Replicating R's prod() function in WinBUGS

Question

Using WinBUGS, how can I calculate the product of all values in a single vector?

I have tried using a for loop over the same vector.

For example:

In R, if A <- [1,2,3,4], prod(A) = 24.

However,

in BUGS, if a <- 2 , and for (i in 1:n){ a <- a * A[i] }, this loop cannot work because 'a' is defined twice.

Answer 1

Hi and welcome to the site!

Remember that BUGS is a declarative syntax and not a programming language, so you cannot over-write variable values as you expect to be able to in a language such as R. So you need to create some intermediate nodes to do what you calculate.

If you have the following data:

A <- [1,2,3,4]
nA <- 4

Then you can include in your model:

sumlogA[1] <- 0
for(i in 1:nA){
    sumlogA[i+1] <- sumlogA[i] + log(A[i])
}
prodA <- exp(sumlogA[nA+1])

Notice that I am working on the log scale and then take the exponent of the sum - this is mathematically equivalent to the product but is a much more computationally stable calculation.

Hope that helps,

Matt