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Source for wiki HashTablesCowan version 7

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cowan

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74.68.121.27

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HashTablesCowan

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== Hash tables ==

This WG2 proposal defines an interface to hash tables, which are widely recognized as a fundamental data structure for a wide variety of applications.  A hash table is a data structure that:

   * Is disjoint from all other types.
   * Provides a mapping from objects known as ''keys'' to corresponding objects known as ''values''.
     * Keys may be any Scheme objects in some kinds of hash tables, but are restricted in other kinds.
     * Values may be any Scheme objects.
   * Has no intrinsic order for the key-value ''associations'' it contains, though an order may be defined as an extension.
   * Provides an ''equivalence function'' which defines when a proposed key is the same as an existing key.  No table may contain more than one value for a given key.
   * Supports mutation as the primary means of setting the contents of a table.
   * Assumes that keys are immutable; mutating a key leads to unspecified behavior.
   * Provides key lookup and destructive update in amortised constant time.

I have added equivalences to SRFI 69 and R6RS facilities.  Systems supporting either should be able to support these hash tables trivially; it will be necessary to write `hash-table-fold` for R6RS systems.  I have adopted SRFI 69's term `hash-table` rather than R6RS's `hashtable` because of our decision in #40.  Besides, the word ''hashtable'' obviously means something that can be ... hashted.

The main annoyances of this proposal for SRFI 69 programmers will be remembering to supply a third argument to `hash-table-ref`, and for R6RS programmers will be remembering to insert a hyphen in `hashtable`.  Both will have to get used to the new constructors.

== Constructors ==

These constructors provide suitable hash functions for the equivalence function specified as part of the constructor name.  This proposal does not allow (semi-)arbitrary equivalence and hash functions to be specified.

`(make-eq-hash-table)`

Returns a newly allocated table with no associations whose equivalence function is `eq?`.  (SRFI-69 `(make-hash-table eq? hash-by-identity)`; R6RS `(make-eq-hashtable)`)

`(make-equal-hash-table)`

Returns a newly allocated with no associations whose equivalence function is `equal?`.  (SRFI-69 `(make-hash-table equal? hash)`; R6RS `(make-hashtable equal? equal-hash)`)

`(make-string-hash-table)`

Returns a newly allocated table with no associations whose equivalence function is `string=?`.  (SRFI-69 `(make-hash-table string=? string-hash)`; R6RS `(make-hashtable string=? string-hash)`)

`(make-string-ci-hash-table)`

Returns a newly allocated table with no associations whose equivalence function is `string-ci=?`.  (SRFI-69 `(make-hash-table string-ci=? string-ci-hash)`; R6RS `(make-hashtable string-ci=? string-ci-hash)`)

Note that there are no hash tables whose equivalence function is `eqv?`, because SRFI 69 does not support them.  Users will have to live with `eq?` or `equal?` hash tables as the case may be.

'''Issue''':  add `(hash-table-unfold `''p f g key-seed value-seed''`)`?  Or possibly just one seed and let `f` generate the two values from a single value.

== Copiers ==

`(hash-table-copy `''hash-table''`)`

Returns a newly allocated hash table with the same equivalence predicate and associations as ''hash-table''. (SRFI-69 `hash-table-copy`; R6RS `(hashtable-copy `''hash-table''` #t)`)

== Predicates ==

`(hash-table? `''obj''`)` 

Returns `#t` if ''obj'' is a hash table.  (SRFI-69 `hash-table?`; R6RS `hashtable?`)

`(hash-table-contains? `''hash-table''` `''key''`)`

Returns `#t` if there is any association to ''key'' in ''hash-table''.  Must execute in amortized O(1) time.  (SRFI-69 `hash-table-exists?`; R6RS `hashtable-contains?`)

== Accessors ==

`(hash-table-ref `''hash-table''` `''key'' ''default-proc''`)`

Returns the value associated to ''key'' in ''table''. If no value is associated to ''key'', ''default-proc'' is applied to no arguments and the result is returned.  Must execute in amortized O(1) time, not counting the time to call ''default-proc'' if necessary.  (SRFI-69 `hash-table-ref/default`; R6RS `hashtable-ref`)

`(hash-table-size `''table''`)`

Returns the number of associations in ''hash-table''.  Must execute in amortized O(1) time.  (SRFI-69 `hash-table-size`; R6RS `hashtable-size`)

== Mutators ==

`(hash-table-set! `''hash-table''` `''key''` `''value''`)`

Creates a new association in ''hash-table'' that associates ''key'' with ''value''. The previous association (if any) is removed.  It is an error if ''hash-table'' is a string or string-ci hash table and ''key'' is not a string.  Must execute in amortized O(1) time.  Returns an unspecified value.  (SRFI-69 `hash-table-set!`; R6RS `hashtable-set!`)

`(hash-table-delete! `''hash-table''` `''key''`)`

Removes any association to ''key'' in ''hash-table''. It is not an error if no association for that ''key'' exists.    Must execute in amortized O(1) time.  Returns an unspecified value.  (SRFI-69 `hash-table-delete!`; R6RS `hashtable-delete!`)

`(hash-table-update! `''hash-table''` `''key''` `''procedure'' ''default-proc'' ]`)`

Semantically equivalent to, but may be implemented more efficiently than, the following code:

`(hash-table-set! `''hash-table''` `''key''` (`''procedure'' `(hash-table-ref `''hash-table''` `''key''` `''default-proc''`)))`

Must execute in amortized O(1) time.  Returns an unspecified value.  (SRFI-69 `hash-table-update!/default`; R6RS `hashtable-update!`)

'''Issue''':  add `hash-table-clear`?


== Iterators ==

`(hash-table-fold `''procedure''` `''init''` `''hash-table''`)`

Calls ''procedure'' for every association in ''hash-table'' with three arguments: the key of the association, the value of the association, and an accumulated value ''val''.  ''Val'' is ''init'' for the first invocation of ''procedure'', and for subsequent invocations of ''procedure'', the return value of the previous invocation.  The value returned by `hash-table-fold` is the return value of the last invocation of f. The order in which ''procedure'' is called for different associations is unspecified.  (SRFI-69 `hash-table-fold`; not in R6RS but trivially implemented: see below.)


== Excluded features ==

The following features are '''not''' part of this proposal for various reasons.

=== Incompatible return values ===

`Hash-table-keys` is present in both SRFI 69 and R6RS, but returns a list of keys in the first, a vector of keys in the second.

`Hash-table-values` returns a list of values in SRFI 69; `hashtable-entries` returns two values, a vector of keys and a vector of values in R6RS.

`Hash-table-hash-function` always returns the hash function of a hash table in SRFI 69, but returns `#f` in the case of `eq?` (and `eqv?`) hash tables in R6RS.

`Hash-table-equivalence-function` returns the equivalence function, but in R6/R7RS you can't reliably check functions for identity, so you don't know what you've got.

=== In R6RS but not in SRFI 69 ===

`Hashtable-clear!`, which removes all associations from a hash table.

Specifying the initial capacity of a hash table.

Immutable hash tables.

Hash tables based on `eqv?`.

=== In SRFI 69 but not in R6RS ===

`Hash-table-walk`, which is the analogue of `for-each` for hash tables.

Conversion from a-lists to hash tables and back.

`Hash-table-merge`, which destructively adds all associations in a source hash table to a destination hash table.

Default values specified in the form of a thunk to call.

=== In Common Lisp but not Scheme ===

The current capacity (as opposed to size) of a hash table.

Rehash size and threshold.

Hash tables based on `equalp` (which is not in Scheme).

`With-hash-table-iterator`, a hash table external iterator implemented as a local macro.

`Sxhash`, a stable hash function.

== R6RS implementation of `hash-table-fold` ==

{{{
(define (hash-table-fold h proc init)
  (let-values (((k v) (hashtable-entries h))
               ((s) (hashtable-size h)))
    (do ((i 0 (+ 1 i))
         (val init (proc (vector-ref k i) (vector-ref v i) val)))
        ((= i s) val))))
}}}

time

2012-11-30 13:48:43