MATLAB: ??? Subscript indices must either be real positive integers or logicals. Using findpeaks

findpeaksMATLAB

I've have been using findpeaks in MatLab to locate the maximum and minimum points of a waveform with no problem, but in the last 20 minutes or so the error:
??? Subscript indices must either be real positive integers or logicals.
Has appeared an I have no idea why. Even trying simple exercises with test data has resulted in the same error. For example if I were to have the dataset:
test = [ 0.1 0.5 0.9 0.5 0.2 0.6 1.0 0.7 0.3 0.1 ]
and used the code:
peaks = test(findpeaks(test));
I would expect the result:
peaks = [0.1 0.9 0.2 1.0 0.1 ]
but for some reason this is no longer the case.
Please advise.
Thanks,
Jared.

Best Answer

findpeaks returns the peak, which is 0.9, if you don't accept both output arguments of it. 0.9 in not an integer and so it cannot be used as as index into test. Since you want the peaks, and that's what findpeaks returns, you can simply do this:
testData = [ 0.1 0.5 0.9 0.5 0.2 0.6 1.0 0.7 0.3 0.1 ]
peaks = findpeaks(testData)
and get the results:
testData =
    0.1000    0.5000    0.9000    0.5000    0.2000    0.6000    1.0000    0.7000    0.3000    0.1000
peaks =
    0.9000    1.0000
These are the peaks, but this does not match what you're calling peaks. It looks like you're including the valleys in how you define peaks. In that case, you might want to run findpeaks again on the inverted data (subtract your data from the max of the data).
maxPeakValue = max(peaks);
% Surround with 0 so you can get the valley
% should it occur at the first or last element.
invertedData = [0, maxPeakValue - testData, 0]
valleys = maxPeakValue - findpeaks(invertedData)
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