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47 lines
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<TITLE>CLHS: Section 18.1.2</TITLE>
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<H1><A REV=MADE HREF="http://www.lispworks.com/"><IMG WIDTH=80 HEIGHT=65 ALT="[LISPWORKS]" SRC="../Graphics/LWSmall.gif" ALIGN=Bottom></A><A REL=TOP HREF="../Front/index.htm"><IMG WIDTH=237 HEIGHT=65 ALT="[Common Lisp HyperSpec (TM)]" SRC="../Graphics/CLHS_Sm.gif" ALIGN=Bottom></A> <A REL=PREV HREF="18_aa.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="18_a.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="18_aba.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>
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18.1.2 Modifying Hash Table Keys</H2> <P>
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The function supplied as the <TT>:test</TT> argument to <A REL=DEFINITION HREF="f_mk_has.htm#make-hash-table"><B>make-hash-table</B></A> specifies the `equivalence test' for the <A REL=DEFINITION HREF="26_glo_h.htm#hash_table"><I>hash table</I></A> it creates. <P>
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An <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> is `visibly modified' with regard to an equivalence test if there exists some set of <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>objects</I></A> (or potential <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>objects</I></A>) which are equivalent to the <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> before the modification but are no longer equivalent afterwards. <P>
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If an <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> O1 is used as a key in a <A REL=DEFINITION HREF="26_glo_h.htm#hash_table"><I>hash table</I></A> H and is then visibly modified with regard to the equivalence test of H, then the consequences are unspecified if O1, or any <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> O2 equivalent to O1 under the equivalence test (either before or after the modification), is used as a key in further operations on H. The consequences of using O1 as a key are unspecified even if O1 is visibly modified and then later modified again in such a way as to undo the visible modification. <P>
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Following are specifications of the modifications which are visible to the equivalence tests which must be supported by <A REL=DEFINITION HREF="26_glo_h.htm#hash_table"><I>hash tables</I></A>. The modifications are described in terms of modification of components, and are defined recursively. Visible modifications of components of the <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> are visible modifications of the <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A>. <P>
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<A REL=CHILD HREF="18_aba.htm"><H2>
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18.1.2.1 Visible Modification of Objects with respect to EQ and EQL</H2></A><P>
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<A REL=CHILD HREF="18_abb.htm"><H2>
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18.1.2.2 Visible Modification of Objects with respect to EQUAL</H2></A><P>
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<A REL=CHILD HREF="18_abc.htm"><H2>
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18.1.2.3 Visible Modification of Objects with respect to EQUALP</H2></A><P>
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<A REL=CHILD HREF="18_abd.htm"><H2>
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18.1.2.4 Visible Modifications by Language Extensions</H2></A><P><P>
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<P>
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<P><HR>The following <A REL=META HREF="../Front/X3J13Iss.htm">X3J13 cleanup issue</A>, <I>not part of the specification</I>, applies to this section:<P><UL><LI> <A REL=CHILD HREF="../Issues/iss187.htm">HASH-TABLE-KEY-MODIFICATION:SPECIFY</A><P></UL><HR>
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