for loop index

I've got a tricky question. If we run the following script

for ii=1:10
clear all
disp('i')
end

We can see 10 i's printed out in the command window, although the index ii has already been cleared (matlab will tell you it is an undefined variable if you add ii line within the loop). This means matlab can remember the number of loops in some implicit form even when the whole workspace has been cleared, including the loop index itself.

My question is, is there any way to retrieve the loop index after the workspace being cleared. The script to do this should be something like the following, and won't give an error msg of "undefined variable ii":

for ii=1:10
clear all
...
...
ii
end

Answer 1

[the cyclist]

I have to admit I have no idea how to do that, and I am extremely curious why you would want the clear all in there in the first place.

You might be interested in the "keep" command from the File Exchange:

http://www.mathworks.com/matlabcentral/fileexchange/181

[Mike]

when the loop is really big and has tons of variables, 'clear all' will be convinient to freed up memories. Of course, I can clear variable one by one. But if the number of variables is really big and the only variable I want to preserve is the loop index, 'clear all' could be very useful, i guess...

[Mike]

Actually it is more of a curiosity than a real question...

[Mike]

The 'Keep' command you suggested looks really helpful. Thanks.

[Oleg Komarov]

Or clearvars -except var1 var2

Answer 2

[Walter Roberson]

No, there is no provided mechanism for that.

The loop index value is held in internal memory, and the statement is executed as if the loop variable is initialized to the current value at the top of the loop. When you "clear all", you lose access to the loop variable, but the internal value is not disturbed.

[Mike]

I am a little cursious about the internal memory you just said. Are you saying, like, it is the memory space where matlab stores data, but users have no access to? Is restart the only way to free that memory?

Thanks.

[Walter Roberson]

Yes, it is internal memory where MATLAB stores data but users have no access to. MATLAB manages it automatically. To free the memory, terminate the "for" loop.

You should not be surprised that MATLAB manages memory that it does not give users access to. Consider, for example, that MATLAB must manage the call stack -- the information about exactly where the present routine was called from -- so that after this routine exits, the previous routine can resume execution. MATLAB is not going to allow the user to overwrite or release that memory.

MATLAB is not a compiler; internally it works as an interpreter. It uses whatever internal state it needs.