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

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Matthew Logan
Matthew Logan 2017-4-24
编辑： Stephen23 2017-4-24
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?

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Rik
Rik 2017-4-24
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).
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Rik
Rik 2017-4-24
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
Stephen23 2017-4-24
编辑：Stephen23 2017-4-24
@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);

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