Global variables to plot variables from ODE45 function

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Marlon 2023-9-7

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评论： Torsten 2023-9-14

Hello,

the following code stands representative for a bigger code, in which i want to plot some variables over time that are calculated in the odefunction but are gonna be passed to the ODE solver. Is there any way to set these variables as global variables, so that I can plot them outside of the ode Function?

Like in this case to plot the variable "a", which is calculatet inside the function.

tspan= 0:.01:10;

x=1;

[t,x] = ode45(@(t,x) odefcn(x,t),tspan,x);

figure;

plot(t,a);

function [dxdt] = odefcn(x,t)

a=cos(t)*x;

dxdt = x*a;

end

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回答（1 个）

Torsten 2023-9-7

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2018151-global-variables-to-plot-variables-from-ode45-function#answer_1304606

编辑：Torsten 2023-9-7

在 MATLAB Online 中打开

t < pi/2 is necessary:

syms t x(t)

eqn = diff(x,t) == x^2*cos(t);

conds = x(0)==1;

dsolve(eqn,conds)

ans =

tspan= 0:.01:10;

x=1;

[t,x] = ode45(@(t,x) odefcn(x,t),tspan,x);

Warning: Failure at t=1.562403e+00. Unable to meet integration tolerances without reducing the step size below the smallest value allowed (3.552714e-15) at time t.

a = zeros(size(t));

for i = 1:numel(t)

[~,a(i)] = odefcn(x(i),t(i));

end

figure;

plot(t,a);

function [dxdt,a] = odefcn(x,t)

a=cos(t)*x;

dxdt = x*a;

end

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Marlon 2023-9-14

Thanks for the quick answer, but when I try to plot the first row of vrv vs time

I get following error:

Error using plot

Vectors must be the same length.

Error in Zustandsraum_lin (line 62)

plot(t,vrv(:,1))

Torsten 2023-9-14

在 MATLAB Online 中打开

Change the code according to your needs:

t = 0:1:100;
x = [t.',t.^2',t.^3']
a = zeros(3,numel(t));
for i = 1:numel(t)
    [~,a(:,i)] = odefcn(x(i,:),t(i));
end
plot(t,a(2,:))
function [dxdt,vrv] = odefcn(x,t)
  rv = [1;1;1];
  delta = 0;
  vrv = [cos(x(3)+delta) sin(x(3)+delta) 0;
        -sin(x(3)+delta) cos(x(3)+delta) 0;
        0 0 1]*rv;
end