Using trapz function to provide answers using loop

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Dyl
Dyl 2022-9-28
评论： Chunru 2022-9-28
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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Chunru
Chunru 2022-9-28
Ran in:
% 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
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
Chunru 2022-9-28
Ran in:
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

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