App designer Problem : error matlab.ui.​control.in​ternal.mod​el.Abstrac​tNumericCo​mponent/se​t.Value (Line 111 ) 'Value' have to be double scalar

    % Properties that correspond to app components
    properties (Access = public)
        UIFigure           matlab.ui.Figure
        numEditFieldLabel  matlab.ui.control.Label
        numEditField       matlab.ui.control.NumericEditField
        resEditFieldLabel  matlab.ui.control.Label
        resEditField       matlab.ui.control.NumericEditField
        diffButton         matlab.ui.control.StateButton
    end
    
    properties (Access = private)
        x % Description
    end
    
    

    % Callbacks that handle component events
    methods (Access = private)

        % Value changed function: diffButton
        function diffButtonValueChanged(app, event)
            fx = (app.x)^2 + app.x;
            difffx = diff(fx,app.x,1);
            app.resEditField.Value = subs(difffx,app.x,app.numEditField.Value);%%%%%%% this is the place where i got error
        end
    end

Answer 1

[Walter Roberson]

app.x is not initialized, so it defaults to empty.

            fx = (app.x)^2 + app.x;

value^2 is only valid if the value is empty or scalar or a square matrix. We cannot tell which from that alone.

            difffx = diff(fx,app.x,1);

The three-parameter version of diff requires that the second parameter be scalar or empty. If app.x is numeric scalar then fx is numeric scalar, and the second parameter would be the number of differences to take; with fx being scalar, if that does not outright fail, the diff is going to return empty unless app.x happens to not be a positive integer, in which case error. If app.x is empty then it would be treated as 1 but since fx would be empty in that case, you would get empty anyhow. So if app.x is numeric then the diff is either going to error (not empty or positive integer) or return empty.

          app.resEditField.Value = subs(difffx,app.x,app.numEditField.Value);%%%%%%%this is the place where i got error

If app.x is empty or numeric then if we got here then subs(empty) is going to be empty and that would not be scalar and you need a scalar at that point.

So... what if app.x just happens to store a scalar symbolic variable? Then the diff() would work and produce a scalar symbolic expression, and the subs would probably work and produce a scalar symbolic expression.

So what is the problem in that case? This: you cannot set that kind of field to a symbolic expression, it is the wrong data type. You need to convert the result to the subs() into a numeric datatype. Like double(subs(...))

Answer 2

[Cris LaPierre]

The result of subs is still symbolic. You need to convert it to a double. You can use the double function for that.

app.resEditField.Value = double(subs(difffx,app.x,app.numEditField.Value));

[Cris LaPierre]

To recreate your error in MATLAB, do the following

fig = uifigure;
edt = uieditfield(fig,'numeric');
syms x
fx = (x)^2 + x;
difffx = diff(fx,x,1);
edt.Value = subs(difffx,x,5)

Error using matlab.ui.control.internal.model.AbstractNumericComponent/set.Value (line 111)

'Value' must be a double scalar.

Now rerun, changing the last line to this

edt.Value = double(subs(difffx,x,5));

[Walter Roberson]

This is probably the problem, but we cannot prove it with the information given; my Answer shows that the result of the subs could be empty. The innards of the test would have to be examined to see whether it fails first for not being a scalar or fails first for not being numeric.

[Cris LaPierre]

True. Oddly enough, there was another post today with the exact same error. See my answer to that one.