How do i use trapz to integrate each element in a nx1 matrix?

Question

John O 2022-3-22

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1678024-how-do-i-use-trapz-to-integrate-each-element-in-a-nx1-matrix

评论： Torsten 2022-3-22

Hello, I'm trying to integrate a nx1 matrix where each element in the matrix is a different equation. So the results should still be an nx1 matrix of the same size as original.

clc

clear

close all

% Parameters

load('XXX.mat')

load('AXX.mat')

v_ac = 10;

v_dc = 40;

% Build model

syms u1(t) u2(t) u3(t) phi1(x) phi2(x) phi3(x)

syms W_s(x)

u = [u1(t) u2(t) u3(t)]

phi = [phi1(x) phi2(x) phi3(x)]

N = 3

mult = phi.*u

W_a(x,t) = W_s(x) + sum(mult(1:N))

phia_1 = ephi1(v_dc+1)

phia_2 = ephi2(v_dc+1)

phia_3 = ephi3(v_dc+1)

phia = [phia_1 phia_2 phia_3]

w_sa = w_ss(v_dc+1)

w_a = subs(W_a,[phi1 phi2 phi3 W_s],[phia_1 phia_2 phia_3 w_sa])

w_a = simplify(w_a)

eqn1 = []

eqn2 = []

XXX = 0:0.05:1;

eqn_mps(x,t) = (diff(w_a(x,t),x,1)^2)+(2*diff(w_a(x,t),x,1)*diff(w_0(x),x,1))

mps = trapz(XXX,eqn_mps(XXX,t))

for i = 1:N

eqn_1(x,t) = phia(i)*diff(w_a,t,2);

eqn1 = [eqn1;eqn_1];

end

eqn1

eqn = trapz(XXX,eqn1(XXX,t))

This is my error

Error using trapz

Point spacing must be a scalar specifying uniform spacing or a vector of x-coordinates for each data point.

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Answer 1

Benjamin Thompson 2022-3-22

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1678024-how-do-i-use-trapz-to-integrate-each-element-in-a-nx1-matrix#answer_924504

trapz is for numerical integration. It does not work on symbolic equations. For symbolic integration use the "int" function from the Symbolic Math Toolbox.

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John O 2022-3-22

It also works if I use trapz individually instead of on the matrix.

Torsten 2022-3-22

Yes, because the matrix eqn_mps is symbolic.

You must substitute numerical values for all the symbolic variables contained in eqn_mps.

If the result of this substitution is a matrix with the number of rows equal to the number of rows of XXX, trapz will work.

Since it is not possible to decipher from your code which symbolic variables or arrays eqn_mps depends on, it's up to you to make the correct substitutions.

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