Trapz not referred to x-axis

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Hi, I'm having some issues regarding the chance to calculate the area between two curves or lines, as the ones following:
clear all
close all
xA=0:4;
A=[-2 -1 2 3 4];
figure(1)
plot(xA,A,'o');
grid on
hold on
%polyfit to build a curve with "A" data
[pA,~,muA] = polyfit(xA, A, 4);
fA = polyval(pA,xA,[],muA);
plot(xA,fA);
xi = linspace(min(xA), max(xA)); % Interpolation Vector
Ai = interp1(xA,A,xi,'linear'); % Interpolated Data
y=xi*(-1)-2;
plot(xi,y);
Areabetweencurve=trapz(abs(y),abs(Ai)); % area between the curves should be around 21
I thought that trapz could have been the right tool, but instead of the x-line as a reference, trapezoids should be considered between the two lines. The point is, I could calculate the area between the two curves with a loop where I consider (b+B)*h/2, but maybe (and probably) there is a smarter way. Any suggestion? thanks
  1 个评论
Scott MacKenzie
Scott MacKenzie 2021-6-14
Check your trapz arguments. For the 2-arg version, the first argument is for the x spacing.

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Cris LaPierre
Cris LaPierre 2021-6-14
编辑:Cris LaPierre 2021-6-14
If you want to use trapz, use it to calculate the area below both lines, and then subtract them to find the area between the lines.

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