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(Hash Table Reference): Decribe rehashing, note

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no hashx-remove!, describe make-hash-table size parameter.
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Kevin Ryde 2003-10-09 00:14:38 +00:00
parent b7be48bc39
commit ea2b9c2f6b
1 changed files with 21 additions and 8 deletions
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29
doc/ref/scheme-compound.texi
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@ -2254,11 +2254,17 @@ with any set of functions, but it's imperative that just one set is
then used consistently, or results will be unpredictable.
@sp 1
Hash tables are implemented as a vector indexed by an integer formed
Hash tables are implemented as a vector indexed by a hash value formed
from the key, with an association list of key/value pairs for each
bucket in case distinct keys hash together. Direct access to the
pairs in those lists is provided by the @code{-handle-} functions.
When the number of table entries goes above a threshold the vector is
increased and the entries rehashed, to prevent the bucket lists
becoming too long and slowing down accesses. When the number of
entries goes below a threshold the vector is decreased to save space.
@sp 1
For the @code{hashx-} ``extended'' routines, an application supplies a
@var{hash} function producing an integer index like @code{hashq} etc
below, and an @var{assoc} alist search function like @code{assq} etc
@ -2268,6 +2274,7 @@ functions implementing case-insensitive hashing of string keys,
@example
(use-modules (srfi srfi-1)
(srfi srfi-13))
(define (my-hash str size)
(remainder (string-hash-ci str) size))
(define (my-assoc str alist)
@ -2281,21 +2288,27 @@ functions implementing case-insensitive hashing of string keys,
@end example
In a @code{hashx-} @var{hash} function the aim is to spread keys
across the vector, so bucket lists don't become long, but the actual
values are arbitrary (so long as they're in the range 0 to
@math{@var{size}-1}). Helpful functions for forming a hash value, in
across the vector, so bucket lists don't become long. But the actual
values are arbitrary as long as they're in the range 0 to
@math{@var{size}-1}. Helpful functions for forming a hash value, in
addition to @code{hashq} etc below, include @code{symbol-hash}
(@pxref{Symbol Keys}), @code{string-hash} and @code{string-hash-ci}
(@pxref{SRFI-13 Comparison}), and @code{char-set-hash} (@pxref{SRFI-14
Predicates/Comparison}).
Note that currently, unfortunately, there's no @code{hashx-remove!}
function, which rather limits the usefulness of the @code{hashx-}
routines.
@sp 1
@deffn {Scheme Procedure} make-hash-table [size]
Create a new hash table, with an optional initial vector @var{size}.
Create a new hash table, with an optional minimum vector @var{size}.
@var{size} doesn't limit the entries in the table, merely gives a
starting size for the internal vector. A prime number bigger than the
expected number of entries would be a good choice.
When @var{size} is given, the table vector will still grow and shrink
automatically, as described above, but with @var{size} as a minimum.
If an application knows roughly how many entries the table will hold
then it can use @var{size} to avoid rehashing when initial entries are
added.
@end deffn
@deffn {Scheme Procedure} hash-ref table key [dflt]