MATLAB: The polyarea function gives the wrong area

MATLABpolyarea

Hi, I have a question with the polyarea function

x = [0 3 3 0 0 1 1 2 2 1]; y = [0 0 3 3 0 1 2 2 1 1];

With this curve the value it gives is 8, which is correct

However I made a very similar curve

x = [0 0.4258 0.4258 0 0.009 0.4168 0.4168 0.009 0.009]; y = [0 0 0.1807 0.1807 0 0.009 0.009 0.1717 0.1717 0.009];

and the value it gives is polyarea (x, y) -> 0.1433. And the value that should give is 0.010593

someone knows why ?

Best Answer

I think MATLAB result is right, your expectation is wrong.

Check area

>> 0.1807*0.4258-(0.1807-2*0.009)*(0.4258-2*0.009)
ans =
       0.010593000000000
>>

which is the same than MATLAB

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MATLAB: Fourier analysis of waveforms and clipping wave, HELP PLEASE!!!

I'm not sure how you are using the term "period" here. If you are talking about only the first 1 of the fundamental frequency which is 1 MHz here, then I think you want at least a few periods to do a spectral analysis even with a clean waveform. The resolution in the DFT (implemented by fft) depends on the number of samples. The more samples, the better the resolution.

In your case, it's easy to see that

xnew = x(1:2800);

gives you the initial waveform, so why not use this?

     Fs = 1/1e-8;
     d = fdesign.lowpass('Fp,Fst,Ap,Ast',1.2e6,1.4e6,0.5,60,Fs);
     Hd = design(d,'butter');
     xnew = x(1:2800);
     y = filter(Hd,detrend(xnew,0));
     ydft = fft(y);
     freq = 0:Fs/length(y):Fs/2;
     ydft = ydft(1:floor(length(y)/2)+1);
     % Plot in MHz
     plot(freq./1e6,abs(ydft))
     axis([0 10 0 max(abs(ydft))+20])
     xlabel('MHz'); ylabel('Magnitude');

If you copy and paste the above code, it works fine.

MATLAB: Mean of indexed data in a matrix

Here it is in just 5 lines (which could be reduced further)

% Your data
data = [1 0.008 0.4168 0.3166 0.3269; 0.008 1 0.4894 0.8251 0.9821; 0.4168 0.4894 1 0.8132 0.6861; 0.3166 0.8251 0.8132 1 0.4929; 0.3269 0.9821 0.6861 0.4929 1]; 
groups = [ 1 1 2 2 1]; 
% Create within-groups index matrix
groupMat = groups == groups'; 
% Isolate the lower half of the correlation matrix
% replacing up half and identity line with NaN
dataTrim = data; 
dataTrim(triu(true(size(data)))) = NaN; 
% Extract within-group and between-group correlations
% and calculate mean ignoring NaN values.
withinGroupMean = nanmean(dataTrim(groupMat)); 
betweenGroupMean = nanmean(dataTrim(~groupMat));

Best Answer

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MATLAB: Mean of indexed data in a matrix

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