R/qGG.R

In LMoFit: Advanced L-Moment Fitting of Distributions

#' Quantile distribution function of the Generalized Gamma (GG) distribution
#'
#' @param u non-exceedance probability
#' @param RP Return Period "don't use in case u is used"
#' @param para parameters as c(scale, shape1, shape2)
#'
#' @return Quantile value/s using the inverse of the cumulative distribution function.
#' @author Mohanad Zaghloul [aut, cre], Simon Michael Papalexiou [aut, ths], Amin Elshorbagy [aut, ths]
#' @export
#' @importFrom stats qgamma
#'
#' @examples
#' 
#' x <- qGG(u = 0.99, para = c(10, 0.25, 0.5))
#' x <- qGG(RP = 100, para = c(10, 0.25, 0.5))
#' 
qGG <- function(u = NULL, RP = 1/(1 - u), para){
  if (is.null(u) & length(RP) >= 1) {u <- 1 - 1/RP}
  scale <- para[1]; shape_1 <- para[2]; shape_2 <- para[3]
  
  #x <- scale*qgamma(p = u, scale = scale, shape = shape_1/shape_2)^(1 / shape_2 ) #GG quantile distribution function
  x <- scale*qgamma(p = u, scale = 1, shape = shape_1/shape_2)^(1 / shape_2 ) #GG quantile distribution function
  
  return(x)
}