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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: Function SET-DIFFERENCE, NSET-DIFFERENCE</TITLE>
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<A NAME="set-difference"><A NAME="nset-difference"><I>Function</I> <B>SET-DIFFERENCE, NSET-DIFFERENCE</B></A></A> <P>
<P><B>Syntax:</B><P>
<P>
<B>set-difference</B> <I>list-1 list-2 <TT>&amp;key</TT> key test test-not</I> =&gt; <I>result-list</I><P>
<B>nset-difference</B> <I>list-1 list-2 <TT>&amp;key</TT> key test test-not</I> =&gt; <I>result-list</I><P>
<P>
<P><B>Arguments and Values:</B><P>
<P>
<I>list-1</I>---a <A REL=DEFINITION HREF="26_glo_p.htm#proper_list"><I>proper list</I></A>. <P>
<I>list-2</I>---a <A REL=DEFINITION HREF="26_glo_p.htm#proper_list"><I>proper list</I></A>. <P>
<I>test</I>---a <A REL=DEFINITION HREF="26_glo_d.htm#designator"><I>designator</I></A> for a <A REL=DEFINITION HREF="26_glo_f.htm#function"><I>function</I></A> of two <A REL=DEFINITION HREF="26_glo_a.htm#argument"><I>arguments</I></A> that returns a <A REL=DEFINITION HREF="26_glo_g.htm#generalized_boolean"><I>generalized boolean</I></A>. <P>
<I>test-not</I>---a <A REL=DEFINITION HREF="26_glo_d.htm#designator"><I>designator</I></A> for a <A REL=DEFINITION HREF="26_glo_f.htm#function"><I>function</I></A> of two <A REL=DEFINITION HREF="26_glo_a.htm#argument"><I>arguments</I></A> that returns a <A REL=DEFINITION HREF="26_glo_g.htm#generalized_boolean"><I>generalized boolean</I></A>. <P>
<I>key</I>---a <A REL=DEFINITION HREF="26_glo_d.htm#designator"><I>designator</I></A> for a <A REL=DEFINITION HREF="26_glo_f.htm#function"><I>function</I></A> of one argument, or <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A>. <P>
<I>result-list</I>---a <A REL=DEFINITION HREF="26_glo_l.htm#list"><I>list</I></A>. <P>
<P><B>Description:</B><P>
<A REL=DEFINITION HREF="#set-difference"><B>set-difference</B></A> returns a <A REL=DEFINITION HREF="26_glo_l.htm#list"><I>list</I></A> of elements of <I>list-1</I> that do not appear in <I>list-2</I>. <P>
<A REL=DEFINITION HREF="#nset-difference"><B>nset-difference</B></A> is the destructive version of <A REL=DEFINITION HREF="#set-difference"><B>set-difference</B></A>. It may destroy <I>list-1</I>. <P>
For all possible ordered pairs consisting of one element from <I>list-1</I> and one element from <I>list-2</I>, the <TT>:test</TT> or <TT>:test-not</TT> function is used to determine whether they <A REL=DEFINITION HREF="26_glo_s.htm#satisfy_the_test"><I>satisfy the test</I></A>. The first argument to the <TT>:test</TT> or <TT>:test-not</TT> function is the part of an element of <I>list-1</I> that is returned by the <TT>:key</TT> function (if supplied); the second argument is the part of an element of <I>list-2</I> that is returned by the <TT>:key</TT> function (if supplied). <P>
If <TT>:key</TT> is supplied, its argument is a <I>list-1</I> or <I>list-2</I> element. The <TT>:key</TT> function typically returns part of the supplied element. If <TT>:key</TT> is not supplied, the <I>list-1</I> or <I>list-2</I> element is used. <P>
An element of <I>list-1</I> appears in the result if and only if it does not match any element of <I>list-2</I>. <P>
There is no guarantee that the order of elements in the result will reflect the ordering of the arguments in any particular way. The result <A REL=DEFINITION HREF="26_glo_l.htm#list"><I>list</I></A> may share cells with, or be <A REL=DEFINITION HREF="f_eq.htm#eq"><B>eq</B></A> to, either of <I>list-1</I> or <I>list-2</I>, if appropriate. <P>
<P><B>Examples:</B><P>
<P>
<PRE>
(setq lst1 (list &quot;A&quot; &quot;b&quot; &quot;C&quot; &quot;d&quot;)
lst2 (list &quot;a&quot; &quot;B&quot; &quot;C&quot; &quot;d&quot;)) =&gt; (&quot;a&quot; &quot;B&quot; &quot;C&quot; &quot;d&quot;)
(set-difference lst1 lst2) =&gt; (&quot;d&quot; &quot;C&quot; &quot;b&quot; &quot;A&quot;)
(set-difference lst1 lst2 :test 'equal) =&gt; (&quot;b&quot; &quot;A&quot;)
(set-difference lst1 lst2 :test #'equalp) =&gt; NIL
(nset-difference lst1 lst2 :test #'string=) =&gt; (&quot;A&quot; &quot;b&quot;)
(setq lst1 '((&quot;a&quot; . &quot;b&quot;) (&quot;c&quot; . &quot;d&quot;) (&quot;e&quot; . &quot;f&quot;)))
=&gt; ((&quot;a&quot; . &quot;b&quot;) (&quot;c&quot; . &quot;d&quot;) (&quot;e&quot; . &quot;f&quot;))
(setq lst2 '((&quot;c&quot; . &quot;a&quot;) (&quot;e&quot; . &quot;b&quot;) (&quot;d&quot; . &quot;a&quot;)))
=&gt; ((&quot;c&quot; . &quot;a&quot;) (&quot;e&quot; . &quot;b&quot;) (&quot;d&quot; . &quot;a&quot;))
(nset-difference lst1 lst2 :test #'string= :key #'cdr)
=&gt; ((&quot;c&quot; . &quot;d&quot;) (&quot;e&quot; . &quot;f&quot;))
lst1 =&gt; ((&quot;a&quot; . &quot;b&quot;) (&quot;c&quot; . &quot;d&quot;) (&quot;e&quot; . &quot;f&quot;))
lst2 =&gt; ((&quot;c&quot; . &quot;a&quot;) (&quot;e&quot; . &quot;b&quot;) (&quot;d&quot; . &quot;a&quot;))
</PRE>
</TT>
<PRE>
;; Remove all flavor names that contain &quot;c&quot; or &quot;w&quot;.
(set-difference '(&quot;strawberry&quot; &quot;chocolate&quot; &quot;banana&quot;
&quot;lemon&quot; &quot;pistachio&quot; &quot;rhubarb&quot;)
'(#\c #\w)
:test #'(lambda (s c) (find c s)))
=&gt; (&quot;banana&quot; &quot;rhubarb&quot; &quot;lemon&quot;) ;One possible ordering.
</PRE>
</TT> <P>
<P><B>Side Effects:</B><P>
<P>
<A REL=DEFINITION HREF="#nset-difference"><B>nset-difference</B></A> may destroy <I>list-1</I>. <P>
<P><B>Affected By:</B> None.
<P>
<P><B>Exceptional Situations:</B><P>
<P>
Should be prepared to signal an error of <A REL=DEFINITION HREF="26_glo_t.htm#type"><I>type</I></A> <A REL=DEFINITION HREF="e_tp_err.htm#type-error"><B>type-error</B></A> if <I>list-1</I> and <I>list-2</I> are not <A REL=DEFINITION HREF="26_glo_p.htm#proper_list"><I>proper lists</I></A>. <P>
<P><B>See Also:</B><P>
<P>
<A REL=CHILD HREF="03_ba.htm">Section 3.2.1 (Compiler Terminology)</A>, <A REL=CHILD HREF="03_f.htm">Section 3.6 (Traversal Rules and Side Effects)</A> <P>
<P><B>Notes:</B><P>
<P>
The <TT>:test-not</TT> parameter is deprecated. <P>
<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/iss345.htm">TEST-NOT-IF-NOT:FLUSH-ALL</A><LI> <A REL=CHILD HREF="../Issues/iss240.htm">MAPPING-DESTRUCTIVE-INTERACTION:EXPLICITLY-VAGUE</A><LI> <A REL=CHILD HREF="../Issues/iss083.htm">CONSTANT-MODIFICATION:DISALLOW</A><P></UL><HR>
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