|Programming and Data Types|
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, for example, you want to study the effect of the interaction coefficients, and , in the Lotka-Volterra predator-prey model.
Create an M-file,
Then interactively enter the statements
global statements make the values assigned to
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.
Creating Global Variables
Each function that uses a global variable must first declare the variable as
global. It is usually best to put global declarations toward the beginning of the function. You would declare global variable
MAXLEN as follows:
If the M-file contains subfunctions as well, then each subfunction requiring access to the global variable must declare it as
global. To access the variable from the MATLAB command line, you must declare it as
global at the command line.
MATLAB global variable names are typically longer and more descriptive than local variable names, and often consist of all uppercase characters. These are not requirements, but guidelines to increase the readability of MATLAB code, and to reduce the chance of accidentally redefining a global variable.
Displaying Global Variables
To see only those variables you have declared as global, use the
whos functions with the literal,
Suggestions for Using Global Variables
There is a certain amount of risk associated with using global variables and, because of this, it is recommended that you use them sparingly. You might, for example, unknowingly give a global variable in one function a name that is already used for a global variable in another function. When you run your application, one function may unintentionally overwrite the variable used by the other. This can be a difficult error to track down.
Another problem comes when you want to change the variable name. To make a change without introducing an error into the application, you must find every occurrence of that name in your code (and other people's code, if you share functions).
Alternatives to Using Global Variables. Instead of using a global variable, you may be able to
|Local Variables||Persistent Variables|