Global Variables
Global variables are variables that you can access from functions or from the command line. They
have their own workspace, which is separate from the base and function workspaces.
However, global variables carry notable risks. For example:
• Any function can access and update a global variable. Other functions that use the variable might
return unexpected results.
• If you unintentionally give a “new” global variable the same name as an existing global variable,
one function can overwrite the values expected by another. This error is difficult to diagnose.
Use global variables sparingly, if at all.
If you use global variables, declare them using the global keyword before you access them within
any particular location (function or command line). For example, create a function in a file called
falling.m:
function h = falling(t)
global GRAVITY
h = 1/2*GRAVITY*t.^2;
Then, enter these commands at the prompt:
global GRAVITY
GRAVITY = 32;
y = falling((0:.1:5)');
The two global statements make the value assigned to GRAVITY at the command prompt available
inside the function. However, as a more robust alternative, redefine the function to accept the value
as an input:
function h = falling(t,gravity)
h = 1/2*gravity*t.^2;
Then, enter these commands at the prompt:
GRAVITY = 32;
y = falling((0:.1:5)',GRAVITY);
Evaluating in Another Workspace
The evalin and assignin functions allow you to evaluate commands or variable names from
character vectors and specify whether to use the current or base workspace.
Like global variables, these functions carry risks of overwriting existing data. Use them sparingly.
evalin and assignin are sometimes useful for callback functions in graphical user interfaces to
evaluate against the base workspace. For example, create a list box of variable names from the base
workspace:
function listBox
figure
lb = uicontrol('Style','listbox','Position',[10 10 100 100],...
'Callback',@update_listBox);
update_listBox(lb)
20 Function Basics
20-12