How to unpack a vector to a struct / local variables

Is there a better way to write a function like this (which is called from an integrator so needs to have dx and x being Vector{Float64}'s and unpack them without excessive copying and mostly without all the magic numerical indexes that need to be maintained?

function eom!(dx,x,p,t,stage,scaling)
  r  = x[1:3]
  v  = x[4:6]
  pv = x[7:9]
  pr = x[10:12]
  m  = x[13]

  rm = norm(r)
  r3 = rm^3
  r5 = rm^5

  u  = normalize(pv)
  At = stage.thrust / ( m * scaling.g_bar )

  dx[1:3]   = v
  dx[4:6]   = - r/r3 + At * u
  dx[7:9]   = - pr
  dx[10:12] = pv / r3 - 3 / r5 * r' * pv * r
  dx[13]    = - stage.thrust / stage.c

  dx
end

Clarification: dx and x needs to be Vector{Float64} or AbstractVector{Float64}? In second case there is a lot of thing that you can do. If it is Vector then it seems that the only thing you can do, is to reduce allocations with the help of @..

Welcome to the forum! I think this is pretty much the prototypical use-case for: GitHub - jonniedie/ComponentArrays.jl: Arrays with arbitrarily nested named components.

2 Likes

Ultimately this is being called by DifferentialEquations.jl which will be wrapped by MINPACK.jl which uses Vector{Float64} but I don’t mind answering the question about AbstractVector{Float64} (and that might drive me to use something other than MINPACK.jl?)

I think, that marius311 already gave an answer for AbstractVector. Since DifferentialEquations.jl support abstract types, I think it makes sense to raise an issue in MINPACK.jl, to make type restrictions less tight (or you can fork this package and do it yourself).

Issue submitted to MINPACK,jl