MATLAB: Error message “Error using EXIST, The first input to exist must be a string.”

errorexistfirst orderode45string inputsystem

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?

Best Answer

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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