MATLAB: Area between two curves that intersect

MATLABtrapz

Hi, i'm trying to find the area between these two plots, i need to find the area of each individual section where the plots overlap. For some reason my 'trapz' commands aren't working. All help would be appreciated. If more info is needed about the program just let me know and i'll see what i can do.
lprofile=xlsread(fname_lprofile);
x1=lprofile(:,1)
y1=lprofile(:,2)
y2=lprofile(:,3)
plot(x1,y1,x1,y2)
x3=2
ex_area=0
fl_area=0
while x3>1 & x3<(n1-1)
    if lprofile(x3,2)>lprofile(x3,3)
    a1=trapz (lprofile(x3,1),lprofile(x3,2),2)
    ex_area=ex_area+a1
    x3=x3+1
    elseif lprofile(x3,3)>lprofile(x3,2)
    a2=trapz(x3,3)
    fl_area=fl_area+a2
    x3=x3+1
    else
    x3=x3+1
        
                        
    end
end

Best Answer

You are using the loop incorrectly, and you don't need it anyway. You are doing an integration over a single point, which is unlikely to work in any context. You could sum the values and divide by the delta x, but by not using array operations you're not taking advantage of the full potential of Matlab.
The method I show below will have a rounding error that increases with the number of crossings, but it shouldn't get too bad, especially if you're using a finer x scale.
% lprofile=xlsread(fname_lprofile);
% x1=lprofile(:,1);
% y1=lprofile(:,2);
% y2=lprofile(:,3);
% plot(x1,y1,x1,y2)
%generate random data instead
x1=linspace(0,2*pi,300);
y1=cos(x1);
y2=sin(x1);
plot(x1,y1,x1,y2)
low=y1-y2;low(low<0)=0;
hi=y2-y1;hi(hi<0)=0;
A_low=trapz(x1,low);
A_hi=trapz(x1,hi);
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