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.
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
