Re: [Pyarie-users] simultaneous solutions

[John P. M. <pen...@en...>]

Nicolas,

This is already functional, it's just not "hooked up" in the model you have.

I'll try to explain and you tell me if this is not what you are seeking. 
Note: I've renamed the methods, so instead of nitrogenBacteria() and 
nitrogenCycling() in testmodel.py, think of them as System() (as in the 
system of equations) and Model() (as in what might hold multiple systems).

The simple case is that at a given timestep, the SimPy iterate() (within 
the model class) function calls the step() method of the chosen 
integrator (runge-kutta, adaptive, etc). The step method requires 4 
arguments: an array of the governing system (the equations), array of 
the current state variable values, the time (SimPy.now()) and the timestep.

The step() method then calls each function in a FOR loop and calculates 
a new state variable value at t given the array of state variables that 
has been calculated at time t-1 (which was passed to it as stated 
above). Since the governing equations can be anything, and have access 
to the entire array of state variables, any equation has full access to 
the values calculated at the previous timestep. This means, using your 
example that stateC could be calculated for time t with values for 
stateA and stateB that were calculated at time t-1.

Now, as I understand it, you wish to calculate stateC at time t with 
values from stateA and stateB that were calculated at time t. This, or 
you wish to mix, using some values from the previous timestep and some 
from the current calculations. Is this correct?

I've attached a new testmodel.py file to illustrate this. It's a real 
simple concept in theory. Basically, you create a list within System 
(cur_state) which holds the values at time t. You can access these 
values from within each function as you see fit (see example state 
functions). The list is empty in Model because it is populated in System 
by copying in the initial conditions. It's important to use Python's 
copy module, so you copy the VALUES and not the REFERENCES into the 
cur_state list.

I don't know if this model will run as is, but it should be a good 
enough example to illustrate how to do what you need, assuming I 
understand correctly.

Your English is perfectly fine. I didn't realize you were a francophone 
until you said that and I looked at your email.

Cheers,
-J

Nicolas Pernetty wrote:

 > For instance, let's say we have a closed system : A and B have effects
 > on C and C has effects on B.
 > In a procedural language, I'll do the loop like this :
 >
 > 10:
 > computeA(stateA, outA)
 > computeB(stateB, outC, outB)
 > computeC(stateC, outA, outB, outC)
 > goto 10
 >
 > In reality we can have a much more complex tree, so do we have to
 > somehow design a solver which deduce the tree call from the informations
 > or do we have to input the tree as well ? I think that somehow it is
 > already solved in SimPy...

-- 
John Metta (yes, my name has indeed changed...)
http://oregonstate.edu/~penningj/
"A computer without windows is like a dog
     without bricks tied to its head."