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matlabfor-loopworkspace

When does the workspace change in MATLAB?

I have a couple questions regarding MATLAB workspaces:

  1. When does MATLAB decide to change the workspace and what conditions prompt it?

  2. Something strange is happening in the following example. I run it with a breakpoint at line 4, then step using F10 to watch the workspace variables. Clearly, I see that m is deleted within the first iteration of the inner loop, but somehow MATLAB still knows to go to the next iteration in the outer loop!

Example: 

something = 2;
somethingelse = 3;
for m = 1 : something
    for n = 1 : somethingelse
        %do something
        clearvars -except n something somethingelse % This clears m, but it still functions
    end
end

The only thing I can think of is that MATLAB probably has not updated the memory locations given that the variable does not show on the list.

Solution

  • In MATLAB there are generally two types of workspaces: the base workspace and function workspaces. The base workspace is what you use when you enter commands at the command line, or when you run scripts (all of which share the base workspace). Conversely, each function has its own unique workspace. Unlike C or C++ (or a number of other languages) you don't have any scoping of variable within loops or conditional structures, just one unique workspace for each instance of a function.

    The issue you're seeing in your example isn't really related to this, it's just an artifact of how for loops behave in MATLAB. Taken from the "Tips" section:

    Avoid assigning a value to the index variable within the loop statements. The for statement overrides any changes made to index within the loop.

    In other words, once an iteration of the loop completes and returns to the beginning, MATLAB ignores any changes to the loop variable and simply increments it to the next value.

    If you'd like to learn more details about MATLAB workspaces and scoping, I'd check out these links: