slider for march chart

Hello! Help adjust the slider for the chart of the marshout (path) of the machine

function SliderValueChanged(app, event)
           app.Slider = app.Slider.Value; 
end
function GPSMenuSelected(app, event)
        gps1=[app.x;app.y];
        plot(app.UIAxes,app.x,app.y,'LineWidth',2);
        hold on
        plot(app.UIAxes,app.x( app.Slider ),app.y(app.Slider ),'b--o',...
     'LineWidth',6)
        hold off
end

I use hold on but when I try to use the slider I get an error

Array indices must be positive integers or logical values.
Error in OperatorMachine01/GPSMenuSelected (line 671)
         plot(app.UIAxes,app.x( app.Slider ),app.y(app.Slider ),'b--o',...
 
Error using matlab.ui.internal.controller.WebMenuController/fireActionEvent (line 67)
Error while evaluating Menu Callback.

Answer 1

[Geoff Hayes]

Lev - why in your SliderValueChanged callback are you overwriting the Slider with its value? I suggest that you remove this function (or at least the code inside of it)

function SliderValueChanged(app, event)
    % app.Slider = app.Slider.Value;   % <----- this isn't necessary and is dangerous!
end

As for the error, it is telling you that you are trying to index into an array with something other than a positive integer or (0 or 1) logical value. I suspect that the slider value is NOT an integer so you will need to ask your self how to convert this number into a valid index for the arrays. For example, your code might look like

function GPSMenuSelected(app, event)
        gps1=[app.x;app.y];
        plot(app.UIAxes,app.x,app.y,'LineWidth',2);
        hold on
        
        sliderValue = ceil(app.Slider.Value);    % <--- round up to the nearest integer
        if sliderValue >= 1 && sliderValue <= length(app.x)
            plot(app.UIAxes,app.x(sliderValue),app.y(sliderValue),'b--o',...
            'LineWidth',6);
        end
        hold off
end

I don't know if that is valid for you so please validate before trying it out.