# Changing the righthand side in a callback

Hi everyone,
I am wondering how to correctly change the right hand side inside a DifferentialEquations callback.
If I don’t reinit the system runs through but gives wrong results (compared to just running new differential equations). If I reinit the integrator I get an infinite loop.
Any pointers?

``````mutable struct ode_switch
original_ode
fault_ode
fault::Bool
end

function (pgs::ode_switch)(u, t, integrator)
println("time \$t") # To make sure the call back is really called
if ! pgs.fault
integrator.f = rhs(pgs.fault_ode)
integrator.u = find_valid_ic(integrator.f, integrator.u) # This takes care of the DAE/Massmatrix aspect
# reinit!(integrator, t0=t) # If I use reinit, I get an infinite loop with the callback being called over and over.
# step!(integrator) # I tried to do a step in order for that not to happen but it didn't help
pgs.fault = true
else
integrator.f = rhs(pgs.original_ode)
integrator.u = find_valid_ic(integrator.f, integrator.u) # This takes care of the DAE/Massmatrix aspect
# reinit!(integrator, t0=t) # If I use reinit, I get an infinite loop with the callback being called over and over.
# step!(integrator) # I tried to do a step in order for that not to happen but it didn't help
pgs.fault = false
end
end

timespan = (0., 10.)
faulttime = (1., 2.)
pgs = ode_switch(func1,func2)
cb = FunctionCallingCallback(pgs; funcat=faultime, func_start = false)
prob = ODEProblem(func1, x0, timespan, callback=cb)
solve(prob, Rodas4(autodiff=false))

``````

I would just use a `DiscerteCallback` to change a parameter that is the boolean in `p`, and then check for that in `f`. By making it integrator-local it won’t have re-use issues.

Thanks for the quick reply, I saw that as a design in the other answers. In this case it’s a bit of an awkward fit because I typically will get an ODEFunction and a fault function that changes the ODEFunction from the user. So it’s a bit of effort to build a wrapper that switches between them, thus I thought I’d try it this way.

For example, the different functions (func1, func2) could have different mass matrices, that could not be handled by a change of parameters, correct?

That probably could be, since it can be a DiffEqOperator with an update_func, but that is not well documented…

In that case I think we will stick to calling solve several times with initial conditions taken from the last simulation.

Is there a documented way to combine several solutions into one? (or the next solve on top of the existing solution object)?

There’s no documented way to do that. Instead of doing that, why not use the integrator interface?