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Module Hashtbl: hash tables and hash functions
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<H2>17.11 Module <TT>Hashtbl</TT>: hash tables and hash functions</H2><A NAME="s:Hashtbl"></A>
<A NAME="@manual323"></A><BLOCKQUOTE>
Hash tables are hashed association tables, with in-place modification.
</BLOCKQUOTE>
<H3>Generic interface </H3>
<PRE>
type ('a, 'b) t
</PRE>
<BLOCKQUOTE>
The type of hash tables from type <CODE>'a</CODE> to type <CODE>'b</CODE>.
</BLOCKQUOTE>
<PRE>
val create : int -> ('a,'b) t
</PRE>
<A NAME="@manual324"></A><BLOCKQUOTE>
<CODE>Hashtbl.create n</CODE> creates a new, empty hash table, with
initial size <CODE>n</CODE>. The table grows as needed, so <CODE>n</CODE> is
just an initial guess. Better results are said to be
achieved when <CODE>n</CODE> is a prime number.
</BLOCKQUOTE>
<PRE>
val clear : ('a, 'b) t -> unit
</PRE>
<A NAME="@manual325"></A><BLOCKQUOTE>
Empty a hash table.
</BLOCKQUOTE>
<PRE>
val add : ('a, 'b) t -> 'a -> 'b -> unit
</PRE>
<A NAME="@manual326"></A><BLOCKQUOTE>
<CODE>Hashtbl.add tbl x y</CODE> adds a binding of <CODE>x</CODE> to <CODE>y</CODE> in table <CODE>tbl</CODE>.
Previous bindings for <CODE>x</CODE> are not removed, but simply
hidden. That is, after performing <CODE>Hashtbl.remove tbl x</CODE>,
the previous binding for <CODE>x</CODE>, if any, is restored.
(Same behavior as with association lists.)
</BLOCKQUOTE>
<PRE>
val find : ('a, 'b) t -> 'a -> 'b
</PRE>
<A NAME="@manual327"></A><BLOCKQUOTE>
<CODE>Hashtbl.find tbl x</CODE> returns the current binding of <CODE>x</CODE> in <CODE>tbl</CODE>,
or raises <CODE>Not_found</CODE> if no such binding exists.
</BLOCKQUOTE>
<PRE>
val find_all : ('a, 'b) t -> 'a -> 'b list
</PRE>
<A NAME="@manual328"></A><BLOCKQUOTE>
<CODE>Hashtbl.find_all tbl x</CODE> returns the list of all data
associated with <CODE>x</CODE> in <CODE>tbl</CODE>.
The current binding is returned first, then the previous
bindings, in reverse order of introduction in the table.
</BLOCKQUOTE>
<PRE>
val mem : ('a, 'b) t -> 'a -> bool
</PRE>
<A NAME="@manual329"></A><BLOCKQUOTE>
<CODE>Hashtbl.mem tbl x</CODE> checks if <CODE>x</CODE> is bound in <CODE>tbl</CODE>.
</BLOCKQUOTE>
<PRE>
val remove : ('a, 'b) t -> 'a -> unit
</PRE>
<A NAME="@manual330"></A><BLOCKQUOTE>
<CODE>Hashtbl.remove tbl x</CODE> removes the current binding of <CODE>x</CODE> in <CODE>tbl</CODE>,
restoring the previous binding if it exists.
It does nothing if <CODE>x</CODE> is not bound in <CODE>tbl</CODE>.
</BLOCKQUOTE>
<PRE>
val iter : ('a -> 'b -> unit) -> ('a, 'b) t -> unit
</PRE>
<A NAME="@manual331"></A><BLOCKQUOTE>
<CODE>Hashtbl.iter f tbl</CODE> applies <CODE>f</CODE> to all bindings in table <CODE>tbl</CODE>.
<CODE>f</CODE> receives the key as first argument, and the associated value
as second argument. The order in which the bindings are passed to
<CODE>f</CODE> is unspecified. Each binding is presented exactly once
to <CODE>f</CODE>.
</BLOCKQUOTE>
<H3>Functorial interface </H3>
<PRE>
module type HashedType =
sig
type t
val equal: t -> t -> bool
val hash: t -> int
end
</PRE>
<A NAME="@manual332"></A><BLOCKQUOTE>
The input signature of the functor <CODE>Hashtbl.Make</CODE>.
<CODE>t</CODE> is the type of keys.
<CODE>equal</CODE> is the equality predicate used to compare keys.
<CODE>hash</CODE> is a hashing function on keys, returning a non-negative
integer. It must be such that if two keys are equal according
to <CODE>equal</CODE>, then they must have identical hash values as computed
by <CODE>hash</CODE>.
Examples: suitable (<CODE>equal</CODE>, <CODE>hash</CODE>) pairs for arbitrary key
types include
(<CODE>(=)</CODE>, <CODE>Hashtbl.hash</CODE>) for comparing objects by structure, and
(<CODE>(==)</CODE>, <CODE>Hashtbl.hash</CODE>) for comparing objects by addresses
(e.g. for mutable or cyclic keys).
</BLOCKQUOTE>
<PRE>
module type S =
sig
type key
type 'a t
val create: int -> 'a t
val clear: 'a t -> unit
val add: 'a t -> key -> 'a -> unit
val remove: 'a t -> key -> unit
val find: 'a t -> key -> 'a
val find_all: 'a t -> key -> 'a list
val mem: 'a t -> key -> bool
val iter: (key -> 'a -> unit) -> 'a t -> unit
end
module Make(H: HashedType): (S with type key = H.t)
</PRE>
<A NAME="@manual333"></A><BLOCKQUOTE>
The functor <CODE>Hashtbl.Make</CODE> returns a structure containing
a type <CODE>key</CODE> of keys and a type <CODE>'a t</CODE> of hash tables
associating data of type <CODE>'a</CODE> to keys of type <CODE>key</CODE>.
The operations perform similarly to those of the generic
interface, but use the hashing and equality functions
specified in the functor argument <CODE>H</CODE> instead of generic
equality and hashing.
</BLOCKQUOTE>
<H3>The polymorphic hash primitive </H3>
<PRE>
val hash : 'a -> int
</PRE>
<A NAME="@manual334"></A><BLOCKQUOTE>
<CODE>Hashtbl.hash x</CODE> associates a positive integer to any value of
any type. It is guaranteed that
if <CODE>x = y</CODE>, then <CODE>hash x = hash y</CODE>.
Moreover, <CODE>hash</CODE> always terminates, even on cyclic
structures.
</BLOCKQUOTE>
<PRE>
val hash_param : int -> int -> 'a -> int
</PRE>
<A NAME="@manual335"></A><BLOCKQUOTE>
<CODE>Hashtbl.hash_param n m x</CODE> computes a hash value for <CODE>x</CODE>, with the
same properties as for <CODE>hash</CODE>. The two extra parameters <CODE>n</CODE> and
<CODE>m</CODE> give more precise control over hashing. Hashing performs a
depth-first, right-to-left traversal of the structure <CODE>x</CODE>, stopping
after <CODE>n</CODE> meaningful nodes were encountered, or <CODE>m</CODE> nodes,
meaningful or not, were encountered. Meaningful nodes are: integers;
floating-point numbers; strings; characters; booleans; and constant
constructors. Larger values of <CODE>m</CODE> and <CODE>n</CODE> means that more
nodes are taken into account to compute the final hash
value, and therefore collisions are less likely to happen.
However, hashing takes longer. The parameters <CODE>m</CODE> and <CODE>n</CODE>
govern the tradeoff between accuracy and speed.
</BLOCKQUOTE>
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