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<HTML LANG="en-us"
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>18.1 Hash Table Concepts | Common Lisp Nova Spec</TITLE
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> <SPAN CLASS="breadcrumb-item"
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>18. Hash Tables</A
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><DIV CLASS="matter"
><SECTION
><H2 ID="sec_18_1" CLASS="HeadII"
>18.1 Hash Table Concepts</H2
><UL CLASS="subtoc"
><LI
><A HREF="18_1_Hash_Table_Concepts.html#sec_18_1_1"
>18.1.1 Hash-Table Operations</A
></LI
><LI
><A HREF="18_1_Hash_Table_Concepts.html#sec_18_1_2"
>18.1.2 Modifying Hash Table Keys</A
></LI
><LI
><A HREF="18_1_Hash_Table_Concepts.html#sec_18_1_2_1"
>18.1.2.1 Visible Modification of Objects with respect to EQ and EQL</A
></LI
><LI
><A HREF="18_1_Hash_Table_Concepts.html#sec_18_1_2_2"
>18.1.2.2 Visible Modification of Objects with respect to EQUAL</A
></LI
><LI
><A HREF="18_1_Hash_Table_Concepts.html#sec_18_1_2_2_1"
>18.1.2.2.1 Visible Modification of Conses with respect to EQUAL</A
></LI
><LI
><A HREF="18_1_Hash_Table_Concepts.html#sec_18_1_2_2_2"
>18.1.2.2.2 Visible Modification of Bit Vectors and Strings with respect to EQUAL</A
></LI
><LI
><A HREF="18_1_Hash_Table_Concepts.html#sec_18_1_2_3"
>18.1.2.3 Visible Modification of Objects with respect to EQUALP</A
></LI
><LI
><A HREF="18_1_Hash_Table_Concepts.html#sec_18_1_2_3_1"
>18.1.2.3.1 Visible Modification of Structures with respect to EQUALP</A
></LI
><LI
><A HREF="18_1_Hash_Table_Concepts.html#sec_18_1_2_3_2"
>18.1.2.3.2 Visible Modification of Arrays with respect to EQUALP</A
></LI
><LI
><A HREF="18_1_Hash_Table_Concepts.html#sec_18_1_2_3_3"
>18.1.2.3.3 Visible Modification of Hash Tables with respect to EQUALP</A
></LI
><LI
><A HREF="18_1_Hash_Table_Concepts.html#sec_18_1_2_4"
>18.1.2.4 Visible Modifications by Language Extensions</A
></LI
></UL
><H3 ID="sec_18_1_1" CLASS="HeadIII"
>18.1.1 Hash-Table Operations</H3
><P CLASS="j"
>Figure 18&#8211;1 lists some <A HREF="26_1_Glossary.html#defined_name"
><EM CLASS="term"
>defined names</EM
></A
> that are applicable to <A HREF="26_1_Glossary.html#hash_table"
><EM CLASS="term"
>hash tables</EM
></A
>. The following rules apply to <A HREF="26_1_Glossary.html#hash_table"
><EM CLASS="term"
>hash tables</EM
></A
>.</P
><UL
><LI
><P CLASS="j"
>A <A HREF="26_1_Glossary.html#hash_table"
><EM CLASS="term"
>hash table</EM
></A
> can only associate one value with a given key. If an attempt is made to add a second value for a given key, the second value will replace the first. Thus, adding a value to a <A HREF="26_1_Glossary.html#hash_table"
><EM CLASS="term"
>hash table</EM
></A
> is a destructive operation; the <A HREF="26_1_Glossary.html#hash_table"
><EM CLASS="term"
>hash table</EM
></A
> is modified.</P
></LI
><LI
><P CLASS="j"
>There are four kinds of <A HREF="26_1_Glossary.html#hash_table"
><EM CLASS="term"
>hash tables</EM
></A
>: those whose keys are compared with <A HREF="f_eq.html" CLASS="funref"
><B
>eq</B
></A
>, those whose keys are compared with <A HREF="f_eql.html" CLASS="funref"
><B
>eql</B
></A
>, those whose keys are compared with <A HREF="f_equal.html" CLASS="funref"
><B
>equal</B
></A
>, and those whose keys are compared with <A HREF="f_equalp.html" CLASS="funref"
><B
>equalp</B
></A
>.</P
></LI
><LI
><P CLASS="j"
><A HREF="26_1_Glossary.html#hash_table"
><EM CLASS="term"
>Hash tables</EM
></A
> are created by <A HREF="f_make-hash-table.html" CLASS="funref"
><B
>make-hash-table</B
></A
>. <A HREF="f_gethash.html" CLASS="funref"
><B
>gethash</B
></A
> is used to look up a key and find the associated value. New entries are added to <A HREF="26_1_Glossary.html#hash_table"
><EM CLASS="term"
>hash tables</EM
></A
> using <A HREF="f_setf.html" CLASS="macref"
><B
>setf</B
></A
> with <A HREF="f_gethash.html" CLASS="funref"
><B
>gethash</B
></A
>. <A HREF="f_remhash.html" CLASS="funref"
><B
>remhash</B
></A
> is used to remove an entry. For example:</P
><PRE CLASS="screen"
>(setq a (make-hash-table)) <SPAN CLASS="cmsy"
><SPAN CLASS="arrow"
>&#8594;</SPAN
></SPAN
> #&lt;HASH-TABLE EQL 0/120 32536573&gt;
(setf (gethash 'color a) 'brown) <SPAN CLASS="cmsy"
><SPAN CLASS="arrow"
>&#8594;</SPAN
></SPAN
> BROWN
(setf (gethash 'name a) 'fred) <SPAN CLASS="cmsy"
><SPAN CLASS="arrow"
>&#8594;</SPAN
></SPAN
> FRED
(gethash 'color a) <SPAN CLASS="cmsy"
><SPAN CLASS="arrow"
>&#8594;</SPAN
></SPAN
> BROWN, <A HREF="26_1_Glossary.html#true"
><EM CLASS="term"
>true</EM
></A
>
(gethash 'name a) <SPAN CLASS="cmsy"
><SPAN CLASS="arrow"
>&#8594;</SPAN
></SPAN
> FRED, <A HREF="26_1_Glossary.html#true"
><EM CLASS="term"
>true</EM
></A
>
(gethash 'pointy a) <SPAN CLASS="cmsy"
><SPAN CLASS="arrow"
>&#8594;</SPAN
></SPAN
> NIL, <A HREF="26_1_Glossary.html#false"
><EM CLASS="term"
>false</EM
></A
></PRE
><P CLASS="j"
>In this example, the symbols <CODE CLASS="f"
>color</CODE
> and <CODE CLASS="f"
>name</CODE
> are being used as keys, and the symbols <CODE CLASS="f"
>brown</CODE
> and <CODE CLASS="f"
>fred</CODE
> are being used as the associated values. The <A HREF="26_1_Glossary.html#hash_table"
><EM CLASS="term"
>hash table</EM
></A
> has two items in it, one of which associates from <CODE CLASS="f"
>color</CODE
> to <CODE CLASS="f"
>brown</CODE
>, and the other of which associates from <CODE CLASS="f"
>name</CODE
> to <CODE CLASS="f"
>fred</CODE
>.</P
></LI
><LI
><P CLASS="j"
>A key or a value may be any <A HREF="26_1_Glossary.html#object"
><EM CLASS="term"
>object</EM
></A
>.</P
></LI
><LI
><P CLASS="j"
>The existence of an entry in the <A HREF="26_1_Glossary.html#hash_table"
><EM CLASS="term"
>hash table</EM
></A
> can be determined from the <A HREF="26_1_Glossary.html#secondary_value"
><EM CLASS="term"
>secondary value</EM
></A
> returned by <A HREF="f_gethash.html" CLASS="funref"
><B
>gethash</B
></A
>.</P
></LI
></UL
><DIV CLASS="displaythree"
><FIGURE CLASS="boxfig"
><DIV CLASS="figbody"
><TABLE CELLSPACING="0" CELLPADDING="0" RULES="GROUPS" STYLE="margin: 1ex 0" WIDTH="100%"
><TBODY
><TR
><TD ALIGN="LEFT" VALIGN="BASELINE" NOWRAP="NOWRAP" STYLE=";padding-right:1em"
><A HREF="f_clrhash.html" CLASS="misc"
><B
>clrhash</B
></A
></TD
><TD ALIGN="LEFT" VALIGN="BASELINE" NOWRAP="NOWRAP" STYLE="padding-left:1em;padding-right:1em"
> <A HREF="f_hash-table-p.html" CLASS="misc"
><B
>hash-table-p</B
></A
></TD
><TD ALIGN="LEFT" VALIGN="BASELINE" NOWRAP="NOWRAP" STYLE="padding-left:1em;"
> <A HREF="f_remhash.html" CLASS="misc"
><B
>remhash</B
></A
></TD
></TR
><TR
><TD ALIGN="LEFT" VALIGN="BASELINE" NOWRAP="NOWRAP" STYLE=";padding-right:1em"
> <A HREF="f_gethash.html" CLASS="misc"
><B
>gethash</B
></A
></TD
><TD ALIGN="LEFT" VALIGN="BASELINE" NOWRAP="NOWRAP" STYLE="padding-left:1em;padding-right:1em"
> <A HREF="f_make-hash-table.html" CLASS="misc"
><B
>make-hash-table</B
></A
></TD
><TD ALIGN="LEFT" VALIGN="BASELINE" NOWRAP="NOWRAP" STYLE="padding-left:1em;"
> <A HREF="f_sxhash.html" CLASS="misc"
><B
>sxhash</B
></A
></TD
></TR
><TR
><TD ALIGN="LEFT" VALIGN="BASELINE" NOWRAP="NOWRAP" STYLE=";padding-right:1em"
> <A HREF="f_hash-table-count.html" CLASS="misc"
><B
>hash-table-count</B
></A
></TD
><TD ALIGN="LEFT" VALIGN="BASELINE" NOWRAP="NOWRAP" STYLE="padding-left:1em;padding-right:1em"
> <A HREF="f_maphash.html" CLASS="misc"
><B
>maphash</B
></A
></TD
><TD ALIGN="LEFT" VALIGN="BASELINE" NOWRAP="NOWRAP" STYLE="padding-left:1em;"
><SPAN CLASS="misc"
></SPAN
></TD
></TR
></TBODY
></TABLE
></DIV
><FIGCAPTION CLASS="caption"
><B
>Figure 18&#8211;1. Hash-table defined names</B
></FIGCAPTION
></FIGURE
></DIV
><H3 ID="sec_18_1_2" CLASS="HeadIII"
>18.1.2 Modifying Hash Table Keys</H3
><P CLASS="j"
>The function supplied as the <SPAN CLASS="kwd"
><SPAN CLASS="cmtt"
>:test</SPAN
></SPAN
> argument to <A HREF="f_make-hash-table.html" CLASS="funref"
><B
>make-hash-table</B
></A
> specifies the &#8216;equivalence test&#8217; for the <A HREF="26_1_Glossary.html#hash_table"
><EM CLASS="term"
>hash table</EM
></A
> it creates. </P
><P CLASS="j"
>An <A HREF="26_1_Glossary.html#object"
><EM CLASS="term"
>object</EM
></A
> is &#8216;visibly modified&#8217; with regard to an equivalence test if there exists some set of <A HREF="26_1_Glossary.html#object"
><EM CLASS="term"
>objects</EM
></A
> (or potential <A HREF="26_1_Glossary.html#object"
><EM CLASS="term"
>objects</EM
></A
>) which are equivalent to the <A HREF="26_1_Glossary.html#object"
><EM CLASS="term"
>object</EM
></A
> before the modification but are no longer equivalent afterwards. </P
><P CLASS="j"
>If an <A HREF="26_1_Glossary.html#object"
><EM CLASS="term"
>object</EM
></A
> <I
>O</I
><SUB CLASS="sub"
><SPAN CLASS="cmr"
>1</SPAN
></SUB
> is used as a key in a <A HREF="26_1_Glossary.html#hash_table"
><EM CLASS="term"
>hash table</EM
></A
> <I
>H</I
> and is then visibly modified with regard to the equivalence test of <I
>H</I
>, then the consequences are unspecified if <I
>O</I
><SUB CLASS="sub"
><SPAN CLASS="cmr"
>1</SPAN
></SUB
>, or any <A HREF="26_1_Glossary.html#object"
><EM CLASS="term"
>object</EM
></A
> <I
>O</I
><SUB CLASS="sub"
><SPAN CLASS="cmr"
>2</SPAN
></SUB
> equivalent to <I
>O</I
><SUB CLASS="sub"
><SPAN CLASS="cmr"
>1</SPAN
></SUB
> under the equivalence test (either before or after the modification), is used as a key in further operations on <I
>H</I
>. The consequences of using <I
>O</I
><SUB CLASS="sub"
><SPAN CLASS="cmr"
>1</SPAN
></SUB
> as a key are unspecified even if <I
>O</I
><SUB CLASS="sub"
><SPAN CLASS="cmr"
>1</SPAN
></SUB
> is visibly modified and then later modified again in such a way as to undo the visible modification. </P
><P CLASS="j"
>Following are specifications of the modifications which are visible to the equivalence tests which must be supported by <A HREF="26_1_Glossary.html#hash_table"
><EM CLASS="term"
>hash tables</EM
></A
>. The modifications are described in terms of modification of components, and are defined recursively. Visible modifications of components of the <A HREF="26_1_Glossary.html#object"
><EM CLASS="term"
>object</EM
></A
> are visible modifications of the <A HREF="26_1_Glossary.html#object"
><EM CLASS="term"
>object</EM
></A
>.</P
><H4 ID="sec_18_1_2_1" CLASS="HeadIV"
>18.1.2.1 Visible Modification of Objects with respect to EQ and EQL</H4
><P CLASS="j"
>No <A HREF="26_1_Glossary.html#standardized"
><EM CLASS="term"
>standardized</EM
></A
> <A HREF="26_1_Glossary.html#function"
><EM CLASS="term"
>function</EM
></A
> is provided that is capable of visibly modifying an <A HREF="26_1_Glossary.html#object"
><EM CLASS="term"
>object</EM
></A
> with regard to <A HREF="f_eq.html" CLASS="funref"
><B
>eq</B
></A
> or <A HREF="f_eql.html" CLASS="funref"
><B
>eql</B
></A
>.</P
><H4 ID="sec_18_1_2_2" CLASS="HeadIV"
>18.1.2.2 Visible Modification of Objects with respect to EQUAL</H4
><P CLASS="j"
>As a consequence of the behavior for <A HREF="f_equal.html" CLASS="funref"
><B
>equal</B
></A
>, the rules for visible modification of <A HREF="26_1_Glossary.html#object"
><EM CLASS="term"
>objects</EM
></A
> not explicitly mentioned in this section are inherited from those in <A HREF="18_1_Hash_Table_Concepts.html#sec_18_1_2_1" CLASS="secref"
><SPAN CLASS="cmr"
>Section</SPAN
> <SPAN CLASS="cmr"
>18.1.2.1</SPAN
> <SPAN CLASS="cmr"
>(Visible</SPAN
> <SPAN CLASS="cmr"
>Modification</SPAN
> <SPAN CLASS="cmr"
>of</SPAN
> <SPAN CLASS="cmr"
>Objects</SPAN
> <SPAN CLASS="cmr"
>with</SPAN
> <SPAN CLASS="cmr"
>respect</SPAN
> <SPAN CLASS="cmr"
>to</SPAN
> <SPAN CLASS="cmr"
>EQ</SPAN
> <SPAN CLASS="cmr"
>and</SPAN
> <SPAN CLASS="cmr"
>EQL)</SPAN
></A
>.</P
><H5 ID="sec_18_1_2_2_1" CLASS="HeadV"
>18.1.2.2.1 Visible Modification of Conses with respect to EQUAL</H5
><P CLASS="j"
>Any visible change to the <A HREF="26_1_Glossary.html#car"
><EM CLASS="term"
>car</EM
></A
> or the <A HREF="26_1_Glossary.html#cdr"
><EM CLASS="term"
>cdr</EM
></A
> of a <A HREF="26_1_Glossary.html#cons"
><EM CLASS="term"
>cons</EM
></A
> is considered a visible modification with regard to <A HREF="f_equal.html" CLASS="funref"
><B
>equal</B
></A
>.</P
><H5 ID="sec_18_1_2_2_2" CLASS="HeadV"
>18.1.2.2.2 Visible Modification of Bit Vectors and Strings with respect to EQUAL</H5
><P CLASS="j"
>For a <A HREF="26_1_Glossary.html#vector"
><EM CLASS="term"
>vector</EM
></A
> of <A HREF="26_1_Glossary.html#type"
><EM CLASS="term"
>type</EM
></A
> <A HREF="t_bit-vector.html" CLASS="typeref"
><B
>bit-vector</B
></A
> or of <A HREF="26_1_Glossary.html#type"
><EM CLASS="term"
>type</EM
></A
> <A HREF="t_string.html" CLASS="typeref"
><B
>string</B
></A
>, any visible change to an <A HREF="26_1_Glossary.html#active"
><EM CLASS="term"
>active</EM
></A
> <A HREF="26_1_Glossary.html#element"
><EM CLASS="term"
>element</EM
></A
> of the <A HREF="26_1_Glossary.html#vector"
><EM CLASS="term"
>vector</EM
></A
>, or to the <A HREF="26_1_Glossary.html#length"
><EM CLASS="term"
>length</EM
></A
> of the <A HREF="26_1_Glossary.html#vector"
><EM CLASS="term"
>vector</EM
></A
> (if it is <A HREF="26_1_Glossary.html#actually_adjustable"
><EM CLASS="term"
>actually adjustable</EM
></A
> or has a <A HREF="26_1_Glossary.html#fill_pointer"
><EM CLASS="term"
>fill pointer</EM
></A
>) is considered a visible modification with regard to <A HREF="f_equal.html" CLASS="funref"
><B
>equal</B
></A
>.</P
><H4 ID="sec_18_1_2_3" CLASS="HeadIV"
>18.1.2.3 Visible Modification of Objects with respect to EQUALP</H4
><P CLASS="j"
>As a consequence of the behavior for <A HREF="f_equalp.html" CLASS="funref"
><B
>equalp</B
></A
>, the rules for visible modification of <A HREF="26_1_Glossary.html#object"
><EM CLASS="term"
>objects</EM
></A
> not explicitly mentioned in this section are inherited from those in <A HREF="18_1_Hash_Table_Concepts.html#sec_18_1_2_2" CLASS="secref"
><SPAN CLASS="cmr"
>Section</SPAN
> <SPAN CLASS="cmr"
>18.1.2.2</SPAN
> <SPAN CLASS="cmr"
>(Visible</SPAN
> <SPAN CLASS="cmr"
>Modification</SPAN
> <SPAN CLASS="cmr"
>of</SPAN
> <SPAN CLASS="cmr"
>Objects</SPAN
> <SPAN CLASS="cmr"
>with</SPAN
> <SPAN CLASS="cmr"
>respect</SPAN
> <SPAN CLASS="cmr"
>to</SPAN
> <SPAN CLASS="cmr"
>EQUAL)</SPAN
></A
>.</P
><H5 ID="sec_18_1_2_3_1" CLASS="HeadV"
>18.1.2.3.1 Visible Modification of Structures with respect to EQUALP</H5
><P CLASS="j"
>Any visible change to a <A HREF="26_1_Glossary.html#slot"
><EM CLASS="term"
>slot</EM
></A
> of a <A HREF="26_1_Glossary.html#structure"
><EM CLASS="term"
>structure</EM
></A
> is considered a visible modification with regard to <A HREF="f_equalp.html" CLASS="funref"
><B
>equalp</B
></A
>.</P
><H5 ID="sec_18_1_2_3_2" CLASS="HeadV"
>18.1.2.3.2 Visible Modification of Arrays with respect to EQUALP</H5
><P CLASS="j"
>In an <A HREF="26_1_Glossary.html#array"
><EM CLASS="term"
>array</EM
></A
>, any visible change to an <A HREF="26_1_Glossary.html#active"
><EM CLASS="term"
>active</EM
></A
> <A HREF="26_1_Glossary.html#element"
><EM CLASS="term"
>element</EM
></A
>, to the <A HREF="26_1_Glossary.html#fill_pointer"
><EM CLASS="term"
>fill pointer</EM
></A
> (if the <A HREF="26_1_Glossary.html#array"
><EM CLASS="term"
>array</EM
></A
> can and does have one), or to the <A HREF="26_1_Glossary.html#dimension"
><EM CLASS="term"
>dimensions</EM
></A
> (if the <A HREF="26_1_Glossary.html#array"
><EM CLASS="term"
>array</EM
></A
> is <A HREF="26_1_Glossary.html#actually_adjustable"
><EM CLASS="term"
>actually adjustable</EM
></A
>) is considered a visible modification with regard to <A HREF="f_equalp.html" CLASS="funref"
><B
>equalp</B
></A
>.</P
><H5 ID="sec_18_1_2_3_3" CLASS="HeadV"
>18.1.2.3.3 Visible Modification of Hash Tables with respect to EQUALP</H5
><P CLASS="j"
>In a <A HREF="26_1_Glossary.html#hash_table"
><EM CLASS="term"
>hash table</EM
></A
>, any visible change to the count of entries in the <A HREF="26_1_Glossary.html#hash_table"
><EM CLASS="term"
>hash table</EM
></A
>, to the keys, or to the values associated with the keys is considered a visible modification with regard to <A HREF="f_equalp.html" CLASS="funref"
><B
>equalp</B
></A
>. </P
><P CLASS="j"
>Note that the visibility of modifications to the keys depends on the equivalence test of the <A HREF="26_1_Glossary.html#hash_table"
><EM CLASS="term"
>hash table</EM
></A
>, not on the specification of <A HREF="f_equalp.html" CLASS="funref"
><B
>equalp</B
></A
>.</P
><H4 ID="sec_18_1_2_4" CLASS="HeadIV"
>18.1.2.4 Visible Modifications by Language Extensions</H4
><P CLASS="j"
><A HREF="26_1_Glossary.html#implementation"
><EM CLASS="term"
>Implementations</EM
></A
> that extend the language by providing additional mutator functions (or additional behavior for existing mutator functions) must document how the use of these extensions interacts with equivalence tests and <A HREF="26_1_Glossary.html#hash_table"
><EM CLASS="term"
>hash table</EM
></A
> searches. </P
><P CLASS="j"
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