Updating the parameter of ODE45

2 vues (au cours des 30 derniers jours)
alpedhuez
alpedhuez le 26 Mai 2020
Commenté : Star Strider le 26 Mai 2020
I have an ODE system such as
b=1;
f = @(t,x) -(b/N)*x(1)*x(2);
ODE45
I want to write a loop that changes the value of b. But it does not seem to me that just changing the value of b will not update the equation f thus the solution.
b=2;
f = @(t,x) -(b/N)*x(1)*x(2)
ODE45
What should be done here?
  2 commentaires
alpedhuez
alpedhuez le 26 Mai 2020
Maybe Pass Extra Parameters to ODE Function at https://www.mathworks.com/help/matlab/ref/ode45.html#bu3uhuk?
Star Strider
Star Strider le 26 Mai 2020
That is the best solution. Include both constants in the argument list:
f = @(t,x,b,N) -(b/N)*x(1)*x(2);
then proceed as in the documentation example.

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Star Strider
Star Strider le 26 Mai 2020
Actually, ‘f’ needs to be a (2x1) matrix, since there are two values of ‘x’, and they both need to be defined, so something like this will work:
f = @(t,x,b,N) [x(1); -(b/N)*x(1)*x(2)];
tspan = [0 5];
ic = rand(2,1);
b = rand;
N = 42;
[t,x] = ode45(@(t,x)f(t,x,b,N), tspan, ic);
I am not certain what you are doing, so this may need to be changed. However to integrate two dependent variables, the ODE function has to have that structure, even if you only use the ‘x(:,2)’ result. I would need to know the original differential equation to know if this is correct for it.
  4 commentaires
alpedhuez
alpedhuez le 26 Mai 2020
Modifié(e) : alpedhuez le 26 Mai 2020
This seems to work:
f = @(t,x,b,d,r,N) [-(b/N)*x(1)*x(2);(b/N)*x(1)*x(2)-(r+d)*x(2);r*x(2);d*x(2)]
Star Strider
Star Strider le 26 Mai 2020
That definitely makes sense!
I am still not certain what you are modeling, however that is definitely the correct way to write the ODE function.

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