Change "Items" in a Discrete Knob with another knob

Hi,

I have a test App in App Designer, with 2 Discrete knobs, Knob and Knob2. I want to change the settings ("Items") on "Knob" by moving "Knob2". This was just set up in App Designer, and the following Callback for "Knob2" added with the switch. Why doesn't this work?

methods (Access = private)

        % Value changed function: Knob2
        function Knob2ValueChanged(app, event)
            value = app.Knob2.Value;
            switch value
                case '1-10'
                    set(app.Knob,'Items',{1,2,3,4,5,6,7,8,9,10},'Value',{5});
                case '11-20'
                    set(app.Knob,'Items',{11,12,13,14,15,16,17,18,19,20},'value',{15});
                case '21-30'
                    set(app.Knob,'Items',{21,22,23,24,25,26,27,28,29,30},'value',{25});
                    
            end
        end
        enda

Thanks!

Doug Anderson

[Adam]

What is 'value' if you put a breakpoint in? What does happen given you say it doesn't work?

I've never worked with knobs in app designer, but it would surprise me if the 'value' field evaluated to '1-10' as a char.

Answer 1

[Adam Danz]

For Knobs (continuous)

There are a few things wrong with your callback function. I've listed them in order of importance.

1. The "Value" of your switch-case is numeric but your cases are character vectors so none of the cases will ever be chosen. Use a series of if-elseif conditionals. You could also use a switch-case with logical comparisons as in this answer.
2. "Items" is not a property of a knob. Instead, use the "MajorTicks" property which expects a vector (not a cell array).
3. In addition to MajorTicks, set the "Limits" propery to maximize the full range of your knob
4. The "value" property expects a scalar, not a cell.

Here's how the callback should appear (showing 1 line, you can adapt the rests)>

if value >=1 && value <= 10
    set(app.Knob,'MajorTicks',[1,2,3,4,5,6,7,8,9,10],'Limits',[1,10],'Value',5);
elseif
    % set()
else
    % set()
end

For "Discrete Knobs"

1. The 'Items" property is expected to be a cell array of strings or chars. You can use num2str with strsplit to convert your numeric vector to a cell array of chars.
2. The "value" property is expected to be a char array that belongs to the updated Items list.

Here's how the callback should appear (showing 1 line, again)

switch value
    case '1-10'
        set(app.Knob3,'Items',strsplit(num2str([1,2,3,4,5,6,7,8,9,10])),'Value','5');

[Douglas Anderson]

Actually, Adam, that did not work.

1. The "value" actually IS '1-10' on a Discrete Knob (I should have indicated that this is for a discrete knob). The text value is taken.
2. For Discrete Knob there aren't ticks but "Items" that indicate the position.
3. I did get the switch/case to work, but then it says: Items' must be a 1-D cell array of character vectors or a string array.
4. That's where I have a problem!I have tried putting this list in square and curly brackets -- something else?

Thanks.

Doug

[Adam Danz]

Well, that changes everything. "Knobs" are continuous. "Discrete Knobs" are not.

I updated my answer with the solution for discrete knobs.

You can use the swtich-case but I always recommend including an "Otherwise" at the end and within that otherwise-case, include an error message. Otherwise there's no indication that a case was not matched unless it causes a different error which would then require you to trace it back to the switch-case.

switch s
    case
    case
    otherwise
        error('No case match for ''%s''.', s)
end

[Douglas Anderson]

Thank you Adam!

The converting of a numerical array to a cell array of chars was something I think I never would have figured out (well, maybe before this excellent help!).

Doug

[Douglas Anderson]

Adam, I am having a different problem now, may be just stupid!

I am making a new discrete knob in Design View, and trying to change the Items by clicking on the ... on the right. I change the "Selected", and it works fine, but if I change the text in the table, it changes what is in the Items box, but has no effect on what is on the screen. I click elswhere -- nada! Back to Off Low Medium High. Matlab 2019b now, haven't installed 2020a.

Thanks!

[Adam Danz]

I suggest making a new question. You can copy the URL to the new question here so I know where to look when I get the time to look at it (tomorrow, likely).