For many students of ocaml, using hashtables is complicated by the functors. Here are a few tips:
For a list of hashtable functions see
To create a hashtable with string keys use String.Table.
let table = String.Table.create () ~size:4 in List.iter ~f:(fun (key, data) -> Hashtbl.set table ~key ~data) [ ("A", 1); ("B", 2); ("C", 3); ]; Hashtbl.find table "C"
Here 4 need only be a guess at the hashtable's future size. There are other similar pre-made hashtables, eg Int63.Table or Host_and_port.Table.
To create a hashtable with a custom key type use Hashable.
module Key = struct module T = struct type t = String.t * Int63.t [@@deriving sexp] let compare = compare let hash = Hashtbl.hash end include T include Hashable.Make (T) end let table = Key.Table.create () ~size:4 in List.iter ~f:(fun (key, data) -> Hashtbl.set table ~key ~data) [ (("pi", Int63.zero), 3.14159); (("e", Int63.minus_one), 2.71828); (("Euler", Int63.one), 0.577215); ]; Hashtbl.find table ("pi", Int63.zero)
Performance may improve if you define
hash explicitly, eg:
let equal (x, y) (x', y') = String.(=) x x' && Int63.(=) y y' let hash (x, y) = String.hash x + Int63.hash y * 65599
[@@deriving sexp_of] but not
[@@deriving sexp] because we want people to
be explicit about the hash and comparison functions used when creating hashtables.
One can use
Hashtbl.Poly.t, which does have
[@@deriving sexp], to use
polymorphic comparison and hashing.
Merge two hashtables.
The result of
merge f h1 h2 has as keys the set of all
k in the
union of the sets of keys of
h2 for which
d(k) is not
h1is to d1, and
h2does not map
h2is to d2, and
h1does not map
k is mapped to a single piece of data x, where
d(k) = Some x.
Returns the list of all data for given hashtable.
filter_inplace t ~f removes all the elements from
t that don't satisfy
map_inplace t ~f applies f to all elements in
t, transforming them in place
filter_map_inplace combines the effects of