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

Question

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

Answer 1

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.