Error “dot indexing..​.comma-sep​arated list with 2 values” doesn’t seem applicable

I am getting this very mysterious error message.

Intermediate dot '.' indexing produced a comma-separated list with 2 values, but it must produce a single value when followed by subsequent indexing operations.

It comes from the marked line in the code below.

methods (Access = public)
    function Display(comp, peak1, peak2)
        % peak1, peak2 are class Peak objects; data are the relevant Y values
        % Use this function to update the underlying components
        comp.Peak1EditField.Value =peak1.ActualAmplitude;  % <<<-----------  ERROR
        comp.Width1EditField.Value= peak1.HWInterval;
        if peak1.IsPair
            comp.Peak2EditField.Value = peak2.ActualAmplitude;
            comp.Width2EditField.Value= peak2.HWInterval;
            comp.TOFEditField.Value = peak1.TOF;
        end          
    end
end

I have read up and understand what this message is about, but I don't understand how it could be applicable here. We are in code for a custom component, whose structure is this:

In the debugger I can see that peak1 is an object of type Peak, and here is its value:

So which part of this could be causing the problem?

[Stephen23]

"So which part of this could be causing the problem?"

Note how X is non-scalar:

X(1).Y.Z = 1;
X(2).Y.Z = 2;
X.Y.Z

Intermediate dot '.' indexing produced a comma-separated list with 2 values, but it must produce a single value when followed by subsequent indexing operations.

[Stephen23]

Of course any type of trailing indexing will throw this error (not just dot-indexing):

X(1).Y = 1;
X(2).Y = 2;
X.Y(:)

Intermediate dot '.' indexing produced a comma-separated list with 2 values, but it must produce a single value when followed by subsequent indexing operations.

Answer 1

[Kenneth]

The real answer, when it came, was far stranger. What actually happened is that at the calling level I had called it with

app.PeakDetail.Display(peak1, peak2)

but I had made a change so that app.PeakDetail was now in fact an ARRAY of PeakDetail custom component instances. To my utter astonishment, Matlab allowed me to make this method call based on an array of the object, rather than a single instance. I know other programming language that would allow this!

So inside the Display function, comp.Peak1EditField.Value referred to multiple edit fields, and an assignment to multiple edit fields at one shot is not supported. Sad to say, the debugger would not show me teh value of comp.Peak1EditField, so it took far too long to analyze the problem.

Wow. That was a doozy!

Answer 2

[Cris LaPierre]

I suspect peak is no longer 1x1 when the error occurs. Can you confirm?

The fix might be to ensure only a single value is returned

comp.Peak1EditField.Value =peak1(1).ActualAmplitude;

[Stephen23]

In that case only the first RHS comma-separated list element is returned without error:

S = struct('A',{1,2})

S = 1×2 struct array with fields:
    A

X = S.A % no error, 1st value is assigned, all others are discarded

X = 1

More generally, a comma-separated list on the LHS will accept <=N of the N elements from the RHS comma-separated list:

C = cell(1,3);
D = {1,2,3,pi,sqrt(2)}

D = 1×5 cell array
    {[1]}    {[2]}    {[3]}    {[3.1416]}    {[1.4142]}

[C{:}] = D{:} % no error

C = 1×3 cell array
    {[1]}    {[2]}    {[3]}

Of course trying to obtain >N items will throw an error:

C = cell(1,6);
[C{:}] = D{:} % fails

Insufficient number of outputs from right hand side of equal sign to satisfy assignment.