# Nicer parameter indexing

**URL:** <https://discourse.julialang.org/t/nicer-parameter-indexing/50704>\
**Category:** Modelling & Simulations\
**Tags:** question, modelingtoolkit\
**Created:** [November 24, 2020, 5:59pm UTC](https://discourse.julialang.org/t/nicer-parameter-indexing/50704 "2020-11-24T17:59:00Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![ohmsweetohm1](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ohmsweetohm1/32/49126_2.png) [@ohmsweetohm1](https://discourse.julialang.org/u/ohmsweetohm1)\
**Post date:** [November 24, 2020, 5:59pm UTC](https://discourse.julialang.org/t/nicer-parameter-indexing/50704/1 "2020-11-24T17:59:00Z")

</div>

Consider the following example:

```julia
import Plots
using ModelingToolkit, DifferentialEquations

@parameters t σ ρ β
@variables x(t) y(t) z(t)
@derivatives D'~t

eqs = [D(x) ~ σ * (y - x),
       D(y) ~ x * (ρ - z) - y,
       D(z) ~ x * y - β * z]

lorenz1 = ODESystem(eqs, name=:lorenz1)
lorenz2 = ODESystem(eqs, name=:lorenz2)

@variables a
@parameters γ
connections = [0 ~ lorenz1.x + lorenz2.y + a * γ]
connected = ODESystem(connections, t, [a], [γ], systems=[lorenz1,lorenz2])

u0 = [lorenz1.x => 1.0,
      lorenz1.y => 0.0,
      lorenz1.z => 0.0,
      lorenz2.x => 0.0,
      lorenz2.y => 1.0,
      lorenz2.z => 0.0,
      a => 2.0]

```

The parameters are than specified as a vector of pairs, corresponding to the parameters of the `ODESystem`.

```julia
p = [lorenz1.σ => 10.0,
      lorenz1.ρ => 28.0,
      lorenz1.β => 8 / 3,
      lorenz2.σ => 10.0,
      lorenz2.ρ => 28.0,
      lorenz2.β => 8 / 3,
      γ => 2.0]

tspan = (0.0, 100.0)
prob = ODEProblem(connected, u0, tspan, p)
sol = solve(prob, Rodas5())

```

It feels a bit inconsistent to speficy `p` as a vector of pairs but inside the integrator it is just a vector.  
In a callback one would have to use e.g. `int.p[1]` which in this case corresponds to gamma.

```julia
julia> prob.p
7-element Array{Float64,1}:
  2.0
 10.0
 28.0
  2.6666666666666665
 10.0
 28.0
  2.6666666666666665

```

It seems to be constructed like this:

```julia
[
    connected.ps...,
    connected.systems[1].ps...,
    connected.systems[2].ps...,
]

```

Is there an easier way of referencing the parameters e.g. for usage in a callback function?

---

<div class="post-metadata">

**Author:** ![ChrisRackauckas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chrisrackauckas/32/77_2.png) [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)\
**Post date:** [November 24, 2020, 9:01pm UTC](https://discourse.julialang.org/t/nicer-parameter-indexing/50704/2 "2020-11-24T21:01:56Z")

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> [@ohmsweetohm1](#):
>
> Is there an easier way of referencing the parameters e.g. for usage in a callback function?

We could use ComponentArrays.jl to make this better, and that might be what we do in the future.

---

<div class="post-metadata">

**Author:** ![airpmb](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/airpmb/32/7826_2.png) [@airpmb](https://discourse.julialang.org/u/airpmb)\
**Post date:** [March 12, 2021, 5:16pm UTC](https://discourse.julialang.org/t/nicer-parameter-indexing/50704/3 "2021-03-12T17:16:08Z")

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Has this been implemented yet (if so, example?) or if not is there an issue that I could subscribe to? A search turned up [https://github.com/SciML/ModelingToolkit.jl/pull/702](https://github.com/SciML/ModelingToolkit.jl/pull/702) which I think mentions this in passing but I can’t tell if it’s any more than that.

---

<div class="post-metadata">

**Author:** ![ChrisRackauckas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chrisrackauckas/32/77_2.png) [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)\
**Post date:** [March 12, 2021, 5:21pm UTC](https://discourse.julialang.org/t/nicer-parameter-indexing/50704/4 "2021-03-12T17:21:29Z")

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You can do it like:

[https://github.com/SciML/ModelingToolkit.jl/issues/318#issuecomment-796632465](https://github.com/SciML/ModelingToolkit.jl/issues/318#issuecomment-796632465)

and that should get automated hopefully soon.