[SciPy-User] Help!!!!!! having problems with ODEINT

[Warren Weckesser]

On Thu, Jul 21, 2011 at 6:22 PM, Laura Matrajt <matrajt at gmail.com> wrote:

> Hi all,
>  I am working with a system of 16 differential equations that simulates an
> epidemic in a city.  Because there are many cities interacting with each
> other, I need to run my ode's for a single day, stop them, modify the
> initial conditions and run them again.  Because the ode is running only for
> a day, I defined my tspan to have only two points, like this:
>
> tspan = tspan = linspace(day, day+1, 2)
>
> I wrote my equations in Python and I am using scipy.odeint to solve it.
>
> Here is my code:
> def advanceODEoneDay(self,day):
>
>         #rename variables for convenience
>         N0, N1 = self.children, self.adults
>         S0,S1,A0,A1,I0,I1,RA0,RA1,RI0,RI1 =
> self.S0,self.S1,self.A0,self.A1,self.I0,self.I1,self.RA0,self.RA1,self.RI0,self.RI1
>
>
>
>         #create a vector of times for integration:
>         tspan = linspace(day, day+1, 2)
>
>
>         #set initial conditions. To do this, I need to look in the array in
> the day
>         initCond = [S0[day,0], S0[day,1],
>                     S1[day,0], S1[day,1],
>                     A0[day,0], A0[day,1], A1[day,0], A1[day,1],
>                     I0[day,0], I0[day,1], I1[day,0], I1[day,1],
>                     RA0[day,0], RA1[day,0], RI0[day,0], RI1[day,0] ]
>
>
>         #run the ode:
>         sir_sol = odeint(sir2groups,initCond, tspan,
> args=(self.p,self.C,self.m,self.VEs,self.VEi,self.VEp,N0,N1))#,self.gamma,self.rho))
>
>
> Most of the time, it works just fine. However, there are some times where
> the following message appears:
>
> Excess work done on this call (perhaps wrong Dfun type).
> Run with full_output = 1 to get quantitative information.
>  lsoda--  at current t (=r1), mxstep (=i1) steps
>        taken on this call before reaching tout
>       In above message,  I1 =       500
>       In above message,  R1 =  0.1170754095027E+03
>
> I should mention that it is NOT only a warning. This is repeated over and
> over (thousands of times) and then it will break the rest of my code
> Ok, after searching in this mailing list, someone else posted a similar
> warning message and it was suggested to him that "
>
> *In your case, you might simply be computing for to coarse a mesh in t,** so "too much work" has to be done for each step. "
>
>
> *Is this what is happening to me? the problem is that I don't get this error every single time, so I don't even know how to run it with full_output = 1 to get the info...
>
>
> I really don't know what to do. Any help will be very very very very appreciated!
> thank you very *
> *
>
>
The function odeint has the keyword argument 'mxstep' that determines the
maximum number of internal steps allowed between requested time values.  The
default is 500.  It is not unusual to have to increase this, especially in a
case like yours where you simply want the value at the end of a long time
interval.  Try increasing it to, say, mxstep=5000.

Warren

*
> *
>
>
>
>
> --
> Laura
>
>
>
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