Matlab error while running - matlab

Question

I am trying to implement the following code in Matlab:
v0=eps;
t0=0; tf=10;
[t,v]=ode45(#const_pow, [t0 tf], v0);
And the const_pow function is:
function f=const_pow(t,v)
f=(P/v-F0-c*v^2)/m;
end
While I am running the program, the following error shows up:
Error using ==> odearguments at 103
CONST_POW returns a vector of length 0, but the length of initial
conditions vector is 1. The vector returned by CONST_POW and the
initial conditions vector must have the same number of elements.
Any suggestions on where the error might be?

Answer 1

From your comments, you're defining some variables globally. In order to access them inside const_pow, you need to tell that function to use the global variables, e.g.:
function f=const_pow(t,v)
global P F0 c m
f=(P/v-F0-c*v^2)/m;
end
Without doing this, the function will not touch the global variables (this is to stop you accidentally changing their values). For example:
function f=const_pow(t,v)
global P F0 c
m = 23.7; % this does not change the global value of m
f=(P/v-F0-c*v^2)/m;
end
An alternative to passing a number of variables without using globals is to define a structure with your required data:
data.P = % initial values
data.F0 =
data.c =
data.m =
This also gives you a little more flexibility if there is more than one set of options. Or, you could pass the variables in directly. To do this with ode45, you would need to parameterize the function using an anonymous function:
function f=const_pow(t,v,P,F0,c,m)
% and so on
end
With P,F0,c,m pre-defined:
odefun = #(t,v) const_pow(t,v,P,F0,c,m); % anonymous function
Then:
[t,v]=ode45(odefun, [t0 tf], v0);