[R-sig-dyn-mod] How to change a parameter after steady?

[Thomas Petzoldt]

On 4/27/2014 8:13 AM, Jinsong Zhao wrote:
>
> Can the following example be used here? Assumed the model reached
> steady when times >= 100. Then I hope to change r to 0.12, or K to
> 15. How to? May forcings in the previous post work? Any other
> solution?

Yes, forcings can be used if your aim is to create a "long-term" 
simulation with changing parameters. Just make r and K time dependent 
forcing functions instead of fixed parameters.

Nevertheless, I still don't understand why you want to do this. If your
aim is to start a model from an equilibrium, you may consider to
identify the equilibrium point with a solver from package rootSolve and
then to use this as the initial value of your simulation.

It is also possible to run several simulations with different parameters
independently, but that's another story.

Thomas


model <- function (time, y, parms) {
     with(as.list(c(y, parms)), {
          dN <- r * N * (1 - N / K)
          list(dN)
     })
}


## find equilibrium
library(rootSolve)
y <- c(N = 0.1)
parms <- c(r = 0.1, K = 10)
times <- c(0, Inf)
equilibrium <- steady(y, times, model, parms, method="runsteady")


## change parameters and run simulation
library(deSolve)
y <- equilibrium$y
parms <- c(r = 0.12, K = 15)
times <- seq(0, 100, 1)
out <- ode(y, times, model, parms)

plot(out)