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#### 7.6.2.23 rnrs hashtables

The `(rnrs hashtables (6))`

library provides structures and
procedures for creating and accessing hash tables. The hash tables API
defined by R6RS is substantially similar to both Guile’s native hash
tables implementation as well as the one provided by SRFI-69;
See section Hash Tables, and SRFI-69 - Basic hash tables, respectively. Note that you can
write portable R6RS library code that manipulates SRFI-69 hash tables
(by importing the `(srfi :69)`

library); however, hash tables
created by one API cannot be used by another.

Like SRFI-69 hash tables—and unlike Guile’s native ones—R6RS hash
tables associate hash and equality functions with a hash table at the
time of its creation. Additionally, R6RS allows for the creation
(via `hashtable-copy`

; see below) of immutable hash tables.

- Scheme Procedure:
**make-eq-hashtable** - Scheme Procedure:
**make-eq-hashtable***k* Returns a new hash table that uses

`eq?`

to compare keys and Guile’s`hashq`

procedure as a hash function. If`k`is given, it specifies the initial capacity of the hash table.

- Scheme Procedure:
**make-eqv-hashtable** - Scheme Procedure:
**make-eqv-hashtable***k* Returns a new hash table that uses

`eqv?`

to compare keys and Guile’s`hashv`

procedure as a hash function. If`k`is given, it specifies the initial capacity of the hash table.

- Scheme Procedure:
**make-hashtable***hash-function equiv* - Scheme Procedure:
**make-hashtable***hash-function equiv k* Returns a new hash table that uses

`equiv`to compare keys and`hash-function`as a hash function.`equiv`must be a procedure that accepts two arguments and returns a true value if they are equivalent,`#f`

otherwise;`hash-function`must be a procedure that accepts one argument and returns a non-negative integer.If

`k`is given, it specifies the initial capacity of the hash table.

- Scheme Procedure:
**hashtable-size***hashtable* Returns the number of keys currently in the hash table

`hashtable`.

- Scheme Procedure:
**hashtable-ref***hashtable key default* Returns the value associated with

`key`in the hash table`hashtable`, or`default`if none is found.

- Scheme Procedure:
**hashtable-set!***hashtable key obj* Associates the key

`key`with the value`obj`in the hash table`hashtable`, and returns an unspecified value. An`&assertion`

condition is raised if`hashtable`is immutable.

- Scheme Procedure:
**hashtable-delete!***hashtable key* Removes any association found for the key

`key`in the hash table`hashtable`, and returns an unspecified value. An`&assertion`

condition is raised if`hashtable`is immutable.

- Scheme Procedure:
**hashtable-contains?***hashtable key* Returns

`#t`

if the hash table`hashtable`contains an association for the key`key`,`#f`

otherwise.

- Scheme Procedure:
**hashtable-update!***hashtable key proc default* Associates with

`key`in the hash table`hashtable`the result of calling`proc`, which must be a procedure that takes one argument, on the value currently associated`key`in`hashtable`—or on`default`if no such association exists. An`&assertion`

condition is raised if`hashtable`is immutable.

- Scheme Procedure:
**hashtable-copy***hashtable* - Scheme Procedure:
**hashtable-copy***hashtable mutable* Returns a copy of the hash table

`hashtable`. If the optional argument`mutable`is a true value, the new hash table will be immutable.

- Scheme Procedure:
**hashtable-clear!***hashtable* - Scheme Procedure:
**hashtable-clear!***hashtable k* Removes all of the associations from the hash table

`hashtable`. The optional argument`k`, which specifies a new capacity for the hash table, is accepted by Guile’s`(rnrs hashtables)`

implementation, but is ignored.

- Scheme Procedure:
**hashtable-keys***hashtable* Returns a vector of the keys with associations in the hash table

`hashtable`, in an unspecified order.

- Scheme Procedure:
**hashtable-entries***hashtable* Return two values—a vector of the keys with associations in the hash table

`hashtable`, and a vector of the values to which these keys are mapped, in corresponding but unspecified order.

- Scheme Procedure:
**hashtable-equivalence-function***hashtable* Returns the equivalence predicated use by

`hashtable`. This procedure returns`eq?`

and`eqv?`

, respectively, for hash tables created by`make-eq-hashtable`

and`make-eqv-hashtable`

.

- Scheme Procedure:
**hashtable-hash-function***hashtable* Returns the hash function used by

`hashtable`. For hash tables created by`make-eq-hashtable`

or`make-eqv-hashtable`

,`#f`

is returned.

A number of hash functions are provided for convenience:

- Scheme Procedure:
**equal-hash***obj* Returns an integer hash value for

`obj`, based on its structure and current contents. This hash function is suitable for use with`equal?`

as an equivalence function.

- Scheme Procedure:
**string-hash***string* - Scheme Procedure:
**symbol-hash***symbol* These procedures are identical to the ones provided by Guile’s core library. See section Hash Table Reference, for documentation.

- Scheme Procedure:
**string-ci-hash***string* Returns an integer hash value for

`string`based on its contents, ignoring case. This hash function is suitable for use with`string-ci=?`

as an equivalence function.

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