Error message "Error using EXIST, The first input to exist must be a string."

system of 1st order ODE for a series/parallel reaction system, A>D>U & A>U

function [xdot] = rates(t,x);
rate_AD = 0.06*x(1); rate_AU = (0.12*x(1)^0.5)/(1 + 0.2*x(3)); rate_DU = (0.25*x(2))/(1 + 1.5*x(2)*x(3)); 
xdot(1,:) = -(rate_AD + rate_AU);
xdot(2,:) = rate_AD - rate_DU ;
xdot(3,:) = - xdot(1) - xdot(2) ; 
end.
i try to evauluate using ode45:

x0 = [2/300;0;0]; % Initial concentrations of components [mol/m^3]
tspan = [0 100] ; % Time domain [s]
[t,x] = ode45(rates,tspan,x0);

what am i missing?

Answer 1

[Rik]

The first input to ode45 must be a function handle. That means that you should have entered this:

[t,x] = ode45(@rates,tspan,x0);

When in doubt, RTFM (or in this case the documentation).

[Matthew Logan]

thanks for the quick reply! I managed to work out that it also works in quotation marks 'rate' :)

[Rik]

Good that you figured it out. Reading documentation of functions you are using often helps a lot when you get errors.

PS If you found my answer useful, please mark it as accepted answer. It will give us both reputation points and will make it easier for other people with the same question to find an answer.

[Stephen23]

@Matthew Logan: using a function handle is the recommended (and better) way to specify the input, exactly as Rik Wisselink showed:

[t,x] = ode45(@rates,tspan,x0);