Check if a variable exists in the workspace, within a function

Hi,

I have a function which needs to check if a variable ('error_N') exists in the work space. If it does not exist, the function must create it.

In the following example, the function does not see the variable 'error_N' from the workspace. I tried to solve it by declaring it as a global variable. However, in the case the variable does not exist in the Workspace, Matlab creates an EMPTY variable called 'error_N' when reading "global error_N", so the function cannot create it with the value that I would like it to get.

Do you have any suggestions?

Thanks!

function output = function_example(input)
    
    % global error_N ???????

    E_1 = exist('error_N','var');
    if E_1 ==1
    else
        error_N = 0.001;
    end
    
    output = function2(input);
    error = output/N;
    while error>error_N
        blablabla        
        output....blablabla
    end

end

[Walter Roberson]

Is it intended to check the variable in the workspace of the function, or is to check the variable in the workspace of the caller of the function?

If the work is to be done in the workspace of the caller of the function, then note that MATLAB provides for "static" workspaces, and you cannot create new variables in static workspaces from inside a called function. Static workspaces exist in all cases in which there is an "end" statement matching the "function" statement. Nested functions are only permitted with static workspaces. Functions that are defined inside script files must have matching "end" statements and so have static workspaces.

[Vogel]

Hi Walter,

thanks for your reply.

It is intended to check and read the variable' 'error_N' in the workspace of the caller of the function.

If it does not exist, the function must create the variable for itsself, but does not have to go to the workspace of the caller.

[Stephen23]

Introspective programming (such as checking for the existence of variables) is slow and leads to complex code. The MATLAB documentation recommends to avoid doing this:

https://www.mathworks.com/help/matlab/matlab_prog/techniques-for-improving-performance.html

Better to simply use nested functions, known variable names, and explicit logic that keeps track of what data you have.

[Walter Roberson]

The path you have defined so far is:

if variable exists in caller
    do nothing in current workspace
else
    create variable in current workspace
end

but in the path you defined, you do not pull out the variable from the caller.

Answer 1

[Walter Roberson]

if evalin('caller', 'exist(''error_N'',''var'')')
    error_N = evalin('caller', 'error_N');
else
    error_N = 0.001;
end

This would not be recommended. Better would be something like,

function output = function_example(x, error_N)
   if ~exist('error_N', 'var') || isempty(error_N)
       error_N = 0.001;
   end
end

That is, the user passes error_N to you if the user wants something other than the default.

[Vogel]

Thank you Walter. It works :)

I usually use the second option you describe. However, it is a function that I have been using for many months in several scripts, so I would prefer not to change the inputs.

I will have to use the first option, although I have read in many pages that it is not recommended...

[Stephen23]

"However, it is a function that I have been using for many months in several scripts, so I would prefer not to change the inputs."

Beginners often write this. At some point, as their projects get larger and more complex, they realise that their code design is inefficient and causing them to waste a lot of time (runtime, programming time, debugging time...).

Making some function rely on what exists in another workspace is inefficient and makes understanding, testing, and maintaining your code more difficult. You might not appreciate this now, but keep it in mind in the future: improving code is a good thing, not something to be avoided!

[Vogel]

Thank you for your advice :)

I will try to keep that in mind!

[Hisham Abbassy]

I'm trying to use it for GUI and it doesn't read the variable in the loaded mat file.

I also tried to use VarLoad but it doesn't work because I get error when the variable doesn't exist in my mat file.

Do you have an idea how to solve that?