Extracting a data vector from a looped ode45

Hi. I have a problem where i have to solve a system of equations numerically. I want to plot the maximum value for each result I get with the the a variable i am using (it is changing each iteration). However, when i try to plot it it seems it only uses the first result and variable.

clc; clear;close all;
% Defining parameters and initial conditions

    f = 0.35;
    r = [0.1:0.1:1.5];
    zeta = 0.25;
    Theta0 = [0.30, 0];
    tspan = [0 20];
    
% Solver options
    opts = odeset('RelTol',1e-6,'AbsTol',[1e-9 1e-9]);
    
% Solver   
   for i = 1:length(r)    
[t,theta] = ode45(@(t,theta) odefcn(t,theta,f,r(i),zeta), tspan,Theta0);
Amp=max(theta(:,1));
plot(r,Amp);hold on;
   end
   

I want to plot is so i have a line showing the change in Amp dependent on the change in r.

Answer 1

[Star Strider]

I cannot run this because ‘odefcn’ is over the horizon.

I would do something like this —

% Defining parameters and initial conditions

f = 0.35;
r = [0.1:0.1:1.5];
zeta = 0.25;
Theta0 = [0.30, 0];
tspan = linspace(0, 20, 50);

% Solver options
opts = odeset('RelTol',1e-6,'AbsTol',[1e-9 1e-9]);

% Solver   
for i = 1:length(r)    
    [t,theta] = ode45(@(t,theta) odefcn(t,theta,f,r(i),zeta), tspan,Theta0);
    Amp(:,i)=max(theta(:,1));
end

figure
surf(t, r, Amp)
grid on

It may be necessary to reverse ‘t’ and ‘r’ in the surf call so that the dimensions will match appropriately.

.

[Frederik Bjerregaard]

Sorry for not including the function, I have included it here

function dthetadt = odefcn(t,theta,f,r,zeta)
    dthetadt = zeros(2,1);
    dthetadt(1)=theta(2);
    dthetadt(2)=f.*sin(r.*t)-2.*zeta.*theta(2)-sin(theta(1));
end

I tried to run the script with the things you included but i can't get it to work.

[Frederik Bjerregaard]

I managed to get the code working using plot rather than surf. Thank you for your help

[Star Strider]

As always, my pleasure!

(I remembered just now that it saves a point and not a vector, so that is appropriate.)

.