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220 lines
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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
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<!Converted with LaTeX2HTML 0.6.5 (Tue Nov 15 1994) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
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<HEAD>
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<TITLE>14.4. Searching Sequences for Items</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Searching Sequences for Items">
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<meta name="keywords" value="clm">
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<meta name="resource-type" value="document">
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<meta name="distribution" value="global">
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<P>
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html3337 HREF="node146.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3335 HREF="node141.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3329 HREF="node144.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3339 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3340 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3338 HREF="node146.html"> Sorting and Merging</A>
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<B>Up:</B> <A NAME=tex2html3336 HREF="node141.html"> Sequences</A>
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<B> Previous:</B> <A NAME=tex2html3330 HREF="node144.html"> Modifying Sequences</A>
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<HR> <P>
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<H1><A NAME=SECTION001840000000000000000>14.4. Searching Sequences for Items</A></H1>
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<P>
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Each of these functions searches a sequence to locate one or more
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elements satisfying some test.
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<P>
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<BR><b>[Function]</b><BR>
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<pre>
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<tt>find <i>item</i> <i>sequence</i> &key :from-end :test :test-not :start :end :key
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find-if <i>predicate</i> <i>sequence</i> &key :from-end :start :end :key
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find-if-not <i>predicate</i> <i>sequence</i> &key :from-end :start :end :key</tt>
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</pre>
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<P>
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If the <i>sequence</i> contains an element satisfying the test,
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then the leftmost such element
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is returned; otherwise <tt>nil</tt> is returned.
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<P>
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If <tt>:start</tt> and <tt>:end</tt> keyword arguments are given,
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only the specified subsequence of <i>sequence</i> is searched.
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<P>
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If a non-<tt>nil :from-end</tt> keyword argument is specified, then the result is
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the <i>rightmost</i> element satisfying the test.
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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X3J13 voted in January 1989
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(MAPPING-DESTRUCTIVE-INTERACTION) <A NAME=15926> </A>
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to restrict user side effects; see section <A HREF="node92.html#STRUCTURETRAVERSALSECTION">7.9</A>.
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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<BR><b>[Function]</b><BR>
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<pre>
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<tt>position <i>item</i> <i>sequence</i> &key :from-end :test :test-not
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:start :end :key
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position-if <i>predicate</i> <i>sequence</i> &key :from-end
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:start :end :key
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position-if-not <i>predicate</i> <i>sequence</i> &key :from-end
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:start :end :key</tt>
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</pre>
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<P>If the <i>sequence</i> contains an element satisfying the test,
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then the index within the sequence of the leftmost such element
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is returned as a non-negative integer; otherwise <tt>nil</tt> is returned.
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<P>
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If <tt>:start</tt> and <tt>:end</tt> keyword arguments are given,
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only the specified subsequence of <i>sequence</i> is searched.
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However, the index returned is relative to the entire sequence,
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not to the subsequence.
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<P>
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If a non-<tt>nil :from-end</tt> keyword argument is specified, then the result is
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the index of the <i>rightmost</i> element satisfying the test. (The index
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returned, however, is an index from the left-hand end, as usual.)
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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X3J13 voted in January 1989
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(MAPPING-DESTRUCTIVE-INTERACTION) <A NAME=15940> </A>
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to restrict user side effects; see section <A HREF="node92.html#STRUCTURETRAVERSALSECTION">7.9</A>.
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<P>
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Here is a simple piece of code that uses several of the sequence
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functions, notably <tt>position-if</tt> and <tt>find-if</tt>,
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to process strings. Note one use of <tt>loop</tt> as well.
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<P><pre>
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(defun debug-palindrome (s)
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(flet ((match (x) (char-equal (first x) (third x))))
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(let* ((pairs (loop for c across s
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for j from 0
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when (alpha-char-p c)
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collect (list c j)))
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(quads (mapcar #'append pairs (reverse pairs)))
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(diffpos (position-if (complement #'match) quads)))
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(when diffpos
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(let* ((diff (elt quads diffpos))
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(same (find-if #'match quads
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:start (+ diffpos 1))))
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(if same
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(format nil
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"/~A/ (at ~D) is not the reverse of /~A/"
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(subseq s (second diff) (second same))
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(second diff)
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(subseq s (+ (fourth same) 1)
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(+ (fourth diff) 1)))
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"This palindrome is completely messed up!"))))))
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</pre><P>
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Here is an example of its behavior.
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<P><pre>
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(setq panama ;A putative palindrome?
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"A man, a plan, a canoe, pasta, heros, rajahs,
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a coloratura, maps, waste, percale, macaroni, a gag,
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a banana bag, a tan, a tag, a banana bag again
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(or a camel), a crepe, pins, Spam, a rut, a Rolo,
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cash, a jar, sore hats, a peon, a canal-Panama!")
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</pre><P>
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<P><pre>
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(debug-palindrome panama)
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=> "/wast/ (at 73) is not the reverse of /, pins/"
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(replace panama "snipe" :start1 73) ;Repair it
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=> "A man, a plan, a canoe, pasta, heros, rajahs,
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a coloratura, maps, snipe, percale, macaroni, a gag,
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a banana bag, a tan, a tag, a banana bag again
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(or a camel), a crepe, pins, Spam, a rut, a Rolo,
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cash, a jar, sore hats, a peon, a canal-Panama!"
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(debug-palindrome panama) => nil ;Copacetic-a true palindrome
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(debug-palindrome "Rubber baby buggy bumpers")
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=> "/Rubber / (at 0) is not the reverse of /umpers/"
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(debug-palindrome "Common Lisp: The Language")
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=> "/Commo/ (at 0) is not the reverse of /guage/"
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(debug-palindrome "Complete mismatches are hard to find")
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=>
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"/Complete mism/ (at 0) is not the reverse of /re hard to find/"
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(debug-palindrome "Waltz, nymph, for quick jigs vex Bud")
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=> "This palindrome is completely messed up!"
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(debug-palindrome "Doc, note: I dissent. A fast never
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prevents a fatness. I diet on cod.")
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=><tt>nil</tt> ;Another winner
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(debug-palindrome "Top step's pup's pet spot") => nil
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</pre><P>
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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<BR><b>[Function]</b><BR>
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<pre>
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<tt>count <i>item</i> <i>sequence</i> &key :from-end :test :test-not :start :end :key
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count-if <i>predicate</i> <i>sequence</i> &key :from-end :start :end :key
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count-if-not <i>predicate</i> <i>sequence</i> &key :from-end :start :end :key</tt>
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</pre>
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<P>The result is always a non-negative integer, the number of
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elements in the specified subsequence of <i>sequence</i> satisfying
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the test.
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<P>
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The <tt>:from-end</tt> argument does not affect the result returned;
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it is accepted purely for compatibility with other sequence functions.
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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X3J13 voted in January 1989
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(MAPPING-DESTRUCTIVE-INTERACTION) <A NAME=15966> </A>
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to restrict user side effects; see section <A HREF="node92.html#STRUCTURETRAVERSALSECTION">7.9</A>.
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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<BR><b>[Function]</b><BR>
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<tt>mismatch <i>sequence1</i> <i>sequence2</i> &key :from-end :test :test-not :key :start1 :start2 :end1 :end2</tt><P>The specified subsequences of
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<i>sequence1</i> and <i>sequence2</i> are compared element-wise.
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If they are of equal length and match in every element, the result is
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<tt>nil</tt>. Otherwise, the result is a non-negative integer.
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This result is the index within
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<i>sequence1</i> of the leftmost position at which the two
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subsequences fail to match; or,
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if one subsequence is shorter than and a matching prefix of the other,
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the result is the index
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relative to <i>sequence1</i> beyond the last position tested.
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<P>
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If a non-<tt>nil :from-end</tt> keyword argument is given, then
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<i>one plus</i> the index of the <i>rightmost</i>
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position in which the sequences differ is returned. In effect, the (sub)sequences
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are aligned at their right-hand ends; then, the last elements are compared,
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the penultimate elements, and so on. The index returned is again
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an index relative to <i>sequence1</i>.
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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X3J13 voted in January 1989
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(MAPPING-DESTRUCTIVE-INTERACTION) <A NAME=15982> </A>
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to restrict user side effects; see section <A HREF="node92.html#STRUCTURETRAVERSALSECTION">7.9</A>.
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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<BR><b>[Function]</b><BR>
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<tt>search <i>sequence1</i> <i>sequence2</i> &key :from-end :test :test-not :key :start1 :start2 :end1 :end2</tt><P>A search is conducted for a subsequence of <i>sequence2</i> that
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element-wise matches <i>sequence1</i>.
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If there is no such subsequence, the result is <tt>nil</tt>; if there is,
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the result is the index into <i>sequence2</i> of the leftmost element
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of the leftmost such matching subsequence.
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<P>
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If a non-<tt>nil :from-end</tt> keyword argument is given,
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the index of the leftmost
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element of the <i>rightmost</i> matching subsequence is returned.
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<P>
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The implementation may choose to search the sequence in any order;
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there is no guarantee on the number of times the test is made.
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For example, <tt>search</tt> with a non-<tt>nil :from-end</tt>
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argument might actually search a list from left to right
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instead of from right to left (but in either case would return
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the rightmost matching subsequence, of course). Therefore it is a good
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idea for a user-supplied predicate to be free of side effects.
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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X3J13 voted in January 1989
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(MAPPING-DESTRUCTIVE-INTERACTION) <A NAME=15998> </A>
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to restrict user side effects; see section <A HREF="node92.html#STRUCTURETRAVERSALSECTION">7.9</A>.
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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<BR> <HR><A NAME=tex2html3337 HREF="node146.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3335 HREF="node141.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3329 HREF="node144.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3339 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3340 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3338 HREF="node146.html"> Sorting and Merging</A>
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<B>Up:</B> <A NAME=tex2html3336 HREF="node141.html"> Sequences</A>
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<B> Previous:</B> <A NAME=tex2html3330 HREF="node144.html"> Modifying Sequences</A>
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<HR> <P>
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<HR>
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<P><ADDRESS>
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AI.Repository@cs.cmu.edu
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</ADDRESS>
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</BODY>
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