MATLAB Programming

3 Basic Program Components
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
• Pass the variable to other functions as an additional argument. In this way,
you make sure that any shared access to the variable is intentional.
If this means that you have to pass a number of additional variables,
you can put them into a structure or cell array and just pass it as one
additional argument.
• Use a persistent variable (described in the next section), if you only need to
make the variable persist in memory from one function call to the next.
Persistent Variables
Characteristics of persistent variables are
• You can declare and use them within M-file functions only.
• Only the function in which the variables are declared is allowed access to it.
• MATLAB does not clear them from memory when the function exits, so
their value is retained from one function call to the next.
You must declare persistent variables before you can use them in a function.
It is usually best to put your persistent declarations toward the beginning of
the function. You would declare persistent variable SUM_X as follows:
persistent SUM_X
If you clear a function that defines a persistent variable (i.e., using clear
functionname or clear all), or if you edit the M-file for that function,
MATLAB clears all persistent variables used in that function.
You can use the mlock function to keep an M-file from being cleared from
memory, thus keeping persistent variables in the M-file from being cleared
as well.
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