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

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.

[Walter Roberson]

Where does it say that the problem is occurring? Which line, which file?

[Jared Rigby]

The problem is occuring at: peaks = test(findpeaks(test));

This code previously returned maximum and minimum peak values in a single matrix does now is problematic

Answer 1

[Image Analyst]

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)

Answer 2

[Richard Brown]

You've probably declared a variable called findpeaks by accident. Try

which findpeaks

and confirm it's actually pointing to a function

[Sean de Wolski]

clear findpeaks

to remove the variable overwriting it

[Jared Rigby]

which findpeaks returns:

C:\Program Files (x86)\MATLAB\R2011a Student\toolbox\signal\signal\findpeaks.m

and I don't have anyother variables in my current workspace other than test and peaks

[Jared Rigby]

I've tried clear findpeaks and running the code again but the problem remains :(

Answer 3

[Wayne King]

   [pks,locs] = findpeaks(test);
   pks
   test(locs)

pks contains the peak values

locs are the indices

    test(locs)

[Jared Rigby]

Is there a way to implement this to return the max and min points?

[Wayne King]

you mean the maximum and minimum peak?

sort(findpeaks(test))

If you just want the min and max elements of the vector, then just use min(), max()

[Jared Rigby]

Sorry I was not clear, the dataset I will eventually be using represents a repetitive exercise over a 30 second period. I need every high peak and every low peak as they represent the beginning and end of an exercise.

[Image Analyst]

He wants the valleys. In my answer I recommended Jared invert the data and run findpeaks again. Valleys will turn into peaks once you've inverted it.

[Richard Brown]

haha, I blame TMW for this confusion (pks vs locs). findpeaks suggests that it returns indices a. la. find, you'd actually want it to be called peaks. But we can't have that, can we!