Prime divisors of a number in ML

Question

In ML i want to get the prime divisors of a number. How can I do this, I am beginner.

Answer 1

There are several general algorithms for finding the prime divisors of an integer: see wikipedia. Trial division with a simple primality test is simplest to understand.

Find or devise an algorithm in pseudocode; only then worry about how to put it into ML.

Answer 2

Using the simple trial division, this starts with p=2 and repeatedly divides n by p, incrementing p as it goes.

open LargeInt  (* if you want to work with huge numbers like 5000000000 *)
infix 7 quot rem
val prime_factors =
  let fun trial_division p n =
    if p > n then nil else
      if n rem p = 0
        then p :: trial_division  p      (n quot p)
        else      trial_division (p + 1)  n
  in trial_division 2 end