MATLAB: Adding conditions / additional input to ode45 function

odeode45

Hello, I'm using ode45 and I want to be able to add an input ('i') which will determine a parameter in the function I put in the ode45:

[T,V] = ode45(@myfun,[0 20],vec_0);

and the function (shortened) is:

function dy = myfun(t,y)

dy=zeros(4,1);

if i==1

I_inj = 20;

elseif i==2

I_inj = 40;

% etc…

end

dy(1) = …

dy(2) = …

dy(3) = …

dy(4) = … + I_inj;

I've tried to add 'i' in the inputs of the function: function dy = hh(t,y,i) but I couldn't understand how to input 'i' in the ode45 function ( [T,V] = ode45(@myfun,[0 20],vec_0); ) I'll be happy to know how to do that or if there are other ways to set conditions to a function for ode45. Thanks a lot!

Best Answer

While this documentation page uses fzero in its examples, the same techniques work for passing additional parameters into the ODE functions that you specify as the first input when you call ode45.

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tspan = [0 12];
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MATLAB: 4th order Runge-Kutta code that can solve for several intial conditions

Here's a version:

% Initial conditions
t0=0;
x0=0:0.2:1;
y0=1:0.2:2;
dt=0.1;
tz=1;
t_range= t0:dt:tz;
X = zeros(numel(x0),numel(t_range));
Y = zeros(numel(y0),numel(t_range));
x_rk=zeros(1,numel(t_range));
y_rk= zeros(1,numel(t_range));
for n = 1:numel(x0)
    x_rk(1)=x0(n);
    y_rk(1)=y0(n);
    for i=1:length(t_range)-1
        [x_rk(i+1), y_rk(i+1)]=rk_method(x_rk(i),y_rk(i),dt);
    end
    X(n,:) = x_rk;
    Y(n,:) = y_rk;
    %% PLots
    subplot(numel(x0)/2,2,n)
    plot (t_range, x_rk,'b-o',t_range, y_rk,'y-o','MarkerSize',5);
    grid
    title([num2str(x0(n)),'   ',num2str(y0(n))])
    legend('x','y')
end
disp('X = '), disp(X)
disp('Y = '), disp(Y)
%% Functions
function [dxdt, dydt]= f(x,y)
dxdt =  5*x-3*y;
dydt = -6*x+2*y;
end 
function [x1, y1]=rk_method(x0,y0,dt)
[k11, k12]=f(x0,y0);     
[k21, k22]=f(x0+0.5*dt*k11,y0+0.5*dt*k12); 
[k31, k32]=f(x0+0.5*dt*k21,y0+0.5*dt*k22); 
[k41, k42]=f(x0+dt*k31, y0+dt*k32); 
x1=x0+dt*((k11/6)+((k21+k31)/3)+(k41/6));
y1=y0+dt*((k12/6)+((k22+k32)/3)+(k42/6));
end

Best Answer

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MATLAB: How To Write ODE45 Function File

MATLAB: 4th order Runge-Kutta code that can solve for several intial conditions

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