Passing a temporary parameter from time step to time step using ode113?

Hello,

i'm using ode113 to solve some differential equations wich i stored in a function called RSDGL:

 [t,F]=ode113(@RSDGL,[t0 tf],initial_conditions);

However in the function RSDGL i am using fzero to calculate zeros of functions wich appear in the differential equations as those zeros are time dependent. The problem i have is that i want to memorize those zeros temporarly to use them as starting points when i use fzero in the next time step (when ode113 calls RSDGL again). This should increase the performance as those zeros only vary little between each timestep. So i want to programm something like:

zero_new=fzero(function,zero_old);

inside my RSDGL function. I dont really have a clue how to store and overwrite those zeros and how to pass them each time ode113 calls my function RSDGL. I hope you understand my problem, as english is not my first language.

Answer 1

[Jan]

This is a job for a persistent variable:

function dx = RSDGL(t, x)
persistent zero_old
if isempty(zero_old)
  zero_old = 1;   % Use a meaningful value!
end

zero_new = fzero(function, zero_old);
zero_old = zero_new;

...
end

It is smart to implement a method to reset this variable, e.g. for different runs of your code. Using clear RSDGL works, but I'd prefer an explicit method:

function dx = RSDGL(t, x)
persistent zero_old
if nargin == 1   % Reset!
  zero_old = [];
  return;
end

if isempty(zero_old)
  zero_old = 1;   % Use a meaningful value!
end

...

Then this resets the persistently store value:

RSDGL('reset')

This is clear and easy to understand during reading the code.

[lu_po]

Thanks a lot! This worked 100% for me. My programm is now running 50% faster wich is crucial for the calculations i want to make.