How can I make a integration with trapz?

v=[2.2696;2.2696; 2.4646;2.4646;2.6775;2.6775; 2.9101;2.9101;3.1644];
x=[0  , -0.0476  , -0.0476 ,  -0.0952  , -0.0952  , -0.1429 ,  -0.1429 , -0.1905   ,-0.1905];
b=x<=0 & x>=-0.08;
xx=[x(b),-0.08];
vv=[v(b);NaN];
trapz(xx,fillmissing(vv,'previous'))
ans = -0.1879

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Davide Masiello 2022-4-21

I think that since the function lies all above the x axis, the area should be positive.

Bruno Luong 2022-4-21

I don't compute the area but the integral with trapz with x going backward, so it return negative of the area, as noted in my answer.

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Answer 2

Davide Masiello 2022-4-21

0
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1701380-how-can-i-make-a-integration-with-trapz#answer_947300

编辑：Davide Masiello 2022-4-21

在 MATLAB Online 中打开

I don't think you really need trapz for this function

clear,clc

x=[0,-0.0476,-0.0476,-0.0952,-0.0952,-0.1429,-0.1429,-0.1905,-0.1905];

y=[2.2696;2.2696; 2.4646;2.4646;2.6775;2.6775; 2.9101;2.9101;3.1644];

plot (x,y)

x1 = [0,-0.0476,-0.0476,-0.08];

y1 = [2.2696;2.2696; 2.4646;2.4646];

dx = diff(unique(x1,'stable'))

dx = 1×2

-0.0476 -0.0324

dy = unique(y1,'stable')'

dy = 1×2

2.2696 2.4646

area = abs(sum(dx.*dy))

area = 0.1879

4 个评论
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Bruno Luong 2022-4-21

编辑：Bruno Luong 2022-4-21

Check again. There are 2 steps I give the manual formula and value above: 0.1879

Davide Masiello 2022-4-21

The answer was indeed wrong because unique sorts the output unless specified otherwise. I fixed it and now it works. I also implemented to calculate the absolute value so the area comes out positive.

Thanks for pointing it out @Bruno Luong!

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