Solving for a value using a while loop.

I have a code that I give input to and it gives me an output. The problem is that I don't know what the input is but I know what the output should be. So I am trying to use a while loop to find the input variable by trying different values for the input variable until the output variable is reached. Specifically, I am trying to increase the input variable by increments of 0.1 starting from zero. I have mage a while loop but I keep getting the error "Function definition not supported in this context. Create functions in code file." for line 4 in my code. This is my code below:

B=0;

A=0;

while A<1.1345

function Lab

clear all; close all; clc;

j=20; % moment of inertia (kg.m^2)

k=30; % rotational stiffness (N.m/rad)

c=10; % damping constant (N.m.s)

xo=0; % initial angular postion

vo=B+0.1; % initial angular velocity

x0=[xo vo]; %Initial condition vector

% Function second_order_ODE

% Numerical simulation Using Runge-Kutta

t1=0; % Starting time (s)

t2=20; %Ending time (s)

dt=0.05; % Integration time step (s)

t=t1:dt:t2;

[t,x]=ode45(@rhs,t,x0,[],j,c,k);

% Plotting the output

figure;

subplot(211);

plot(t,x(:,1),'b-');

xlabel('Time (s)'); ylabel('Angular Displacement (rad)');

subplot(212);

plot(t,x(:,2),'b-');

xlabel('Time (s)'); ylabel('Angular Velocity (rad/s)')

function G=rhs(t,x,j,c,k)

G(1,1)=x(2);

G(2,1)=(-k*x(1)-c*x(2))/j;

end

A=max(x(:,1));

end

end

Answer 1

[Star Strider]

You can get around the function problem by creating ‘rhs’ as an anonymous function:

rhs = @(t,x,j,c,k) [x(2); (-k*x(1)-c*x(2))/j];

Then put it before the beginning of the while loop, and change the ode45 call to:

[t,x]=ode45(@(t,x)rhs(t,x,j,c,k),t,x0);

There are likely much more efficient ways to optimise your differential equation that the exhaustive-serach approach.

It would help to know what you are doing, and what you want to optimise for, and what parameters you are changing to do the optimisation. That is not clear from your code.

EDIT — (11 Oct 2020 at 14:00)

In the interim, I would change to this version of your code:

B=0;
A=0;
t1=0; % Starting time (s)
t2=20; %Ending time (s)
dt=0.05; % Integration time step (s)
t=t1:dt:t2;
rhs = @(t,x,j,c,k) [x(2); (-k*x(1)-c*x(2))/j];

k = 1
while A<1.1345
    j=20; % moment of inertia (kg.m^2)
    k=30; % rotational stiffness (N.m/rad)
    c=10; % damping constant (N.m.s)
    xo=0; % initial angular postion
    vo=B+0.1; % initial angular velocity
    x0=[xo vo]; %Initial condition vector
    % Function second_order_ODE
    % Numerical simulation Using Runge-Kutta
    
    [t,x]=ode45(@(t,x)rhs(t,x,j,c,k),t,x0);
    
    A=max(x(:,1));
    Av(k) = A;
    k = k + 1;
end

% Plotting the output
figure
subplot(211);
plot(t,x(:,1),'b-');
xlabel('Time (s)'); ylabel('Angular Displacement (rad)');
subplot(212);
plot(t,x(:,2),'b-');
xlabel('Time (s)'); ylabel('Angular Velocity (rad/s)')

.