Not enough input arguments, ODE 45, System of ODEs

Hello everybody! I seem to be very unlucky, I can not find any solution to my problem - I have:

Error using chu2 (line 9)
Not enough input arguments.

Here is my code:

      function dy = chu2(y)

      m0 = -3/7;
      m1 = 4/7;
      alpha = 9;
      beta = 14;

      dy = zeros(3,1);
      dy(1) = alpha*(y(2) - y(1) - (.5)*(m0 - m1)*(abs(y(1) + 1) - abs(y(1) - 1)));
      dy(2) = y(1) - y(2) + y(3);
      dy(3) = -beta*y(2); 

      [~,Y] = ode45(@chu2,[0 1 1]);

      plot3(Y)

Firstly, I thought the problem is because I gave not enough info, but I have checked everything.

Also I have a question about ODE45 - what method Matlab uses as it ? Runge–Kutta method ?* * Thank you for answering such dull question, but I am so desperate to find any mistake in my code, I have googled but that did not help.

Solution to this problem by Star Strider is to call not the chu2(y) but chu2(~,y)

Answer 1

[Star Strider]

First, it seems that you are calling your ODE function ‘chu2’ from inside the function, with these lines:

      [~,Y] = ode45(@chu2,[0 1 1]);

      plot3(Y)

Do not do that! Call it from outside your function, in the main script file.

Second, the ODE solvers require 3 arguments, the first one being your ODE function (that you supplied), the second one being vector containing a range [beginning end] or vector of solution times, and the third your initial conditions. It seems you omitted your time vector, the reason you got the error.

[Ievgenii]

Dear Star Strider, I have changed calling the ODE

[~,Y] = ode45(@chu2,[0 10],[0 1 1]);

But now I receive another error:

Undefined function 'y' for input arguments of type 'double'.

Error in chu2 (line 9)
  dy(1) = alpha.*(y(2) - y(1) - (1/2).*(m0 - m1).*(abs(y(1) + 1) - abs(y(1) - 1)));

I have also had multiplication like * and here is .* but that does not help.. My main question is - What is wrong with line 9?

All this happens when dy = zeros(3,1); - COMMENTED

Now, if I uncomment it I have another issue :

Error using chu2
Too many input arguments.

Error in odearguments (line 88)
f0 = feval(ode,t0,y0,args{:});   % ODE15I sets args{1} to yp0.

Error in ode45 (line 114)
[neq, tspan, ntspan, next, t0, tfinal, tdir, y0, f0, odeArgs, odeFcn, ...

[Star Strider]

We need to see the full text of ‘chu2’. I don’t see it in what you posted, but then it’s 01.30 (Z-7) here just now. I’ll see if I can reproduce your error with your ‘chu2’ file and the ODE solver call. If MATLAB believes the subscripts are function arguments, it’s getting confused somewhere. Also, ‘chu2’ has to have two input arguments, (t,y). If you’re not using t, calling it with (~,y) will work, but with only ‘y’ it may interpret that as the first argument ‘t’, and be oblivious to ‘y’, considering subscripts to be function arguments. That is my best guess, anyway.

[Ievgenii]

Here is full code to chu2.m

function dy = chu2( * *!! you mean here I should write input arguments?!!* * )

m0 = -3/7;
m1 = 4/7;
alpha = 9;
beta = 14;

dy = zeros(3,1); % 0 0 0
dy(1) = alpha*(y(2) - y(1) - (1/2)*(m0 - m1)*(abs(y(1) + 1) - abs(y(1) - 1)));
dy(2) = y(1) - y(2) + y(3);
dy(3) = -beta*y(2); 

end

And I am calling it in command line like this :

[T,Y] = ode45(@chu2,[0 10],[0 1 1]);

If I write here function dy = chu2( ~,y ) then program has no ERRORs!

One more question - How can plot my system ODE if the output, as far as I understand is: Column vector of time points and Solution array. Each row in Y corresponds to the solution at a time returned in the corresponding row of T. To plot 3d plot I need to have each projection of Y to plot on its own axis, and I will have such numbers (y1,y2,y3) - T times. But how to separate from the Y projections?

[Star Strider]

Quoting:

‘function dy = chu2( * !! you mean here I should write input arguments?!! * )’

Yes!

‘If I write here function dy = chu2( ~,y ) then program has no ERRORs!’

Success!!

If you want to plot each y-value on its own axis, I would use the subplot function:

figure(1)
subplot(2,1,1)
plot(t, y(:,1))
title('Y_1')
grid
subplot(2,1,2)
plot(t, y(:,2))
title('Y_2')
grid
subplot(2,1,3)
plot(t, y(:,3))
title('Y_3')
grid

That should work for your plots.