rzigp {ZIGP}R Documentation

ZIGP random variable generator

Description

'rzigp' generates ZIGP random variables using the inversion method.

Usage

rzigp(n, mu, phi, omega)

Arguments

n length of output random vector.
mu mean
phi dispersion parameter
omega zero inflation parameter

Value

Generates a ZIGP random vector of length n.

Examples

# set seed for random variable generator
set.seed(2)
rzigp(3, 2, 1.5, 0.2)
#[1] 3 1 0

## The function is currently defined as
function(n, mu = stop("no mu arg"), phi = stop("no phi arg"), omega = stop("no omega arg"))
{
# check if parameters are valid
if(omega < 0) {return("omega has to be in [0,1]!")}
if(omega > 1) {return("omega has to be in [0,1]!")}

# inversion method
   x <- double(n)
   u <- runif(n, 0, 1)
   upper <- max(u)
   s <- double(1000)
   #P(X=0)
   p <- omega + (1-omega) * exp(-mu/phi)
   s[1] <- p
   if (upper > 0) {
      rekursive <- FALSE
      i <- 1
      while (s[i] < upper) {
        #P(X=x)
        if (rekursive==FALSE) {
          p <- (1-omega)*mu*(mu+(phi-1)*i)^(i-1)/exp(lgamma(i+1))*
               phi^(-i)*exp(-1/phi*(mu+(phi-1)*i))}
        if (p==Inf) { 
          rekursive <- TRUE 
          log.p.alt <- log( (1-omega)*mu*(mu+(phi-1)*(i-1))^(i-2)/exp(lgamma(i-1+1))*
                       phi^(-(i-1))*exp(-1/phi*(mu+(phi-1)*(i-1))))
          }
        if (rekursive==TRUE) {
          log.p <- log( (mu+(i-1)*(phi-1))/(phi*i)*
                   (1+(phi-1)/(mu+(i-1)*(phi-1)))^(i-1)*
                   exp(1/phi-1) ) + log.p.alt
          log.p.alt <- log.p
          p <- exp(log.p)
        }
        if (ceiling(i/1000)==floor(i/1000)) {
          temp <- double(1000)
          s <- c(s,temp)
        }
        s[i+1] <- s[i] + p
        i <- i+1
      }
   }
  for (j in 1:length(u)) {
     i <- 1
     while (u[j] > s[i]) {   
        i <- i+1
     }
     x[j] <- i-1
   }
   return(x)
}

[Package ZIGP version 1.1 Index]