R/output_utils.R

In readsdr: Translate Models from System Dynamics Software into 'R'

#' Estimate the net change of a stock in discrete times
#'
#' @param sim_df A data frame with the simulation output
#' @param cumulative_var A string that indicates to which variable the discrete
#'   change will be estimated
#'
#' @return A dataframe.
#' @export
#'
#' @examples
#' test_output <- data.frame(time = seq(0, 2, by = 0.25),
#'                           C    = c(0, rep(5,4), rep(20, 4)))
#' sd_net_change(test_output, "C")

sd_net_change <- function(sim_df, cumulative_var) {

  temp_df       <- sim_df[, c("time", cumulative_var)]
  temp_df       <- temp_df[temp_df$time - trunc(temp_df$time) == 0, ]
  cml_vals      <- temp_df[ , cumulative_var]
  temp_df$value <- cml_vals  - dplyr::lag(cml_vals )
  temp_df       <- dplyr::slice(temp_df, -1)
  temp_df$var   <- paste0("delta_", cumulative_var)

  temp_df[, c("time", "value", "var")]
}