Question

I am having a very strange issue with global variables in Matlab.

Usually, when you declare a variable to be global before assigning any value to it, it will be left as an empty variable. I have a variable R which I want to declare as global. But after I type clear and global R, in the variable list R is already set to be a 1*18 array, with some zero’s and other numbers filled in it.

I do have some other functions and scripts which share the global variable R, but I made sure I did not call any of the scripts or functions after I typed clear, and the variable list is already empty when I typed global R from the prompt.

But still, the problem persists. I guess I must have some serious misunderstanding of the rules about global variables. Can anybody explain why this is happening?

Thanks in advance.

Answer 1

The clear command does not clear global variables. It removes the variable from your local workspace, but it is still there. So if you assigned some value to it previously, declaring it again simply ‘shows’ the global variable in your local scope. You have to use clear all or clear global. From the documentation of clear:

If variable name is global, then clear removes it from the current workspace, but it remains in the global workspace.

Consider the following example:

>> clear all; 
>> global v;
>> v = 1:100;  % assign global variable
>> whos        % check if it is there

  Name      Size             Bytes  Class     Attributes

  v         1x100              800  double    global    

>> clear;
>> whos       % nothing declared in local workspace
>> global v;
>> whos       % ups, v is not empty!!

  Name      Size             Bytes  Class     Attributes

  v         1x100              800  double    global    

>> clear global;     % you have to clear it properly
>> whos
>> global v          % now it is empty
>> whos
  Name      Size            Bytes  Class     Attributes

  v         0x0                 0  double    global