[R] DeSolve Package and Moving Average

[Werning, Jan-Philipp]

Dear all,


I am using the DeSolve Package to simulate a system dynamics model. At the problematic point in the model, I basically want to decide how many products shall be produced to be sold. In order to determine the amount a basic forecasting model of using the average of the last 12 time periods shall be used. My code looks like the following.

“ […]

# Time units in month
START<-0; FINISH<-120; STEP<-1

# Set seed for reproducability

 set.seed(123)

# Create time vector
simtime  <- seq(START, FINISH, by=STEP)

# Create a stock vector with initial values
stocks   <- c([…])

# Create an aux vector for the fixed aux values
auxs    <- c([…])


model <- function(time, stocks, auxs){
  with(as.list(c(stocks, auxs)),{

[… “lots of aux, flow, and stock functions” … ]


aMovingAverage  <-  ifelse(exists("ResultsSimulation")=="FALSE",10000,movavg(ResultsSimulation$TotalSales, 12, type = "s”))


return (list(c([…]))

  })
}

# Call Solver, and store results in a data frame
ResultsSimulation <-  data.frame(ode(y=stocks, times=simtime, func = model,
                      parms=auxs, method="euler"))

[…]”

My problem is, that the moving average (function: movavg) is only computed once and the same value is used in every timestep of the model. I.e. When running the model for the first time, 10000 is used, running it for the next time the total sales value of the first timestep is used. Since only one timestep exists, this is logical. Yet  I would expect the movavg function to produce a new value in each of the 120 timesteps, as it is the case with all other flow, stock and aux calculations as well.

It would be great if you could help me with fixing this problem.


Many thanks in advance!

Yours,

Jan





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