R/pmt.R

In eaoestergaard/UNPIE: UNPIE Understanding Pensions In Europe

#' Returns the payment (eg. on loan) based on constant payments and a constant interest rate
#'
#' @param rate The interest rate per period. Default is zero. Must be entered as decimal or ts
#' @param inflation The inflation rate per period. Default is zero. Must be entered as decimal or ts
#' @param nper The total number of payment periods. Default is one period. If rate and inflation are entered as ts nper is ignored.
#' @param fv The future value of single payment (spending) made in the future. Default is assumed to be zero. Must be entered as a negative number
#' @seealso \code{\link{pv}}
#' @export
#' @examples
#' pmt(rate=0.05,nper=35,fv=-1000)
#' pmt(rate=0.01,inflation=0,nper=10,fv=-1000)

pmt <- function(rate=0.04,inflation=.02, nper=35,fv=-1000){
  if(!(is.ts(inflation) || is.scalar(inflation))) return(stop("inflation must either be of type scalar or ts.",call. = FALSE))
  if(!(is.ts(rate) || is.scalar(rate))) return(stop("rate must either be of type scalar or ts",call. = FALSE))
  if(!is.scalar(nper)|| nper<1 ) return(stop("nper must be of type integer larger than zero",call. = FALSE))
  if(!is.scalar(fv)) return(stop("fv must be of type scalar",call. = FALSE))

  #Find start, end and frequency
  if(is.ts(rate)){
    start = start(rate)
    end = end(rate)
    frequency = frequency(rate)
  }else if(is.ts(inflation)) {
    start = start(inflation)
    end = end(inflation)
    frequency = frequency(inflation)
  }else{
    start = c(1,1)
    end = c(nper,1)
    frequency = 1
  }

  if(is.scalar(rate)){
    rate = ts(rep(rate,nper), frequency = 1, start = start, end = end)
  }
  if(is.scalar(inflation)){
    inflation = ts(rep(inflation,nper), frequency = 1, start = start, end = end)
  }

  realRate = rate.real(rate,inflation)
  pmt  = fv * realRate/(1-(1+realRate)^(-nper))
  return(pmt)
}