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.\"	@(#)ch17.n	6.2 (Berkeley) 5/14/86
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." $Header: ch17.n,v 40.1 84/08/08 21:36:08 layer Exp $
." (c) Copyright 1984, Franz Inc., Berkeley California
.Lc Hash\ Tables 17
.sh 2 Overview \n(ch 1
.pp
A hash table is an object that can efficiently map a given object to another.
Each hash table is a collection of entries,
each of which associates a unique \fIkey\fP with a \fIvalue\fP.
There are elemental functions to add, delete, and find entries
based on a particular key.
Finding a value in a hash table is relatively fast compared to
looking up values in, for example, an assoc list or property list.
.pp
Adding a key to a hash table modifies the hash table, and so
it is a descructive operation.
.pp
There are two different kinds of hash tables:  those that use the
function \fIequal\fP for the comparing of keys, and those that
use \fIeq\fP, the default.
If a key is "eq" to another object, then a match is assumed.
Likewise with "equal".
.sh 2 Functions
.Lf makeht "'x_size ['s_test]" 
.Re
A hash table of x_size hash buckets.
If present, s_test is used as the test to compare keys in the
hash table, the default being \fBeq\fP.
\fIEqual\fP might be used to create a hash table where the
keys are to be lists (or any lisp object).
.No
At this time, hash tables are implemented on top of vectors.
.Lf hash-table-p "'H_arg"
.Re
t if H_arg is a hash table.
.No
Since hash tables are really vectors, the lisp type of a hash table
is a vector, so that given a vector, this function will return t.
.Lf gethash "'g_key 'H_htable ['g_default]"
.Re
the value associated the key g_key in hash table H_htable.
If there is not an entry given by the key and g_default is specified,
then g_default is returned, otherwise, a symbol that is unbound
is returned.
This is so that \fBnil\fP can be a associated with a key.
.No
\fIsetf\fP may be used to set the value assocaited with a key.
.Lf remhash "'g_key 'H_htable"
.Re
t if there was an entry for g_key in the hash table
H_htable, nil otherwise.  In the case of a match, the
entry and associated object are removed from the hash
table.
.Lf maphash "'u_func 'H_htable"
.Re
nil.
.No
The function u_func is applied to every element in the
hash table H_htable.  The function is called with two arguments:
the key and value of an element.
The mapped function should not add or delete object from the
table because the results would be unpredicable.
.Lf clrhash "'H_htable"
.Re
the hash table cleared of all entries.
.Lf hash-table-count "'H_htable"
.Re
the number of entries in H_htable.  Given a new hash table
with no entries, this function returns zero.
.Eb
; make a vanilla hash table using "eq" to compare items...
\-> (setq black-box (makeht 20))
hash-table[26]
\-> (hash-table-p black-box)
t
\-> (hash-table-count black-box)
0
\-> (setf (gethash 'key black-box) '(the value associated with the key))
key
\-> (gethash 'key black-box)
(the value associated with the key)
\-> (hash-table-count black-box)
1
\-> (addhash 'composer black-box 'franz)
composer
\-> (gethash 'composer black-box)
franz
\-> (maphash '(lambda (key val) (msg "key " key " value " val N)) black-box)
key composer value franz
key key value (the value associated with the key)
nil
\-> (clrhash black-box)
hash-table[26]
\-> (hash-table-count black-box)
0
\-> (maphash '(lambda (key val) (msg "key " key " value " val N)) black-box)
nil

; here is an example using "equal" as the comparator
\-> (setq ht (makeht 10 'equal))
hash-table[16]
\-> (setf (gethash '(this is a key) ht) '(and this is the value))
(this is a key)
\-> (gethash '(this is a key) ht)
(and this is the value)
; the reader makes a new list each time you type it...
\-> (setq x '(this is a key))
(this is a key)
\-> (setq y '(this is a key))
(this is a key)
; so these two lists are really different lists that compare "equal"
; not "eq"
\-> (eq x y)
nil
; but since we are using "equal" to compare keys, we are OK...
\-> (gethash x ht)
(and this is the value)
\-> (gethash y ht)
(and this is the value)
.Ee
