Cannot seem to save data to a file within a class

I have a class which needs to write data to files. However, when I try to write the data, it gives a "variable not found" error, despite the variable being declared in the lines immediately preceding the call. I can manually run the code from the matlab command line and it does what I expect, so I think that the code is fine, but by being put into a class it somehow changes where the variables live. This is a shortened (working) version of what I have:

classdef SaveFiles < handle
    properties
        data;
        fileLoc;
    end

      methods
          function s = SaveFiles(loc)
              s.fileLoc = loc;
          end

          function s = addData(s, data)
              s.data = data;
          end

          function s = writeData(s)
              data = s.data;
              evalin('base', sprintf('save(''%s'', ''data'')', s.fileLoc));
          end
      end
  end

From the matlab command line:

>> sf = SaveFiles('<file_loc>');
>> a = 1;
>> sf.addData(a);
>> sf.writeData();

This last line causes the following exception:

Error using save
Variable 'data' not found.

Error in SaveFiles/writeData (line 18)
            evalin('base', sprintf('save(''%s'', ''data'')', s.fileLoc));

I'm completely stumped. Is there something obvious I'm just overlooking?

Answer 1

[per isakson]

Yes, there is. To make it work, replace

evalin('base', sprintf('save(''%s'', ''data'')', s.fileLoc));

by

save( s.fileLoc, 'data' )

Why do you think you need to execute save in the base workspace? The variable data is not available in the base workspace, but in the workspace of the method.

.

Then there is a second issue. With handle objects, there is no need to return the object variable in methods, e.g. replace

function s = addData(s, data)

by

function  addData(s, data)

However, the constructor shall return the object variable. See Comparison of Handle and Value Classes

[matquest]

Thanks for the info. I was able to get it to work simply by replacing evalin with eval (since as you noted, the data variable only exists in the method workspace), but your suggested method also works perfectly.

Unfortunately, I "need" to return the object for consistency with the rest of the codebase, but your point is well taken.

[Stephen23]

"I was able to get it to work simply by replacing evalin with eval."

Why bother using eval for such a trivial command as this? per Isakson showed you a much better way (i.e. simply without eval).

https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

[matquest]

I was merely mentioning that using eval rather than evalin also worked for the same reason, not suggesting that it was "better" in any sense.

[per isakson]

"I "need" to return the object for consistency with the rest of the codebase" Fine, then you do it for a reason, not by mistake - which I suspected.