R/PMT.R

In optiRum: Financial Functions & More

#' Calculates the repayment for a loan
#'
#' Based on period interest rate, number of periods, and loan amount, this function calculates 
#' the repayment of the loan such that it would be paid off fully at the end of the loan.
#' This function is designed to be equivalent to the Excel function PMT. 
#' It calculates based on a fixed interest rate, FV=0, and charging is 
#' at the end of the period. Response is rounded to 2dp
#'
#' @param rate The nominal interest rate per period (should be positive)
#' @param nper Number of periods
#' @param pv Present value i.e. loan advance (should be positive)
#' @return pmt Instalment per period (should be negative)
#'
#' @keywords financial pv pmt
#' @seealso \code{\link{PV}}  \code{\link{RATE}} 
#' @family finance
#' @export
#' 
#' @examples
#' PMT(0.1,12,3000) # =-440.29 taken from excel
#' 
#' df<-data.frame(rate=c(.1,.2),nper=c(12,24),pv=c(3000,1000))
#' PMT(df$rate,df$nper,df$pv) # =-440.29,-202.55 taken from excel

PMT <- function(rate, nper, pv) {
    stopifnot(rate > 0, rate < 1, nper >= 1, pv > 0)
    return(round(-pv * rate/(1 - 1/(1 + rate)^nper), 2))
}