| rzigp {ZIGP} | R Documentation |
'rzigp' generates ZIGP random variables using the inversion method.
rzigp(n, mu, phi, omega)
n |
length of output random vector. |
mu |
mean |
phi |
dispersion parameter |
omega |
zero inflation parameter |
Generates a ZIGP random vector of length n.
# 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)
}