question about R

# density function
desp <- function(x,lambda) {
  return(lambda * exp(-lambda*x)*(x>=0))
}
# cum. probability function
pesp <- function(q,lambda) {
  return((1-exp(-lambda*q)))
}
# quantile function
qesp <- function(p,lambda) {
  return(-lambda^{-1}*log(1-p))
}
# random sample
resp <- function(n,lambda) {
  return(qesp(runif(n),lambda))
}
resp(100,lambda=2) # generates 100 samples from the exponential
distribution with par 2

Hi,
At each iteration in my program,I need to generate ?tree vectors,X1,X2,X3,
from exponential distribution with parameters a1,a2,a3. Can you help me
please how can I do it such that it take a little time?

thank you
khazaei

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Hi,

Hi,
At each iteration in my program,I need to generate ?tree vectors,X1,X2,X3,
from exponential distribution with parameters a1,a2,a3. Can you help me
please how can I do it such that it take a little time?

So, at each iteration you're sampling from three different exponential
distributions, each with rate parameters a1, a2, a3 respectively?

Say you have 100 iterations, you can create vectors of length 100
having samples from each distro like so:

R> dist1 <- rexp(100, rate=a1)
R> dist2 <- rexp(100, rate=a2)
R> dist3 <- rexp(100, rate=a3)

... and use them as you would expect where you need them.

-steve

Steve Lianoglou
Graduate Student: Computational Systems Biology
 | Memorial Sloan-Kettering Cancer Center
 | Weill Medical College of Cornell University
Contact Info: http://cbio.mskcc.org/~lianos/contact