82 lines
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82 lines
6.3 KiB
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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: Accessor GETHASH</TITLE>
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<A NAME="gethash"><I>Accessor</I> <B>GETHASH</B></A> <P>
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<P><B>Syntax:</B><P>
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<P>
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<B>gethash</B> <I>key hash-table <TT>&optional</TT> default</I> => <I>value, present-p</I><P>
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<TT>(setf (</TT><B>gethash</B> <I>key hash-table <TT>&optional</TT> default</I><TT>) </TT><I>new-value</I><TT>)</TT><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>key</I>---an <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A>. <P>
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<I>hash-table</I>---a <A REL=DEFINITION HREF="26_glo_h.htm#hash_table"><I>hash table</I></A>. <P>
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<I>default</I>---an <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A>. The default is <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A>. <P>
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<I>value</I>---an <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A>. <P>
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<I>present-p</I>---a <A REL=DEFINITION HREF="26_glo_g.htm#generalized_boolean"><I>generalized boolean</I></A>. <P>
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<P><B>Description:</B><P>
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<P>
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<I>Value</I> is the <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> in <I>hash-table</I> whose <A REL=DEFINITION HREF="26_glo_k.htm#key"><I>key</I></A> is the <A REL=DEFINITION HREF="26_glo_s.htm#same"><I>same</I></A> as <I>key</I> under the <I>hash-table</I>'s equivalence test. If there is no such entry, <I>value</I> is the <I>default</I>. <P>
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<I>Present-p</I> is <A REL=DEFINITION HREF="26_glo_t.htm#true"><I>true</I></A> if an entry is found; otherwise, it is <A REL=DEFINITION HREF="26_glo_f.htm#false"><I>false</I></A>. <P>
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<A REL=DEFINITION HREF="m_setf_.htm#setf"><B>setf</B></A> may be used with <A REL=DEFINITION HREF="#gethash"><B>gethash</B></A> to modify the <A REL=DEFINITION HREF="26_glo_v.htm#value"><I>value</I></A> associated with a given <A REL=DEFINITION HREF="26_glo_k.htm#key"><I>key</I></A>, or to add a new entry. When a <A REL=DEFINITION HREF="#gethash"><B>gethash</B></A> <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A> is used as a <A REL=DEFINITION HREF="m_setf_.htm#setf"><B>setf</B></A> <I>place</I>, any <I>default</I> which is supplied is evaluated according to normal left-to-right evaluation rules, but its <A REL=DEFINITION HREF="26_glo_v.htm#value"><I>value</I></A> is ignored. <P>
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<P><B>Examples:</B><P>
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<P>
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<PRE>
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(setq table (make-hash-table)) => #<HASH-TABLE EQL 0/120 32206334>
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(gethash 1 table) => NIL, <A REL=DEFINITION HREF="26_glo_f.htm#false">false</A>
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(gethash 1 table 2) => 2, <A REL=DEFINITION HREF="26_glo_f.htm#false">false</A>
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(setf (gethash 1 table) "one") => "one"
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(setf (gethash 2 table "two") "two") => "two"
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(gethash 1 table) => "one", <A REL=DEFINITION HREF="26_glo_t.htm#true">true</A>
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(gethash 2 table) => "two", <A REL=DEFINITION HREF="26_glo_t.htm#true">true</A>
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(gethash nil table) => NIL, <A REL=DEFINITION HREF="26_glo_f.htm#false">false</A>
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(setf (gethash nil table) nil) => NIL
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(gethash nil table) => NIL, <A REL=DEFINITION HREF="26_glo_t.htm#true">true</A>
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(defvar *counters* (make-hash-table)) => *COUNTERS*
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(gethash 'foo *counters*) => NIL, <A REL=DEFINITION HREF="26_glo_f.htm#false">false</A>
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(gethash 'foo *counters* 0) => 0, <A REL=DEFINITION HREF="26_glo_f.htm#false">false</A>
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(defmacro how-many (obj) `(values (gethash ,obj *counters* 0))) => HOW-MANY
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(defun count-it (obj) (incf (how-many obj))) => COUNT-IT
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(dolist (x '(bar foo foo bar bar baz)) (count-it x))
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(how-many 'foo) => 2
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(how-many 'bar) => 3
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(how-many 'quux) => 0
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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> None.
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<P>
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<P><B>See Also:</B><P>
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<P>
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<A REL=DEFINITION HREF="f_remhas.htm#remhash"><B>remhash</B></A> <P>
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<P><B>Notes:</B><P>
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<P>
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The <A REL=DEFINITION HREF="26_glo_s.htm#secondary_value"><I>secondary value</I></A>, <I>present-p</I>, can be used to distinguish the absence of an entry from the presence of an entry that has a value of <I>default</I>. <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/iss306.htm">SETF-GET-DEFAULT:EVALUATED-BUT-IGNORED</A><P></UL><HR>
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