expected left pointing arrow < - or twiddles ~ error

So this seemingly helpful OpenBUGS error was throwing me off. Code:


n <- 100
p <- 5
x <- matrix(rnorm(n*p),n,p)
b <- rnorm(p,0,.1)
rho <- exp(x%*%b)
N <- rpois(n,rho)
y <- rbinom(n,N,.5)

library(R2OpenBUGS)

model <- function() {
  for (i in 1:n) {
    y[i] ~ dbin(0.5,N[i])
    N[i] ~ dpois(rho[i])
    rho[i] <- exp(inprod(x[i,],b))
    #rho[i] <- exp(x[i,1]*b[1]+x[i,2]*b[2]+x[i,3]*b[3]+x[i,4]*b[4]+x[i,5]*b[5])
  }
  for (i in 1:p) {
    b[i] ~ dnorm(0,1)
  }
}

fn <- file.path(tempdir(), "model.txt")
write.model(model,fn)

data <- list(n=n,p=p,y=y,x=x)
inits <- list(list(b=rep(0,p),N=y))
parameters <- c("b","N")

sim <- bugs(data,inits,parameters,fn,n.chains=1,n.burnin=1e3,n.thin=1,n.iter=1e4)

caused this error:


OpenBUGS version 3.2.1 rev 781
type 'modelQuit()' to quit
OpenBUGS> expected left pointing arrow <  - or twiddles ~ error pos 197 (error on line 10)
OpenBUGS> model must be checked before data is loaded
OpenBUGS> model must be checked before compiling
OpenBUGS> model must have been compiled but not updated to be able to change RN generator
OpenBUGS> model must be compiled before initial values loaded
OpenBUGS> model must be compiled before generating initial values
OpenBUGS> model must be initialized before updating
OpenBUGS> model must be initialized before monitors used
OpenBUGS> model must be initialized before monitors used
OpenBUGS> model must be initialized before monitors used
OpenBUGS> model must be initialized before monitors used
OpenBUGS> model must be initialized before monitors used
OpenBUGS> model must be initialized before monitors used
OpenBUGS> model must be initialized before DIC can be monitored
OpenBUGS> model must be initialized before updating
OpenBUGS> model must be initialized before monitors used
OpenBUGS> model must be initialized before monitors used
OpenBUGS> DIC monitor not set
OpenBUGS>

which was caused by


rho[i] <- exp(inprod(x[i,],b))

and can be remedied by either


rho[i] <- exp(inprod(x[i,],b[]))


rho[i] <- exp(x[i,1]*b[1]+x[i,2]*b[2]+x[i,3]*b[3]+x[i,4]*b[4]+x[i,5]*b[5])

the former is what I was going for.

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Published

07 October 2011

expected left pointing arrow < - or twiddles ~ error

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