# How to catch integrator problems

**URL:** <https://discourse.julialang.org/t/how-to-catch-integrator-problems/108512>\
**Category:** Modelling & Simulations\
**Tags:** question\
**Created:** [January 8, 2024, 12:33pm UTC](https://discourse.julialang.org/t/how-to-catch-integrator-problems/108512 "2024-01-08T12:33:48Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![Niall](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/niall/32/22517_2.png) [@Niall](https://discourse.julialang.org/u/Niall)\
**Post date:** [January 8, 2024, 12:33pm UTC](https://discourse.julialang.org/t/how-to-catch-integrator-problems/108512/1 "2024-01-08T12:33:48Z")

</div>

I am using a `CoupledODEs` as a test for the results of a genetic algorithm. In other words, the parameter values (here 0.5 and 1.8) are chosen at random by the GA, then tested by solving the ODEs. This means that occasionally the parameter values chosen are a bit silly and lead to explosive results. In many cases I can catch these using various tests, but this one appears to go into an infinite loop, or at least, it doesn’t get caught by the `try`-block. Is there any way of catching and handling this?

```julia
using DynamicalSystems

function demo()
	odes = CoupledODEs(
		function (du,u,p,t)
			q = sum(p[1:2].*u[1:2])

			du[1] = (q/(q+u[3]) * (1 - p[1]/(p[1]+u[3])) - 0.1) * u[1]
			du[2] = (q/(q+u[3]) * (1 - p[2]/(p[2]+u[3])) - 0.1) * u[2]
			du[3] = 1 - u[3] - q
			nothing
		end,
		[2.0,2.0,1.0],
		[0.5,1.8]
	)

	for t in 0:0.1:0.4
        while t > current_time(odes)
			try
				step!(odes)
			catch (e)
				println("Caught!")
			end
        end
	end
end

```

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**Author:** ![Datseris](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/datseris/32/13406_2.png) [@Datseris](https://discourse.julialang.org/u/Datseris)\
**Post date:** [January 8, 2024, 12:51pm UTC](https://discourse.julialang.org/t/how-to-catch-integrator-problems/108512/2 "2024-01-08T12:51:52Z")

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Recently I was in exactly the same situation as you and CHris recommended me to add a `dtmin` option into the the ODE solver keywords. Something very small compared to the timescales of the system, e.g., `1e-3` or `1e-6` the typical timescale. What could happen is that the solver makes smaller and smaller steps.

Can you try this and let me know?

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<div class="post-metadata">

**Author:** ![Datseris](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/datseris/32/13406_2.png) [@Datseris](https://discourse.julialang.org/u/Datseris)\
**Post date:** [January 8, 2024, 12:52pm UTC](https://discourse.julialang.org/t/how-to-catch-integrator-problems/108512/3 "2024-01-08T12:52:26Z")

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Notice that this `try` block doesn’t really help because the whole purpose of `step!` is to never explicitly throw an error, which is what the `try` blocks try to catch.

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<div class="post-metadata">

**Author:** ![Niall](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/niall/32/22517_2.png) [@Niall](https://discourse.julialang.org/u/Niall)\
**Post date:** [January 8, 2024, 3:30pm UTC](https://discourse.julialang.org/t/how-to-catch-integrator-problems/108512/4 "2024-01-08T15:30:23Z")

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Thanks. Can you tell me where I would insert this `dtmin` option? I’m having some difficulty finding my way around the DynamicalSystems documentation - in particular regarding the `step!` function. When I click on the link to `step!` in the tutorial documentation, it generates a 404 File Not Found error. The code in `ArbitrarySteppable` gives me no clue, and indeed, in the above code, VSE underlines my call to step! as being a possible method call error. I get the feeling I’m doing something wrong somewhere. 🤔

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<div class="post-metadata">

**Author:** ![Niall](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/niall/32/22517_2.png) [@Niall](https://discourse.julialang.org/u/Niall)\
**Post date:** [January 8, 2024, 3:52pm UTC](https://discourse.julialang.org/t/how-to-catch-integrator-problems/108512/5 "2024-01-08T15:52:19Z")

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OK, sorry - think I’ve found what you mean in trajectory.jl. I’ll try it out.

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<div class="post-metadata">

**Author:** ![Niall](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/niall/32/22517_2.png) [@Niall](https://discourse.julialang.org/u/Niall)\
**Post date:** [January 8, 2024, 5:30pm UTC](https://discourse.julialang.org/t/how-to-catch-integrator-problems/108512/6 "2024-01-08T17:30:55Z")

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Nope - I can’t work it out. I inserted the extra argument `diffeq = (dtmin = 1e-4)` after the parameters in the `CoupledODEs` constructor, but then I get the error:

```julia
MethodError: no method matching haskey(::Float64, ::Symbol)

```

generated from `continuous_time_ode.jl:16`

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<div class="post-metadata">

**Author:** ![Niall](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/niall/32/22517_2.png) [@Niall](https://discourse.julialang.org/u/Niall)\
**Post date:** [January 8, 2024, 5:45pm UTC](https://discourse.julialang.org/t/how-to-catch-integrator-problems/108512/7 "2024-01-08T17:45:59Z")

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OK, finally got the `diffeq` issue sorted - finally realised I also had to specify the solver algorithm. Inserted this argument in the `CoupledODEs()` constructor:

```julia
diffeq = (alg = Tsit5(), dtmin = 1e-6)

```

But I’m afraid it still hangs at the same crucial time. However, the messages may be of use to you, so here they are:

```julia
┌ Warning: dt(1.0e-6) <= dtmin(1.0e-6) at t=0.38987336473510126, and step error estimate = 54.93179322897898. Aborting. There is either an error in your model specification or the true solution is unstable.
└ @ SciMLBase C:\Users\hswt136nia\.julia\packages\SciMLBase\eK30d\src\integrator_interface.jl:599

```