In [9]:

    

let compositesInRange fromNum toNum =
    [|2 .. toNum/fromNum|]
    |> Array.map (fun x -> fromNum * x)

let primesTo n =
    // start off assuming every number from 0 .. n is prime
    let isPrime = Array.init (1+n) (fun _ -> true)
    
    // simple sieve of eratosthenes - update "primeness" of
    // each number in the isPrime array
    for x in 2 .. int(sqrt(double(n))) do 
        if isPrime.[x] then             
            compositesInRange x n
            |> Array.map (fun i -> isPrime.[i] <- false)
            |> ignore
                
    // now return numbers which are "true" in isPrime array
    [|2..n|]
    |> Array.filter (fun x -> isPrime.[x])
    
// pick a big number, hopefully we have enough primes!!
let primes = primesTo 1000000   

primes.[10000]


    

    
Out[9]:





104743

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