using the trapz function

Question

Eliraz Nahum 2018-10-11

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/423523-using-the-trapz-function

评论： Walter Roberson 2018-10-11

hey, I am trying to understand how the trapz works. I am trying to compare its results to an exact solution, but for some reason can't get the same results.

clear all
close all
clc
syms x1 y1
y1=x1^2;
y1_int=int(y1);
y1_int_f=matlabFunction(y1_int)
y_int_exact=[];
del=0.1;
x=[-5:del:5];
y=x.^2;
counter=0;
for Xval=x
    counter=counter+1;
    y_int_exact(counter)=y1_int_f(Xval)-y1_int_f(x(1));
    y_temp=y(1:counter);
    y_int_calc(counter)=trapz(y_temp);
end
plot(x,y,'b',x,y_int_calc,'--r',x,y_int_exact,'g')

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Answer 1

Eliraz Nahum 2018-10-11

0
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/423523-using-the-trapz-function#answer_340971

according to https://uk.mathworks.com/help/matlab/ref/trapz.html I can only give the trapz function the y values of y(x). That's what I tried to do...each time giving the function a y_temp vector that contains the [y(1),y(2),...,y(counter)] values so it will find the area in each step

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Walter Roberson 2018-10-11

在 MATLAB Online 中打开

You should have read slightly further

    Q = trapz(X,Y) integrates Y with respect to the coordinates or scalar spacing specified by X.
    If X is a vector of coordinates, then length(X) must be equal to the size of the first dimension of Y whose size does not equal 1.
    If X is a scalar spacing, then trapz(X,Y) is equivalent to X*trapz(Y).