Max indices in for loop not changing after initial run (index exceeds the number or array elements)
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I am getting the error
Warning: Error occurred while executing the listener callback for event ElementsAvailable defined for class daq.Buffer:
Index exceeds the number of array elements. Index must not exceed 3.
this error is occuring on my set function line in the code below. After pressing the "Start Button" on my UI(as seen below) and run it the first time after opening it it runs smoothly. after checking off additional channels it leads to errors. After the daq is done collecting and the user presses stop it writes the values to the workspace and I can see that it is still collecting those values even though they are not being displayed.
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/1648531/image.png)
Any help is greatly appreciated!
function updateLivePlot(app)
if isempty(app.DataFIFOBufferch1) || isempty(app.SelectedChannels)
return
end
% Disable interactivity
disableDefaultInteractivity(app.LiveAxes);
% Keep the colors the same after each new data point
app.LiveAxes.ColorOrderIndex = 1;
if isempty(app.LivePlotLine)
% First-Time Setup
app.LivePlotLine = plot(app.LiveAxes, app.TimestampsFIFOBuffer, app.DataFIFOBufferch1(:,app.SelectedChannels));
else
% Update existing plot
for j = 1:numel(app.SelectedChannels)
disp(j)
% Check if the line needs to be extended
if any(j <= numel(app.LivePlotLine)) && isempty(app.LivePlotLine(j))
% Use existing axes to plot
app.LivePlotLine(j) = plot(app.LiveAxes, app.TimestampsFIFOBuffer, app.DataFIFOBufferch1(:, j));
else
% Update existing line
set(app.LivePlotLine(j), 'XData', app.TimestampsFIFOBuffer, 'YData', app.DataFIFOBufferch1(:, j));
end
end
end
if numel(app.TimestampsFIFOBuffer) > 1
xlim(app.LiveAxes, [app.TimestampsFIFOBuffer(1), app.TimestampsFIFOBuffer(end)]);
end
end
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Akzeptierte Antwort
Voss
am 21 Mär. 2024
Looks like you already found an apparent solution, but I'll go ahead and point out the things I noticed, since they may still be useful for future debugging.
First, this condition is always false:
% Check if the line needs to be extended
if any(j <= numel(app.LivePlotLine)) && isempty(app.LivePlotLine(j))
The any is unnecessary because you are comparing two scalars, but also isempty(app.LivePlotLine(j)) is always false for the following reason:
app.LivePlotLine is a vector of line objects, so if j <= numel(app.LivePlotLine) then app.LivePlotLine(j) is a scalar, (i.e., a single line object). Since a scalar is not empty by definition, that condition is always false and the if branch there (i.e., the "Use existing axes to plot" line) is never executed.
So, ignoring that line and focusing on the "First-Time Setup" line and the "Update existing line" line, the relation between app.LivePlotLine and app.SelectedChannels seems inconsistent between the two.
When the first lines are created, you create as many lines as there are selected channels:
% First-Time Setup
app.LivePlotLine = plot(app.LiveAxes, app.TimestampsFIFOBuffer, app.DataFIFOBufferch1(:,app.SelectedChannels));
so that app.LivePlotLine(1) corresponds to app.SelectedChannels(1), app.LivePlotLine(2) corresponds to app.SelectedChannels(2), and so on. For example, if app.SelectedChannels is [2,5,9], then LivePlotLine(1) has data from channel 2, LivePlotLine(2) is channel 5, and LivePlotLine(3) is channel 9. No problem (yet).
However, later, when updating those same lines (suppose app.SelectedChannels is still [2,5,9]) in the "Update existing plot" branch, j runs from 1 to numel(app.SelectedChannels), so that this:
% Update existing line
set(app.LivePlotLine(j), 'XData', app.TimestampsFIFOBuffer, 'YData', app.DataFIFOBufferch1(:, j));
would be updating app.LivePlotLine(1) from channel 1 (not channel 2), app.LivePlotLine(2) from channel 2 (not channel 5), and app.LivePlotLine(3) from channel 3 (not channel 9).
Seems like it should be the following, which would be consistent with how the lines were created in the First-Time Setup:
% Update existing line
set(app.LivePlotLine(j), 'XData', app.TimestampsFIFOBuffer, 'YData', app.DataFIFOBufferch1(:,app.SelectedChannels(j)));
More generally, I think it's worth considering a simpler code design if possible, where you create as many lines as you have channels (all channels, not selected channels) when the app starts, giving you a nice 1:1 correspondence between channels and lines. Initially the lines can be invisible and have no data. When a channel is selected, the line becomes visible; when a channel is deselected, the line becomes invisible. When you update the lines' data, you only need to update the selected channels' lines, whose index in app.LivePlotLine is the same as app.SelectedChannels, because they are 1:1 now.
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