Can the "Value" display of variables be customized?

Consider this simple class that contains data and a data set ID:

classdef myClass
    properties
        id
        data
    end
    methods
        function obj = myClass(id)
            obj.id = id;
        end
    end
end

When a class instance is created with obj = myClass(1), MATLAB's workspace window displays the "Value" of the resulting variable as "1x1 myClass". Likewise, a vector of objects obj = [myClass(1), myClass(2)] is displayed in MATLAB's variable viewer as "1x1 myClass" in each table cell.

I would like MATLAB to display the value of each object's id property instead of "1x1 myClass". Is this possible?

I read about custom display, but all this seems to only affect methods like disp and details. Or am I missing something?

[Captain Karnage]

I dug a little further, and it looks like it is possible, using Java. When I have time, I will see if I can code something up myself, but I wanted to share this link as soon as I could, as it will probably take me a while to actually generate working code. Java is the language I've used the least so I'd have to brush up on my Java skills again.

Here is the link: https://undocumentedmatlab.com/articles/customizing-matlabs-workspace-table

Two caveats with this link:

1. They are customizing the "Bytes" column - but I assume it would be a trivial change to make it the "Value" column.
2. The code in the article changes the column display for every variable. Some non-trivial code would need to be added to seek out variables of one of your custom classes and change the display specific to that class.

Or, alternately to #2, you could have it seek ANY custom class, seek out a certain property name and, if it exists, display that property name (e.g. the "id" property in your case).

If it were me, I would actually make a hidden, dependent property called "Value" or something generic like that for it to seek out, then I would add a getter for that property in each custom class and set it equal to the value I want to display in the Workspace pane. Then code the Java to display the "Value" property in the "Value" column for all custom classes that have a "Value" property.

Maybe there is a way with code in the Class's .m file, but if there is - like you, I haven't found it yet nor I have I found evidence it's not possible at all.

Answer 1

[Steven Lord]

You are missing something. The methods that your class inherits when you subclass from matlab.mixin.CustomDisplay are used when displaying the object even if you don't explicitly call disp or display.

classdef myClass < matlab.mixin.CustomDisplay
    properties
        id
        data
    end
    methods
        function obj = myClass(id)
            obj.id = id;
        end
    end
    methods(Access=protected)
        function displayScalarObject(obj)
            if isnumeric(obj.id) || ischar(obj.id)
                S = string(obj.id);
            else
                S = obj.id;
            end
                
            fprintf('id: %s\n', S);
        end
    end
end

Try constructing this using a number or text data as the ID.

>> Q = myClass(42)

Q = 

id: 42
>> Q = myClass('abracadabra')

Q = 

id: abracadabra
>> Q = myClass("MATLAB")

Q = 

id: MATLAB

You may need or want to implement more of the methods inherited from matlab.mixin.CustomDisplay than just displayScalarObject. See this page for a more detailed description of when and how each of those methods are used when displaying your object.

[Frederick Zittrell]

Thank you for pointing this out, but I was already aware of this customization and the information in the link you provided. While this customization affects the variable display in the Command Window, it does not affect the "Value" display in the Workspace Window or when a multi-element array of objects is inspected (by double clicking the variable name in the Workspace Window). Consider this object array:

>> X = [myClass(1);myClass(2)]
X = 
  2×1 myClass array with properties:

    id
    data

In the Workspace Window, X has the "Value" 2x1 myClass, and the variable inspector shows this:

I would like to change the display behavior in this instance (and ideally also in the Workspace Window), not in the Command Window.

An example where MATLAB does something similar to what I would like to achieve is the display of cell arrays in the variable inspector. Consider these variables:

>> c1 = {'a'}
c1 =
  1×1 cell array
    {'a'}
>> c2 = {'a','b'}
c2 =
  1×2 cell array
    {'a'}    {'b'}
>> c3 = {c1;c2}
c3 =
  2×1 cell array
    {1×1 cell}
    {1×2 cell}

When inspecting c3, the variable inspector shows this:

I could not find a hint in the documentation on how the display behavior in the Workspace Window and in the variable inspector can be customized.

[Fabian Müller]

Any updates on this? Did you figure out if it's possible to customize this "value" display? Would help me also a lot. Thanks

[Frederick Zittrell]

No, unfortunately not -- I did not find a solution and I did not find a clear answer that it is definitely not possible.

Answer 2

[Justin Rehak]

Can you make your class inherit from double? Here's an example of something I made recently. It appears as a double in the workspace, but I can override the display methods to show metric suffixes in the command window.

classdef capacitance < double & matlab.mixin.CustomDisplay
    properties (Hidden)
        str
    end
    methods
        function obj = capacitance(value)
            obj = obj@double(value)
            obj.str = obj.getSuffix();
        end
        function str = getSuffix(obj)
            if obj > 1e-3
                str = obj*1e3 + "m";
            elseif obj > 1e-6
                str = obj*1e6 + "µ";
                % and so on %
            end
        end
    end
    %% Custom display methods
    methods (Access = protected)
        % Custom display here %
    end
end

The following documentation was helpful when I ran into issues with arrays of this class. Subclasses of Built-In Types with Properties