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)).

update_and_return t key ~f is update, but returns the result of 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.

Just like find_and_call, but takes an extra argument which is passed to if_found and if_not_found, so that the client code can avoid allocating closures or using refs to pass this additional information. This function is only useful in code which tries to minimize heap allocation.

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 (`Left d1) if k in h1 maps to d1, and h2 does not have data for k;

• f ~key:k (`Right d2) if k in h2 maps to d2, and h1 does not have data for k;

• f ~key:k (`Both (d1, 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 f t1 t2 and similar f t1 t2 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.