R/pochhammer.R

In thiyangt/fformpp: Feature-based FORecast Model Performance Prediction

##' Pochhammer symbol
##'
##' The Pochhammer symbol is evaluated via the expression (a)n  =
##' gamma(a+n)/gamma(a) for positive integer n.
##' @param a "vector".
##' @param n "positive integer vector".
##' @param log "logical"
##'
##'        If TRUE, return the logarithm of Pochhammer symbol. Else return the
##' usual one.
##'
##' @return "matrix"
##'
##'        The (i, j)th element of the matrix indicates the Pochhammer symbol
##' for (a[i])n[j].
##'
##' @references NA
##' @author Feng Li, Department of Statistics, Stockholm University, Sweden.
##' @note Created: Tue Jan 15 11:20:51 CET 2013;
##'       Current: Tue Jan 15 11:21:00 CET 2013.
##' @export
pochhammer <- function(a, n, log)
  {
    a.len <- length(a)
    n.len <- length(n)

    a.mat <- matrix(a, a.len, n.len)
    n.mat <- matrix(n, a.len, n.len, byrow = TRUE)
    out.log <- lgamma(a.mat+n.mat) - lgamma(a.mat)

    if(log)
      {
        out <- out.log
      }
    else
      {
        out <- exp(out.log)
      }

    return(out)
  }