Using trapz function to provide answers using loop

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Dyl 2022-9-28

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1813455-using-trapz-function-to-provide-answers-using-loop

评论： Chunru 2022-9-28

采纳的回答： Chunru

Hey,

I am trying to use the trapz function in a loop:

y_approx = zeros ([21 1]);

for i = 1:length(x)

y_approx = trapz(x(i),y)

end

It is providing me the 21 answers but they are incorrect. I think it is using the 21 y for each iteration of x. 21 y values for 1 x value, 21 y values for 2 x values etc. How do I alter the code to make it use the same number of y values as x values?

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Answer 1

Chunru 2022-9-28

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https://ww2.mathworks.cn/matlabcentral/answers/1813455-using-trapz-function-to-provide-answers-using-loop#answer_1062150

在 MATLAB Online 中打开

% The way to use trapz
x= 1:20;                    % x
y = randn(20, 1);           % y
ytrapz = trapz(x, y)        % integral y dx
ytrapz = -0.0920

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Dyl 2022-9-28

I need the trapz fuction as I am estimating y values using the trapezoidal method. Basically, I need a loop that does:

y2 = trapz(x(1:2),y(1:2))

y3 = trapz(x(1:3),y(1:3))

...

y19 = trapz(x(1:20),y(1:20))

y20 = trapz(x(1:21),y(1:21))

Chunru 2022-9-28

在 MATLAB Online 中打开

That is not really necessary since the looping code above is doing the trapz job in a more efficient way when you want the intermidiate results.

However, if you insisit, you can do the following (definitely not recommended):

x= 1:20;                    % x
y = randn(20, 1);           % y
ytrapz = trapz(x, y)        % integral y dx
ytrapz = 3.5232
yloop = zeros(size(x));
for i=2:length(x)
    yloop(i) = yloop(i-1) + (y(i) + y(i-1))/2*(x(i)-x(i-1));
end
yloop([1:3 end-3:end])
ans = 1×7
         0   -0.7511   -0.5029    5.0386    4.9629    4.9692    3.5232
% Not recommended:
yloop1 = zeros(size(x));
for i=2:length(x)
    yloop1(i) = trapz(x(1:i), y(1:i));
end
yloop1([1:3 end-3:end])
ans = 1×7
         0   -0.7511   -0.5029    5.0386    4.9629    4.9692    3.5232