MATLAB: Extract values from ODE45

ode45outputfcn

function [] = Example2()
clc 
clear 
close all
y0 = [100.; 0; 0]; % Initial values for the dependent variables 
tspan= linspace(0,20,50);
options = optimset('display','off');
[t,x]= ode45(@ODEfun,tspan,y0,options);
plot(t,x)
legend('Fa','Fb','Fc')
end 
function dYdv = ODEfun(v,s); 
Fa = s(1); 
Fb = s(2); 
Fc = s(3); 
Kc = 0.01; 
Ft = Fa + Fb + Fc; 
Co = 1; 
K = 10; 
Kb = 40; 
ra = 0 - (K * Co / Ft * (Fa - (Co ^ 2 * Fb * Fc ^ 2 / (Kc * Ft ^ 2)))); 
Ka = 1; 
Ra = Ka * Co * Fa / Ft;
Rb = Kb * Co * Fb / Ft;
dFadv = ra - Ra; 
dFbdv = 0 - ra - Rb; 
dFcdv = -2 * ra; 
dYdv = [dFadv; dFbdv; dFcdv];
end
How Do I extract the values of Ra and Rb and plot them ?

Best Answer

‘How Do I extract the values of Ra and Rb and plot them ?’
You don’t ‘extract’ them. Just recalculate them from the integrated values.
Make a few changes to your ‘Example2’ function: 
function [] = Example2()
% clc 
% clear 
% close all
y0 = [100.; 0; 0]; % Initial values for the dependent variables 
tspan= linspace(0,20,50);
options = optimset('display','off');
[t,x]= ode45(@ODEfun,tspan,y0,options);
figure
plot(t,x)
legend('Fa','Fb','Fc')
Fa = x(:,1); 
Fb = x(:,2); 
Fc = x(:,3);
Kc = 0.01; 
Ft = Fa + Fb + Fc; 
Co = 1; 
K = 10; 
Kb = 40; 
ra = 0 - (K .* Co / Ft .* (Fa - (Co .^ 2 * Fb .* Fc .^ 2 ./ (Kc * Ft .^ 2)))); 
Ka = 1; 
Ra = Ka * Co * Fa / Ft;
Rb = Kb * Co * Fb / Ft;
figure
plot(t, [Ra Rb])
legend('R_a', 'R_b')
end
This will produce the plot you want. You could plot them in the same figure as the first plot (using the hold function), however you will likely not be able to see them well. I would use separate plots.
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