How to bring differential equations to wanted value with DiffEqFlux?

Specific Domains Numerics
17

I have been looking at example 2 of the vignette since it covers a Lotka-Volterra system. This example has a main function that resembles the one I called growth in my second post. The problem is that the example than sets another function:

function g(du,u,p,t)
  du[1] = p[3]*u[1]
  du[2] = p[4]*u[2]
end

What is the use of this function in my case?
I just want to test over u[3] in growth. How do I write that down?
shall I do:

function g(du,u,p,t)
  du[1] = p[3]*u[1]
  du[2] = p[4]*u[2]
  du[3] = p[5]*u[3]
end

or simply

function g(du,u,p,t)
  du[3] = p[1]*u[3]
end

What about the indices?