# [R] missing value where TRUE/FALSE needed ERROR

Maram SAlem marammagdysalem at gmail.com
Tue Feb 9 14:01:51 CET 2016

```Hi all,

I'm trying to write a function to implement a Metropolis-within-Gibbs
algorithm for two parameters.I'm including a naive version here so as to be
able to spot the error I got. So I first generate the vectors, X and R,
that will help to start the algorithm using (for example):

n=8; m=5; p=0.1; t=0.9 ; JH=10;

R <- numeric(m)

W <- numeric(m)

V <- numeric(m)

U <- numeric(m)

X <- numeric(m)

Bay.alpha<- numeric (JH)

Bay.beta<- numeric (JH)

Bay.Surv <- numeric (JH)

hyp=c(3,15,6,22.5)

theta<-c(0.2,2)

alpha.curr<-theta

beta.curr<- theta

R<-rbinom(1, n-m, p)

for (i in 2:m-1) {

R[i]<-rbinom(1,n-m-sum(R[1:i-1]),p)

}

R[m]<-n-m-sum(R[1:m-1])

W<-runif(m, min = 0, max = 1)

for (i in 1:m){

V[i]<-W[i]^(1/(i+sum(R[(m-i+1):m])))

}

for (i in 1:m){

U[i]<- 1- prod(V[(m-i+1):m])

}

for (i in 1:m){

X[i]<- ((-1/theta)*log(1-U[i]))^(1/theta)

}

Then, I defined three functions 1- alpha.update() for updating alpha (Gibbs
step) 2- bettarg(), for the target distribution of beta 3- beta.update()
for updating beta using a Metropolis Hastings technique.

alpha.update=function(X, R, alpha.curr, beta.curr ,m, hyp)

{

o<-numeric(m)

for (i in 1:m) {

o[i]<- (1+R[i])*((X[i])^(beta.curr))

}

sh<-sum(o) + hyp + (hyp* beta.curr)

rg<-rgamma(1, shape= m+hyp+hyp , rate = sh )

return(rg)

}

bettarg<- function(X, R, alpha.curr, beta.curr ,m, hyp)

{

o<-numeric(m)

for (i in 1:m) {

o[i]<- (1+R[i])*((X[i])^( beta.curr))

}

bt<- beta.curr ^(m+hyp-1) * prod((X)^( beta.curr -1)) *
exp(-1*alpha.curr *(sum(o) +      (hyp* beta.curr)))

return(bt)

}

beta.update<- function (X, R, alpha.curr, beta.curr ,m, hyp, cand.sd)

{

beta.cand<- rnorm(1, mean = beta.curr, sd = cand.sd)

AB<- bettarg (X, R, alpha.curr, beta.curr = beta.cand ,m, hyp)

CD<- bettarg (X, R, alpha.curr, beta.curr ,m, hyp)

accept.prob <- AB/CD

if (runif(1) <= accept.prob)

beta.cand

else  beta.curr

}

Then I started a tiny chain using ten iterations but got this ERROR:

for (k in 1:JH)

{

alpha.curr<- alpha.update(X, R, alpha.curr, beta.curr ,m, hyp)

Bay.alpha[k]<-alpha.curr

beta.curr<- beta.update (X, R, alpha.curr, beta.curr ,m, hyp, cand.sd=2)

Bay.beta[k]<- beta.curr

Bay.Surv [k]<- exp (-1*Bay.alpha[k] * (t)^Bay.beta[k])

}

Error in if (runif(1) <= accept.prob) beta.cand else beta.curr :

missing value where TRUE/FALSE needed

In rgamma(1, shape = m + hyp + hyp, rate = sh) : NAs produced

I ran the code several times for only one iteration but everything was fine
with no errors or warnings, so I don't know from where does the missing
value/ NA come from?

In addition, I want to calculate the acceptance rate for the
Metropolis-Hastings step, is this possible?

Any help would be appreciated.
Thanks,

Maram Salem

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