# Looking for findfirst or similar for sol objects of DifferentialEquations
**URL:** https://discourse.julialang.org/t/looking-for-findfirst-or-similar-for-sol-objects-of-differentialequations/37134
**Category:** Modelling & Simulations
**Created:** [April 7, 2020, 12:08am UTC](https://discourse.julialang.org/t/looking-for-findfirst-or-similar-for-sol-objects-of-differentialequations/37134 "2020-04-07T00:08:18Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@klaff](https://discourse.julialang.org/u/klaff)
#### Post date: [April 7, 2020, 12:08am UTC](https://discourse.julialang.org/t/looking-for-findfirst-or-similar-for-sol-objects-of-differentialequations/37134/1 "2020-04-07T00:08:18Z")
Hello!
I’m doing some control systems work and wanting to find things like risetime of responses, so I want to find where a simulation variable crosses some value. `findfirst` works but it searches among the simulation points rather than finding where the interpolation crosses the desired point, which if things are smooth might be some distance from the point I would like to find.
I suspect someone has crossed this bridge already, but I’m struggling to find it.
I could do it using callbacks, but that seems a bit overkill for what feels more natural to be done with analysis after the simulation has been run.
Thanks for any pointers.
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### Author:  [@dpsanders](https://discourse.julialang.org/u/dpsanders)
#### Post date: [April 7, 2020, 12:56am UTC](https://discourse.julialang.org/t/looking-for-findfirst-or-similar-for-sol-objects-of-differentialequations/37134/2 "2020-04-07T00:56:59Z")
This is a root-finding problem – try Roots.jl maybe? (Untested)
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### Author:  [@dpsanders](https://discourse.julialang.org/u/dpsanders)
#### Post date: [April 7, 2020, 12:57am UTC](https://discourse.julialang.org/t/looking-for-findfirst-or-similar-for-sol-objects-of-differentialequations/37134/3 "2020-04-07T00:57:24Z")
Note that you can call the DiffEq solution object as if it were a function.
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### Author:  [@klaff](https://discourse.julialang.org/u/klaff)
#### Post date: [April 7, 2020, 1:49am UTC](https://discourse.julialang.org/t/looking-for-findfirst-or-similar-for-sol-objects-of-differentialequations/37134/4 "2020-04-07T01:49:39Z")
That would certainly work. I was just trying to find out if someone had done the work before me.
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### Author:  [@dpsanders](https://discourse.julialang.org/u/dpsanders)
#### Post date: [April 7, 2020, 2:02am UTC](https://discourse.julialang.org/t/looking-for-findfirst-or-similar-for-sol-objects-of-differentialequations/37134/5 "2020-04-07T02:02:15Z")
There’s no work to do, you just call the relevant function from Roots.jl (I believe).
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### Author:  [@dpsanders](https://discourse.julialang.org/u/dpsanders)
#### Post date: [April 7, 2020, 2:10am UTC](https://discourse.julialang.org/t/looking-for-findfirst-or-similar-for-sol-objects-of-differentialequations/37134/6 "2020-04-07T02:10:55Z")
@klaff Here’s an example using the first code from the DiffEq docs:
```julia
using DifferentialEquations
f(u, p, t) = 1.01 * u
u0 = 1 / 2
tspan = (0.0, 1.0)
prob = ODEProblem(f, u0, tspan)
sol = solve(prob, Tsit5(), reltol=1e-8, abstol=1e-8)
julia> using Roots
julia> initial = 3.0;
julia> desired = 5.0;
julia> root = fzero(x -> sol(x) - desired, initial)
2.2906833822317667
julia> sol(root)
5.000000014083912
```
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### Author:  [@klaff](https://discourse.julialang.org/u/klaff)
#### Post date: [April 7, 2020, 6:21pm UTC](https://discourse.julialang.org/t/looking-for-findfirst-or-similar-for-sol-objects-of-differentialequations/37134/7 "2020-04-07T18:21:52Z")
Thanks, this works nicely.
```julia
"find 10-90 risetime of sol in interval ti to tf, 100% step from yi to yf; n(th) state var"
function findTr(sol, ti, tf, yi, yf, n=1)
v10,v90 = [0.1,0.9].*(yf-yi).+yi
t10 = fzero(t -> sol(t)[n] - v10, [ti,tf])
t90 = fzero(t -> sol(t)[n] - v90, [ti,tf])
return t90-t10
end
```