index.r

#' Calculate the real return on an index fund over a single year.
#'
#' @param yield - percent as a decimal
#' @param expense_ratio - percent as a decimal
#' @param balance 
#'
#' @return
#' @examples
real_return <- function(yield, expense_ratio, balance) {
  # if there was no expense ratio, this would be your new balance
  return_ <- balance * (1 + yield) 
  # expenses are the difference in the return (with no expenses)
  # and a return with a yield subtracted by the expense ratio
  expense <- return_ - balance * (1 + yield - expense_ratio)
  return_ - expense
}