Programming and Data Types

Local and Global Variables

The same guidelines that apply to MATLAB variables at the command line also apply to variables in M-files:

• You do not need to type or declare variables. Before assigning one variable to another, however, you must be sure that the variable on the right-hand side of the assignment has a value.
• Any operation that assigns a value to a variable creates the variable if needed, or overwrites its current value if it already exists.
• MATLAB variable names consist of a letter followed by any number of letters, digits, and underscores. MATLAB distinguishes between uppercase and lowercase characters, so A and a are not the same variable.
• MATLAB uses only the first 31 characters of variable names.

Ordinarily, each MATLAB function, defined by an M-file, has its own local variables, which are separate from those of other functions, and from those of the base workspace. However, if several functions, and possibly the base workspace, all declare a particular name as global, then they all share a single copy of that variable. Any assignment to that variable, in any function, is available to all the other functions declaring it global.

Suppose you want to study the effect of the interaction coefficients, and , in the Lotka-Volterra predator-prey model.

Create an M-file, lotka.m.

function yp = lotka(t,y)
%LOTKA   Lotka-Volterra predator-prey model.
global ALPHA BETA
yp = [y(1) - ALPHA*y(1)*y(2); -y(2) + BETA*y(1)*y(2)];

Then interactively enter the statements

global ALPHA BETA
ALPHA = 0.01
BETA = 0.02
[t,y] = ode23('lotka',0,10,[1; 1]);
plot(t,y)

The two global statements make the values assigned to ALPHA and BETA at the command prompt available inside the function defined by lotka.m. They can be modified interactively and new solutions obtained without editing any files.

For your MATLAB application to work with global variables:

• Declare the variable as global in every function that requires access to it. To enable the workspace to access the global variable, also declare it as global from the command line.
• In each function, issue the global command before the first occurrence of the variable name. The top of the M-file is recommended.

MATLAB global variable names are typically longer and more descriptive than local variable names, and sometimes consist of all uppercase characters. These are not requirements, but guidelines to increase the readability of MATLAB code and reduce the chance of accidentally redefining a global variable.

Passing Variable Numbers of Arguments

Persistent Variables