Use a Hash Table With Given Key and Value Types using the Standard Library
Task
Data Structures & Algorithms / Hash Tables / Use a Hash Table With Given Key and Value Types
No packages used
This recipe uses only the OCaml Standard Library.Code
let open Format in
Hashtbl.t is a hash table with in-place modification.
Hashtbl.create ( -1 ) creates a table of default size.
If you know how large the table would become, it is better to set a size on creation.
But the table grows as needed, so setting the size accurately is technically not necessary.
let numbers = Hashtbl.create ( -1 ) in
Adds an element 1 to the table with the key "one".
Hashtbl.add numbers "one" 1;
Hashtbl.add numbers "two" 2;
Hashtbl.add numbers "three" 3;
Hashtbl.add numbers "secret" 42;
printf "%i\n%!" (Hashtbl.find numbers "secret");
Hashtbl.find looks up for an element by its key.
(try printf "%i\n%!" (Hashtbl.find numbers "four")
with Not_found -> printf "There is no four.\n%!");
Hashtbl.find_opt does the same but returns an option.
(match Hashtbl.find_opt numbers "four" with
| Some value -> printf "%i\n%!" value
| None -> printf "There is no four.\n%!");
Hashtbl.add adds an element, but hides the previous element under the same key.
The classical bahavior of replacing the previous element under the same key.
Hashtbl.add numbers "secret" 543;
printf "%i\n%!" (Hashtbl.find numbers "secret");
Removes an element by its key.
Hashtbl.remove numbers "secret";
The previous values is back.
printf "%i\n%!" (Hashtbl.find numbers "secret");
Replace the previous value.
Hashtbl.replace numbers "secret" 543;
Hashtbl.remove numbers "secret";
The element is removed.
(try printf "%i\n%!" (Hashtbl.find numbers "secret")
with Not_found -> printf "There is no secret.\n%!");
This becomes a bit tideous, so sometimes people define operators.
let ( .%{} ) tbl key = Hashtbl.find tbl key
and ( .%?{} ) tbl key = Hashtbl.find_opt tbl key
and ( .%{}<- ) tbl key value = Hashtbl.replace tbl key value
in
numbers.%{"secret"} <- 42;
printf "%i\n%!" numbers.%{"secret"};
Iterate over the whole table.
numbers |> Hashtbl.iter (printf "%s = %i\n%!")