Attempting to modify (set, remove, etc.) the hashtable during iteration (fold, iter, iter_keys, iteri) will raise an exception.

Iterates over both keys and values.

Example:

let h = Hashtbl.of_alist_exn (module Int) [(1, 4); (5, 6)] in
Hashtbl.iteri h ~f:(fun ~key ~data ->
  print_endline (Printf.sprintf "%d-%d" key data));;
1-4
5-6
- : unit = ()

Sets the given key to data.

add and add_exn leave the table unchanged if the key was already present.

change t key ~f changes t's value for key to be f (find t key).

update t key ~f is change t key ~f:(fun o -> Some (f o)).

map t f returns a new table with values replaced by the result of applying f to the current values.

Example:

let h = Hashtbl.of_alist_exn (module Int) [(1, 4); (5, 6)] in
let h' = Hashtbl.map h ~f:(fun x -> x * 2) in
Hashtbl.to_alist h';;
- : (int * int) list = [(5, 12); (1, 8)]

Like map, but the function f takes both key and data as arguments.

Returns a new table by filtering the given table's values by f: the keys for which f applied to the current value returns Some are kept, and those for which it returns None are discarded.

Example:

let h = Hashtbl.of_alist_exn (module Int) [(1, 4); (5, 6)] in
Hashtbl.filter_map h ~f:(fun x -> if x > 5 then Some x else None)
|> Hashtbl.to_alist;;
- : (int * int) list = [(5, 6)]

Like filter_map, but the function f takes both key and data as arguments.

Returns new tables with bound values partitioned by f applied to the bound values.

Like partition_map, but the function f takes both key and data as arguments.

Returns a pair of tables (t1, t2), where t1 contains all the elements of the initial table which satisfy the predicate f, and t2 contains the rest.

Like partition_tf, but the function f takes both key and data as arguments.

find_or_add t k ~default returns the data associated with key k if it is in the table t, and otherwise assigns k the value returned by default ().

Like find_or_add but default takes the key as an argument.

find t k returns Some (the current binding) of k in t, or None if no such binding exists.

find_exn t k returns the current binding of k in t, or raises Caml.Not_found or Not_found_s if no such binding exists.

find_and_call t k ~if_found ~if_not_found

is equivalent to:

match find t k with Some v -> if_found v | None -> if_not_found k

except that it doesn't allocate the option.

find_and_remove t k returns Some (the current binding) of k in t and removes it, or None is no such binding exists.

Merges 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 h1 and h2 for which d(k) is not None, where:

d(k) =

• f ~key:k (Some d1) None if k in h1 maps to d1, and h2 does not have data for k;

• f ~key:k None (Some d2) if k in h2 maps to d2, and h1 does not have data for k;

• f ~key:k (Some d1) (Some d2) otherwise, where k in h1 maps to d1 and k in h2 maps to d2.

Each key k is mapped to a single piece of data x, where d(k) = Some x.

Example:

let h1 = Hashtbl.of_alist_exn (module Int) [(1, 5); (2, 3232)] in
let h2 = Hashtbl.of_alist_exn (module Int) [(1, 3)] in
Hashtbl.merge h1 h2 ~f:(fun ~key:_ -> function
  | `Left x -> Some (`Left x)
  | `Right x -> Some (`Right x)
  | `Both (x, y) -> if x=y then None else Some (`Both (x,y))
) |> Hashtbl.to_alist;;
- : (int * [> `Both of int * int | `Left of int | `Right of int ]) list =
[(2, `Left 3232); (1, `Both (5, 3))]

Every key in src will be removed or set in dst according to the return value of f.

Returns the list of all keys for given hashtable.

Returns the list of all data for given hashtable.

filter_inplace t ~f removes all the elements from t that don't satisfy f.

map_inplace t ~f applies f to all elements in t, transforming them in place.

filter_map_inplace combines the effects of map_inplace and filter_inplace.

equal t1 t2 f and similar t1 t2 f both return true iff t1 and t2 have the same keys and for all keys k, f (find_exn t1 k) (find_exn t2 k). equal and similar only differ in their types.

Returns the list of all (key, data) pairs for given hashtable.

remove_if_zero's default is false.

add_multi t ~key ~data if key is present in the table then cons data on the list, otherwise add key with a single element list.

remove_multi t key updates the table, removing the head of the list bound to key. If the list has only one element (or is empty) then the binding is removed.

find_multi t key returns the empty list if key is not present in the table, returns t's values for key otherwise.