CODEWITHSUNDEEP
matlab
bzm3r
bzm3r

Reputation: 4596

How do I pass ode45 an odefun that takes two arguments?

I have a use case as follows:

Inside F.m I have a function F that takes as its argument a 2 x 1 matrix x. F needs to matrix multiply the matrix kmat by x. kmat is a variable that is generated by a script.

So, what I did was set kmat to be global in the script:

global kmat;
kmat = rand(2);

In F.m:

function result = F(x)
    global kmat;
    result = kmat*x;
end

Then finally, in the script I have (x_0 has already been defined as an appropriate 2 x 1 matrix, and tstart and tend are positive integers):

xs = ode45(F, [tstart, tend], x_0);

However, this is causing the error:

Error using F (line 3)
Not enough input arguments.

Error in script (line 12)
xs = ode45(F, [tstart, tend], x_0);

What is going on here, and what can I do to fix it? Alternatively, what is the right way to pass kmat to F?

Upvotes: 0

Views: 1301

Answers (2)

David
David

Reputation: 8459

Firstly, the proper way to handle kmat is to make it an input argument to F.m

function result = F(x,kmat)
    result = kmat*x;
end

Secondly, the input function to ode45 must be a function with inputs t and x (possibly vectors, t is the dependent variable and x is the dependent). Since your F function doesn't have t as an input argument, and you have an extra parameter kmat, you have to make a small anonymous function when you call ode45

ode45(@(t,x) F(x,kmat),[tstart tend],x_0)

If your derivative function was function result=derivative(t,x), then you simply do ode45(@derivative,[tstart tend],x_0) as Erik said.

Upvotes: 1

Erik
Erik

Reputation: 4563

I believe F in ode45(F,...) should be a function handle, i.e. @F. Also, you can have a look at this page of the MATLAB documentation for different methods to pass extra parameters to functions.

Upvotes: 0

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