Problem 1: Arrays and Random

For random_target.

Arrays vs. lists

An array is a fixed-size, indexable block of values, written with [| ... |]. Unlike a list, reading element i is instant no matter how big the array is -- there's no walking down a chain of cells to get there.

let a = [| "red"; "green"; "blue" |]
let _ = a.(0)   (* = "red" *)
let _ = Array.length a   (* = 3 *)

word_list is an array (not a list) for exactly this reason: picking a random word means picking a random index, and that only stays fast if indexing is instant.

Random.int

Random.int n returns a random integer, uniformly distributed, from 0 up to but not including n.

let () =
  let n = Random.int 6 in
  assert (n >= 0 && n < 6)

Combine the two: a random index into an array of length n is Random.int n, and a.(Random.int n) is a random element of a.

Why not List.nth?

List.nth l i has to walk i cells into the list before it reaches the answer -- reading the last element of a 4,615-word list means walking through all 4,614 before it. Array.get/a.(i) computes the address of element i directly, so it costs the same whether i is 0 or 4,614.