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95 lines
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<!-- Common Lisp HyperSpec (TM), version 7.0 generated by Kent M. Pitman on Mon, 11-Apr-2005 2:31am EDT -->
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<TITLE>CLHS: Macro WITH-HASH-TABLE-ITERATOR</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="f_maphas.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="c_hash_t.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="f_clrhas.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<A NAME="with-hash-table-iterator"><I>Macro</I> <B>WITH-HASH-TABLE-ITERATOR</B></A> <P>
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<P>
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<P>
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<P><B>Syntax:</B><P>
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<P>
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<B>with-hash-table-iterator</B> <I>(name hash-table) <I>declaration</I><B>*</B> <I>form</I><B>*</B></I> => <I><I>result</I><B>*</B></I><P>
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<P>
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<P><B>Arguments and Values:</B><P>
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<P>
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<I>name</I>---a name suitable for the first argument to <A REL=DEFINITION HREF="s_flet_.htm#macrolet"><B>macrolet</B></A>. <P>
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<I>hash-table</I>---a <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A>, evaluated once, that should produce a <A REL=DEFINITION HREF="26_glo_h.htm#hash_table"><I>hash table</I></A>. <P>
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<I>declaration</I>---a <A REL=DEFINITION HREF="s_declar.htm#declare"><B>declare</B></A> <A REL=DEFINITION HREF="26_glo_e.htm#expression"><I>expression</I></A>; not evaluated. <P>
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<I>forms</I>---an <A REL=DEFINITION HREF="26_glo_i.htm#implicit_progn"><I>implicit progn</I></A>. <P>
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<I>results</I>---the <A REL=DEFINITION HREF="26_glo_v.htm#value"><I>values</I></A> returned by <I>forms</I>. <P>
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<P><B>Description:</B><P>
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<P>
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Within the lexical scope of the body, <I>name</I> is defined via <A REL=DEFINITION HREF="s_flet_.htm#macrolet"><B>macrolet</B></A> such that successive invocations of <TT>(</TT><I>name</I><TT>)</TT> return the items, one by one, from the <A REL=DEFINITION HREF="26_glo_h.htm#hash_table"><I>hash table</I></A> that is obtained by evaluating <I>hash-table</I> only once. <P>
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An invocation <TT>(</TT><I>name</I><TT>)</TT> returns three values as follows: <P>
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<P><DL><DT>1. A <A REL=DEFINITION HREF="26_glo_g.htm#generalized_boolean"><I>generalized boolean</I></A> that is <A REL=DEFINITION HREF="26_glo_t.htm#true"><I>true</I></A> if an entry is returned. <DD><DT>2. The key from the <I>hash-table</I> entry. <DD><DT>3. The value from the <I>hash-table</I> entry. <P><DD></DL>After all entries have been returned by successive invocations of <TT>(</TT><I>name</I><TT>)</TT>, then only one value is returned, namely <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A>. <P>
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It is unspecified what happens if any of the implicit interior state of an iteration is returned outside the dynamic extent of the <A REL=DEFINITION HREF="#with-hash-table-iterator"><B>with-hash-table-iterator</B></A> <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A> such as by returning some <A REL=DEFINITION HREF="26_glo_c.htm#closure"><I>closure</I></A> over the invocation <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A>. <P>
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Any number of invocations of <A REL=DEFINITION HREF="#with-hash-table-iterator"><B>with-hash-table-iterator</B></A> can be nested, and the body of the innermost one can invoke all of the locally <I>established</I> <A REL=DEFINITION HREF="26_glo_m.htm#macro"><I>macros</I></A>, provided all of those <A REL=DEFINITION HREF="26_glo_m.htm#macro"><I>macros</I></A> have <A REL=DEFINITION HREF="26_glo_d.htm#distinct"><I>distinct</I></A> names. <P>
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<P><B>Examples:</B><P>
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<P>
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The following function should return <A REL=DEFINITION HREF="a_t.htm#t"><B>t</B></A> on any <A REL=DEFINITION HREF="26_glo_h.htm#hash_table"><I>hash table</I></A>, and signal an error if the usage of <A REL=DEFINITION HREF="#with-hash-table-iterator"><B>with-hash-table-iterator</B></A> does not agree with the corresponding usage of <A REL=DEFINITION HREF="f_maphas.htm#maphash"><B>maphash</B></A>. <P>
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<PRE>
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(defun test-hash-table-iterator (hash-table)
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(let ((all-entries '())
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(generated-entries '())
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(unique (list nil)))
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(maphash #'(lambda (key value) (push (list key value) all-entries))
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hash-table)
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(with-hash-table-iterator (generator-fn hash-table)
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(loop
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(multiple-value-bind (more? key value) (generator-fn)
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(unless more? (return))
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(unless (eql value (gethash key hash-table unique))
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(error "Key ~S not found for value ~S" key value))
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(push (list key value) generated-entries))))
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(unless (= (length all-entries)
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(length generated-entries)
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(length (union all-entries generated-entries
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:key #'car :test (hash-table-test hash-table))))
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(error "Generated entries and Maphash entries don't correspond"))
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t))
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</PRE>
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</TT> <P>
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The following could be an acceptable definition of <A REL=DEFINITION HREF="f_maphas.htm#maphash"><B>maphash</B></A>, implemented by <A REL=DEFINITION HREF="#with-hash-table-iterator"><B>with-hash-table-iterator</B></A>. <P>
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<PRE>
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(defun maphash (function hash-table)
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(with-hash-table-iterator (next-entry hash-table)
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(loop (multiple-value-bind (more key value) (next-entry)
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(unless more (return nil))
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(funcall function key value)))))
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</PRE>
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</TT> <P>
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<P><B>Side Effects:</B> None.
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<P>
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<P><B>Affected By:</B> None.
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<P>
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<P><B>Exceptional Situations:</B><P>
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<P>
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The consequences are undefined if the local function named <I>name</I> <I>established</I> by <A REL=DEFINITION HREF="#with-hash-table-iterator"><B>with-hash-table-iterator</B></A> is called after it has returned <A REL=DEFINITION HREF="26_glo_f.htm#false"><I>false</I></A> as its <A REL=DEFINITION HREF="26_glo_p.htm#primary_value"><I>primary value</I></A>. <P>
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<P><B>See Also:</B><P>
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<P>
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<A REL=CHILD HREF="03_f.htm">Section 3.6 (Traversal Rules and Side Effects)</A> <P>
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<P><B>Notes:</B> None.
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<P>
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<P>
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<P><HR>The following <A REL=META HREF="../Front/X3J13Iss.htm">X3J13 cleanup issues</A>, <I>not part of the specification</I>, apply to this section:<P><UL><LI> <A REL=CHILD HREF="../Issues/iss240.htm">MAPPING-DESTRUCTIVE-INTERACTION:EXPLICITLY-VAGUE</A><LI> <A REL=CHILD HREF="../Issues/iss188.htm">HASH-TABLE-PACKAGE-GENERATORS:ADD-WITH-WRAPPER</A><LI> <A REL=CHILD HREF="../Issues/iss097.htm">DECLS-AND-DOC</A><P></UL><HR>
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