Area Under Curve (or Between Curve)

How to find area under curve, I try trapz, not working.

sample=data(:,2); w=data(:,3); z=13.2 plot(sample, w, sample, z); area(w,z)

how to calculate area of shaded region?

[Torsten]

Why does trapz not work ?

What's the error message ?

Best wishes

Torsten.

[Jena Jenna]

I want to find area filled so, i try areaunder=trapz (sample,w)-trapz (sample, z). I got answer for trapz(sample,w) but not for trapz(sample,z) . error message was

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

Error in ==> ipermute at 23 inverseorder(order) = 1:numel(order); % Inverse permutation order

Error in ==> trapz at 78 if ~isempty(perm), z = ipermute(z,perm); end

Answer 1

[John BG]

I understand the time reference is in sample=data(:,2)

the function to integrate is in w=data(:,3) correct?

and the z=13.2 is some kind of squelch you want to remove while integrating? there is no need to plot 2 functions.

The area you are looking for is trapz(sample,w)

you have assigned z a scalar, but it should match the length of the data to integrate.

The integration goes from around 15 to 250, so the reference vector is tref=[15:1:250]

if you really want to remove those 13.2 (volts?) while integrating, this DC is actually

13.2*ones(1,length(tref)) so if you want to use two trapz, then

 trapz(sample,w)-z*ones(1,length(tref))

does it make sense?

regards

John