MATLAB: Save/Load an Object Array

classloadobject arrayoopsave

I have a script that imports a large dataset from a .csv file and converts it to Matlab objects of a custom class. The resulting output of the script is an array of these objects. The script takes roughly 8 hours to complete. Once I've imported and structured the data into the object array I'd like to save it so I only have to do it once and can utilize it in other scripts by simply loading the saved object array. I believe I'm able to do the save part properly – after the script is finished I just call the save('filename', 'variable') function from the Command Window to save my object array from the workspace. Where I'm running into problems is when I'm trying to load it in another script. For example, lets say I saved the object array as 'testObjArray.mat'. If I load it and try to call an attribute from it using the following code:
thisObjArray = load('testObjArray.mat')
thisObjArray.name{1}
…I get a failure stating that I'm referencing a non-existent field 'name'. It seems like my problem is that when assigning thisObjArray with the load function I'm ending up with a struct rather than my object array. Is there a different way to load the file so that it remains an object array?
Thanks, Scott

Best Answer

Does the MATLAB session in which you're trying to LOAD the objects have access to the class definition file for your object? If you don't give it the "blueprints" for your house, it's not going to be able to build you a house (the object) -- it'll leave a pile of construction supplies on your lawn (the struct array.) It should have issued a warning in this scenario, though.
Here's a sample object. It's not on my path but is in my current directory, so I can instantiate it.
 >> which -all myobjectForSaveLoadExample.m
 c:\temp\sampleObject\myobjectForSaveLoadExample.m  % myobjectForSaveLoadExample constructor

 >> dbtype myobjectForSaveLoadExample.m
 1     classdef myobjectForSaveLoadExample
 2         properties
 3             x;
 4         end
 5         methods
 6             function obj = myobjectForSaveLoadExample(x)
 7                 obj.x = x;
 8             end
 9         end
 10    end
 >> y = myobjectForSaveLoadExample(1:10); % It exists!
Now let's go up a level, taking the class out of scope.
 >> cd ..
 >> which -all myobjectForSaveLoadExample
 Not on MATLAB path  % myobjectForSaveLoadExample constructor
 >> save('mySampleObjectMatfile.mat', 'y');
If I clear the instance out of memory, then try to LOAD it from the MAT-file, I receive a warning.
 >> clear all
 >> which -all myobjectForSaveLoadExample
 'myobjectForSaveLoadExample' not found.
 >> load mySampleObjectMatfile
 Warning: Variable 'y' originally saved as a myobjectForSaveLoadExample cannot be instantiated
   as an object and will be read in as a uint32.
[The line break in the warning is mine, to make it fit in the Answer window.]
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