# Ring Probabilities in F#

A few months back I took a look at Elixir. More recently I’ve been exploring F# and I’m very pleased with the experience so far. Here is the ring probabilities algorithm implemented using F#. It’s unlikely that I will ever use Elixir again because having a powerful static type system provided by F# at my disposal is just too good.

let rec calcStateProbs (prob: float, i: int, currProbs: float [], newProbs: float []) = if i < 0 then newProbs else let maxIndex = currProbs.Length-1 // Match prev, next probs based on the fact that this is a // ring structure. let (prevProb, nextProb) = match i with | i when i = maxIndex -> (currProbs.[i-1], currProbs.[0]) | 0 -> (currProbs.[maxIndex], currProbs.[i+1]) | _ -> (currProbs.[i-1], currProbs.[i+1]) let newProb = prob * prevProb + (1.0 - prob) * nextProb Array.set newProbs i newProb calcStateProbs(prob, i-1, currProbs, newProbs) let calcRingProbs parsedArgs = // Probs at S = 0. // Make certain that we are positioned at only start location. // e.g. P(Start Node) = 1 let startProbs = Array.concat [ [| 1.0 |] ; [| for _ in 1 .. parsedArgs.nodes - 1 -> 0.0 |] ] let endProbs = List.fold (fun probs _ -> calcStateProbs(parsedArgs.probability, probs.Length-1, probs, Array.create probs.Length 0.0)) startProbs [1..parsedArgs.states] endProbs

Here’s the code.

No promises this time but I may follow this sequential version up with a parallelized version.