAB: Simple Guide

In MATLAB, use the global keyword to declare a variable as global inside every function or script that needs access to it. This allows sharing the same variable value across different functions without passing it as an argument.

Syntax

To use a global variable in MATLAB, declare it with the global keyword in each function or script that accesses it. This tells MATLAB to link the variable to the same memory location everywhere.

• global varName: Declares varName as a global variable.
• Use the variable normally after declaration.

matlab

global myVar;

Example

This example shows how two functions share a global variable. One function sets the value, and the other reads it.

matlab

function setGlobalVar(val)
    global sharedVar
    sharedVar = val;
end

function val = getGlobalVar()
    global sharedVar
    val = sharedVar;
end

% Usage
setGlobalVar(42);
result = getGlobalVar();
disp(['Global variable value is: ' num2str(result)])

Output

Global variable value is: 42

Common Pitfalls

Common mistakes when using global variables in MATLAB include:

• Not declaring the variable as global in every function that uses it, causing separate copies.
• Overusing global variables, which can make code hard to debug and maintain.
• Assuming global variables keep their value without initialization.

Always declare global in each function and initialize the variable before use.

matlab

function wrongUsage()
    sharedVar = 10; % This is a local variable, not global
end

function rightUsage()
    global sharedVar
    sharedVar = 10; % Correct global assignment
end

Quick Reference

Action	Syntax	Notes
Declare global variable	global varName	Must be in every function/script using the variable
Assign value	varName = value;	After declaring global
Access value	use varName directly	After declaring global
Avoid	Overusing globals	Use function arguments when possible

Key Takeaways

Declare the variable as global in every function or script that uses it.

Global variables share the same value across functions without passing arguments.

Always initialize global variables before using them.

Avoid overusing global variables to keep code clear and maintainable.

Use function inputs and outputs when possible instead of globals.