Version: 4.2.1
3.13 Hash Tables
Hash Tables in Guide: PLT Scheme introduces hash tables.
A hash table (or simply hash) maps each of its
keys to a single value. For a given hash table, keys are equivalent
via equal?, eqv?, or eq?, and keys are
retained either strongly or weakly (see Weak Boxes). A hash
table is also either mutable or immutable. Immutable tables support
constant-time access and update, just like mutable hash tables; the
constant on immutable operations is usually larger, but the
functional nature of immutable hash tables can pay off in certain
algorithms.
A hash table can be used as a two-valued sequence (see
Sequences). The keys and values of the hash table serve as
elements of the sequence (i.e., each element is a key and its
associated value). If a mapping is added to or removed from the hash
table during iteration, then an iteration step may fail with
exn:fail:contract, or the iteration may skip or duplicate
keys and values. See also in-hash, in-hash-keys,
in-hash-values, and in-hash-pairs.
Two hash tables cannot be equal? unless they use the same
key-comparison procedure (equal?, eqv?, or
eq?), both hold keys strongly or weakly, and have the same
mutability.
Caveats concerning concurrent
modification: A mutable hash table can be manipulated with
hash-ref, hash-set!, and hash-remove!
concurrently by multiple threads, and the operations are protected by
a table-specific semaphore as needed. Three caveats apply, however:
If a thread is terminated while applying hash-ref,
hash-set!, hash-remove!, hash-ref!,
or has-update! to a hash table that
uses equal? or eqv? key comparisons, all current
and future operations on the hash table may block indefinitely.
The hash-map and hash-for-each procedures do
not use the table’s semaphore. Consequently, if a hash table is
extended with new keys by another thread while a map or for-each
traversal is in process, arbitrary key–value pairs can be dropped
or duplicated in the traversal. Similarly, if a map or for-each
procedure itself extends the table, arbitrary key–value pairs can
be dropped or duplicated. However, key mappings can be deleted or
remapped by any thread with no adverse affects (i.e., the change
does not affect a traversal if the key has been seen already,
otherwise the traversal skips a deleted key or uses the remapped
key’s new value).
The hash-update! and hash-set! functions
use a table’s semaphore
independently for the hash-ref and hash-set! parts
of its functionality, which means that the update as a whole is not
“atomic.”
Caveat concerning mutable
keys: If a key in an equal?-based hash table is mutated
(e.g., a key string is modified with string-set!), then the
hash table’s behavior for insertion and lookup operations becomes
unpredictable.
Returns
#t if
hash compares keys with
eqv?,
#f if it compares with
equal? or
eq?.
Returns
#t if
hash compares keys with
eq?,
#f if it compares with
equal? or
eqv?.
Returns #t if hash retains its keys weakly,
#f if it retains keys strongly.
Creates an empty mutable hash table that holds keys strongly and that
uses
equal? to compare keys. See also
make-custom-hash.
Creates an empty mutable hash table that holds keys strongly and that
uses
eqv? to compare keys.
Creates an empty mutable hash table that holds keys strongly and that
uses
eq? to compare keys.
Creates an empty mutable hash table that holds keys weakly and that
uses
eqv? to compare keys.
Creates an empty mutable hash table that holds keys weakly and that
uses
eq? to compare keys.
Creates an immutable hash table that compares keys with
equal?. In each element of
assocs, the
car
of each pair is a key, and the
cdr is the corresponding
value. The mappings are added to the table in the order that they
appear in
assocs, so later mappings can hide earlier
mappings.
Maps key to v in hash, overwriting
any existing mapping for key.
See also the caveats concerning concurrent modification and the caveat concerning mutable keys above.
Functionally extends hash by mapping key to
v, overwriting any existing mapping for key, and
returning the extended hash table.
See also the caveat concerning mutable keys above.
Returns the value for key in hash. If no value
is found for key, then failure-result determines the
result:
If failure-result is a procedure, it is called
(through a tail call) with no arguments to produce the result.
Otherwise, failure-result is returned as the result.
See also the caveats concerning concurrent modification and the caveat concerning mutable keys above.
Returns the value for
key in
hash. If no value is
found for
key, then
to-set determines the result as
in
hash-ref (i.e., it is either a thunk that computes a value
or a plain value), and this result is stored in
hash for the
key. (Note that if
to-set is a thunk, it is not
invoked in tail position.)
See also the caveats concerning concurrent modification and the caveat concerning mutable keys above.
Returns #t if hash contains a value for the given
key, #f otherwise.
Composes
hash-ref and
hash-set! to update an
existing mapping in
hash, where the optional
failure-result argument is used as in
hash-ref when
no mapping exists for
key already. See the caveat above about
concurrent updates.
See also the caveats concerning concurrent modification and the caveat concerning mutable keys above.
Composes
hash-ref and
hash-set to functionally
update an existing mapping in
hash, where the optional
failure-result argument is used as in
hash-ref when
no mapping exists for
key already.
See also the caveat concerning mutable keys above.
Removes any existing mapping for key in hash.
See also the caveats concerning concurrent modification and the caveat concerning mutable keys above.
Functionally removes any existing mapping for key in
hash, returning the fresh hash table.
See also the caveat concerning mutable keys above.
Applies the procedure proc to each element in
hash in an unspecified order, accumulating the results
into a list. The procedure proc is called each time with a
key and its value. See the caveat above about concurrent
modification.
See also the caveats concerning concurrent modification above.
Applies proc to each element in hash (for the
side-effects of proc) in an unspecified order. The procedure
proc is called each time with a key and its value. See the
caveat above about concurrent modification.
See also the caveats concerning concurrent modification above.
Returns the number of keys mapped by
hash. If
hash
is not created with
'weak, then the result is computed in
constant time and atomically. If
hash is created with
'weak, see the
caveats concerning concurrent modification above.
Returns #f if hash contains no elements, otherwise
it returns an integer that is a index to the first element in the hash
table; “first” refers to an unspecified ordering of the table
elements, and the index values are not necessarily consecutive
integers. For a mutable hash, this index is guaranteed to
refer to the first item only as long as no items are added to or
removed from hash.
Returns either an integer that is an index to the element in
hash after the element indexed by
pos (which is not
necessarily one more than
pos) or
#f if
pos
refers to the last element in
hash. If
pos is not a
valid index, then the
exn:fail:contract exception is raised. For a mutable
hash, the result index is guaranteed to refer to its item
only as long as no items are added to or removed from
hash.
Returns the key for the element in
hash at index
pos. If
pos is not a valid index for
hash, the
exn:fail:contract exception is raised.
Returns the value for the element in
hash at index
pos. If
pos is not a valid index for
hash, the
exn:fail:contract exception is raised.
Returns a mutable hash table with the same mappings, same
key-comparison mode, and same key-holding strength as hash.
Returns an exact integer; for any two
eq? values, the
returned integer is the same. Furthermore, for the result integer
k and any other exact integer
j,
(= k j)
implies
(eq? k j).
Returns an exact integer; for any two
eqv? values, the
returned integer is the same. Furthermore, for the result integer
k and any other exact integer
j,
(= k j)
implies
(eq? k j).
Returns an exact integer; for any two
equal? values, the
returned integer is the same. Furthermore, for the result integer
k and any other exact integer
j,
(= k j)
implies
(eq? k j). A has code is computed even when
v contains a cycle through pairs, vectors, boxes, and/or
inspectable structure fields. See also
prop:equal+hash.
Like
equal-hash-code, but computes a secondary value suitable
for use in double hashing.