# Multiple Condition for DifferentialEquations callback

**URL:** <https://discourse.julialang.org/t/multiple-condition-for-differentialequations-callback/89308>\
**Category:** General Usage\
**Tags:** question, events, differentialequation, diffeqcallbacks\
**Created:** [October 26, 2022, 4:22pm UTC](https://discourse.julialang.org/t/multiple-condition-for-differentialequations-callback/89308 "2022-10-26T16:22:44Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![pimler73](https://avatars.discourse-cdn.com/v4/letter/p/8e8cbc/32.png) [@pimler73](https://discourse.julialang.org/u/pimler73)\
**Post date:** [October 26, 2022, 4:22pm UTC](https://discourse.julialang.org/t/multiple-condition-for-differentialequations-callback/89308/1 "2022-10-26T16:22:44Z")

</div>

I am solving an IVP with the `DifferentialEquations` package and I am looking to find `n` events with a specific `condition`. The issue rises from the fact that my events require 2 conditions to be satisfied. I am using `ContinuousCallback` since I want to find the exact moment when the event occurs.

Previously, if I needed to terminate at the conditions, x=0 and \dot{x}\>0 I would use

```julia
# Stopping conditions:
condition(u, t, integrator) = u[1] # when x=0

```

```julia
function affect!(integrator)
    # if xdot > 0, terminate the integration
    integrator.u[4] > 0 ? terminate!(integrator) : nothing
end

```

However, in my new case, I want to find the first `n` events which satisfy these conditions. My current solution is to keep `condition(u,t,integrator)` the same and have `affect!(integrator)` be

```julia
function affect!(integrator)
    # extract counter from the "cache"
    event_num = integrator.p[2]
    # if xdot > 0, add a crossing, else keep the same
    integrator.u[4] > 0 ? event_num += 1 : event_num
    # if crossings > n_crossings, terminate, else nothing
    event_num == n_cross ? terminate!(integrator) : nothing
    # update the "cache" value
    integrator.p[2] = event_num
end

```

Here, as suggested in another [post](https://stackoverflow.com/a/67636791) ([also here](https://stackoverflow.com/questions/55585048/is-there-an-idiomatic-way-to-terminate-integration-after-n-callbacks-in-differen)), I use one of my user-defined variables as a “cache” which counts the amount of desired events, and then I set the `termination!` once I reach the desired amount of events.

However, when I set the options `save_everystep=false`, `save_start=false`, and `save_end=false` in `solve`, I can see that the returned states show x-axis crossings when \dot{x} is both positive AND negative. It seems to return the states when only the `condition()` is satisfied. Would there be a way to have `condition()` only consider an event when \dot{x} \> 0? I know the difficulty rises from this having to be a function which is zero at the event.

Yes, I know that there is a post-processing approach which I could use to filter out the undesired states. But I would like to keep this contained if possible.

Thank you all for your time.

---

<div class="post-metadata">

**Author:** ![pimler73](https://avatars.discourse-cdn.com/v4/letter/p/8e8cbc/32.png) [@pimler73](https://discourse.julialang.org/u/pimler73)\
**Post date:** [October 26, 2022, 5:11pm UTC](https://discourse.julialang.org/t/multiple-condition-for-differentialequations-callback/89308/2 "2022-10-26T17:11:46Z")

</div>

So as it turns out, one can differentiate between up-crossings (negative to positive) and down-crossings (positive to negative). Since I am only interested in up-crossings, I can set up the following to meet my needs:

```julia
# Stopping conditions:
condition(u, t, integrator) = u[1] # when x=0

```

```julia
function affect!(integrator)
    # extract counter from the "cache"
    event_num = integrator.p[2]
    # update crossing amount
    event_num += 1
    # if crossings = n_crossings, terminate, else nothing
    event_num == n_cross ? terminate!(integrator) : nothing
    # update the "cache" value
    integrator.p[2] = event_num
end

```

```julia
cb = ContinuousCallback(condition, affect!;
            affect_neg! = nothing)

```