Not Enough Input Arguments

I'm solving a third order non-linear ODE using ode45() function. My script is below. The function 'eqn' is saved in a separate file. I'm getting an error of 'Not enough input arguments.' Any help in resolving this is appreciated.

Thank you.

    beta = 0;
    guess = 0.4696;
    eta_span = [0 5];
    init_cond = [0 0 guess];

    [eta, F] = ode45(eqn, eta_span, init_cond);

    function F = eqn(eta_span, init_cond)

    F = zeros(size(init_cond));

    F(1) = init_cond(2);
    F(2) = init_cond(3);
    F(3) = -beta - init_cond(1) * init_cond(3) + beta * init_cond(2) * init_cond(2);

Answer 1

[Walter Roberson]

[eta, F] = ode45(@eqn, eta_span, init_cond);

Answer 2

[Star Strider]

I didn’t run your code, but two items immediately present themselves.

First, considering that you’re letting ‘init_cond’ determine the size of the vector returned by your ODE function ‘eqn’, it must be a column vector.

Second, you need to create a function handle for ‘eqn’ using the ‘@’ operator in your ode45 call, since it is in its own function file.

See if these changes work:

init_cond = [0 0 guess]';

[eta, F] = ode45(@eqn, eta_span, init_cond);

[Karen Melikov]

The init_cond vector works fine. It was the @ in front of the function handle that did it. Thanks.

I have another question, however: in my script I have a variable called beta. I would like to be able to vary beta in the script and have the function eqn take beta as an input variable. However, when I put beta in the list of eqn function variables, the code does not work. Is there a way for me to feed the beta value into the function instead of manually changing the beta value in the function? Thanks.

[Star Strider]

You can easily pass ‘beta’ to your ‘eqn’ function. You have to make changes in it and in your call to it in your ode45 call:

[eta, F] = ode45(@(eta_span, init_cond) eqn(eta_span, init_cond, beta), eta_span, init_cond);

function F = eqn(eta_span, init_cond, beta)