Problem 2: Array.init and strings as characters

For score_guess, first pass.

Building an array from a function

Array.init n f builds a new array of length n whose element i is f i, for every i from 0 to n - 1.

let squares = Array.init 5 (fun i -> i * i)
(* [| 0; 1; 4; 9; 16 |] *)

This is the array equivalent of List.init, and it's the natural shape for "compute one output per position, from that position's own index" -- exactly what scoring a guess does: one letter_status per letter position.

A string is indexable, one character at a time

s.[i] reads the character at position i of string s (0-indexed, like an array). String.length s gives the number of characters.

let s = "hello"
let _ = s.[0]            (* = 'h' *)
let _ = String.length s  (* = 5 *)

Characters have their own type, char, written with single quotes ('h', not "h"). Comparing two characters with = works exactly like comparing two integers or strings.

Searching a string

String.contains s c answers "does character c occur anywhere in string s?" directly, with no loop to write.

let _ = String.contains "house" 'u'   (* = true *)
let _ = String.contains "house" 'z'   (* = false *)

Comparing guess.[i] against target.[i] checks one exact position; String.contains target guess.[i] checks the whole string instead, with no notion of position at all.