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The procedures described in this section are the basic operations on hash tables. They provide the functionality most often needed by programmers. Subsequent sections describe other operations that provide additional functionality needed by some applications.

- procedure:
**hash-table?***object* -
Returns

`#t`

if`object`is a hash table, otherwise returns`#f`

.

- procedure:
**hash-table-set!***hash-table key datum* - obsolete procedure:
**hash-table/put!***hash-table key datum* Associates

`datum`with`key`in`hash-table`and returns an unspecified result.The average time required by this operation is bounded by a constant.

- procedure:
**hash-table-ref***hash-table key [get-default]* Returns the datum associated with

`key`in`hash-table`. If there is no association for`key`, and`get-default`is provided, it is called with no arguments and the value it yields is returned; if`get-default`is not provided, an error is signaled.The average time required by this operation is bounded by a constant.

- procedure:
**hash-table-ref/default***hash-table key default* - obsolete procedure:
**hash-table/get***hash-table key default* Equivalent to

(hash-table-ref

`hash-table``key`(lambda ()`default`))

- procedure:
**hash-table-delete!***hash-table key* - obsolete procedure:
**hash-table/remove!***hash-table key* If

`hash-table`has an association for`key`, removes it. Returns an unspecified result.The average time required by this operation is bounded by a constant.

- procedure:
**hash-table-clear!***hash-table* - obsolete procedure:
**hash-table/clear!***hash-table* Removes all associations in

`hash-table`and returns an unspecified result.The average and worst-case times required by this operation are bounded by a constant.

- procedure:
**hash-table-size***hash-table* - obsolete procedure:
**hash-table/count***hash-table* Returns the number of associations in

`hash-table`as an exact non-negative integer. If`hash-table`does not hold its keys and data strongly, this is a conservative upper bound that may count some associations whose keys or data have recently been reclaimed by the garbage collector.The average and worst-case times required by this operation are bounded by a constant.

- procedure:
**hash-table->alist***hash-table* Returns the contents of

`hash-table`as a newly allocated alist. Each element of the alist is a pair`(`

where`key`.`datum`)`key`is one of the keys of`hash-table`, and`datum`is its associated datum.The average and worst-case times required by this operation are linear in the number of associations in the table.

- procedure:
**hash-table-keys***hash-table* - obsolete procedure:
**hash-table/key-list***hash-table* Returns a newly allocated list of the keys in

`hash-table`.The average and worst-case times required by this operation are linear in the number of associations in the table.

- procedure:
**hash-table-values***hash-table* - obsolete procedure:
**hash-table/datum-list***hash-table* Returns a newly allocated list of the datums in

`hash-table`. Each element of the list corresponds to one of the associations in`hash-table`; if the table contains multiple associations with the same datum, so will this list.The average and worst-case times required by this operation are linear in the number of associations in the table.

- procedure:
**hash-table-walk***hash-table procedure* - obsolete procedure:
**hash-table/for-each***hash-table procedure* `Procedure`must be a procedure of two arguments. Invokes`procedure`once for each association in`hash-table`, passing the association’s`key`and`datum`as arguments, in that order. Returns an unspecified result.`Procedure`must not modify`hash-table`, with one exception: it is permitted to call`hash-table-delete!`

to remove the association being processed.

The following procedure is useful when there is no sensible default
value for `hash-table-ref`

and the caller must choose between
different actions depending on whether there is a datum associated
with the key.

- obsolete procedure:
**hash-table/lookup***hash-table key if-found if-not-found* `If-found`must be a procedure of one argument, and`if-not-found`must be a procedure of no arguments. If`hash-table`contains an association for`key`,`if-found`is invoked on the datum of the association. Otherwise,`if-not-found`is invoked with no arguments. In either case, the result yielded by the invoked procedure is returned as the result of`hash-table/lookup`

(`hash-table/lookup`

*reduces*into the invoked procedure, i.e. calls it tail-recursively).The average time required by this operation is bounded by a constant.

- procedure:
**hash-table-update!***hash-table key procedure [get-default]* `Procedure`must be a procedure of one argument and`get-default`, if supplied, must be a procedure of zero arguments. Applies`procedure`to the datum associated with`key`in`hash-table`or to the value of calling`get-default`if there is no association for`key`, associates the result with`key`, and returns an unspecified value. If`get-default`is not supplied and there’s no association for`key`, an error is signaled.The average time required by this operation is bounded by a constant.

- procedure:
**hash-table-update!/default***hash-table key procedure default* - obsolete procedure:
**hash-table/modify!***hash-table key default procedure* Equivalent to

(hash-table-update!

`hash-table``key``procedure`(lambda ()`default`))

- procedure:
**hash-table-intern!***hash-table key get-default* - obsolete procedure:
**hash-table/intern!***hash-table key get-default* `Get-default`must be a procedure of zero arguments. Ensures that`hash-table`has an association for`key`and returns the associated datum. If`hash-table`did not have a datum associated with`key`,`get-default`is called and its value is used to create a new association for`key`.The average time required by this operation is bounded by a constant.

The following procedure is sometimes useful in conjunction with weak and ephemeral hash tables. Normally it is not needed, because such hash tables clean themselves automatically as they are used.

- procedure:
**hash-table-clean!***hash-table* - obsolete procedure:
**hash-table/clean!***hash-table* If

`hash-table`is a type of hash table that holds its keys or data weakly or ephemerally, this procedure recovers any space that was being used to record associations for objects that have been reclaimed by the garbage collector. Otherwise, this procedure does nothing. In either case, it returns an unspecified result.