9.7: What Could Go Wrong?

Don’t forget the @ on the function handle. If you leave it out, like:

[T, Y] = ode45(rate_func, [0, 365], 1000)

MATLAB treats the first argument as a function call and calls rate_func without providing arguments. Then you get an error message:

Not enough input arguments.

Error in rats>rate_func (line 18)
    res = a * y * (1 - cos(omega * t));

Error in rats (line 6)
    [T, Y] = ode45(rate_func, [0, 365], 1000);

Also, the rate function you write has to take two input variables, t and y, in that order, and return one output variable, res.

If you’re working with a rate function like:

\[\frac{dy}{dt}(t) = a y(t)\notag\]

you might be tempted to write this:

function res = rate_func(y)        % WRONG
    a = 0.002;
    res = a * y;
end

But that would be wrong. So very wrong. Why? Because when ode45 calls rate_func, it provides two arguments. If you only take one input variable, you’ll get an error. So you have to write a function that takes t as an input variable, even if you don’t use it:

function res = rate_func(t, y)     % RIGHT
    a = 0.002;
    res = a * y;
end

Another common error is to write a function that doesn’t make an assignment to the output variable. If you write something like:

function res = rate_func(t, y)
    a = 0.002;
    omega = 2*pi / 365;
    r = a * y * (1 - cos(omega * t));    % WRONG
end

and then call it from ode45, you get

Output argument "res" (and maybe others) not assigned during call
to "rate_func".

I hope these warnings save you some time debugging.