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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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<TITLE>A.2.6. Alteration of Series</TITLE>
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<meta name="description" value=" Alteration of Series">
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html6138 HREF="node356.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html6136 HREF="node349.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html6132 HREF="node354.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html6140 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html6141 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html6139 HREF="node356.html"> Optimization</A>
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<B>Up:</B> <A NAME=tex2html6137 HREF="node349.html"> Series Functions</A>
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<B> Previous:</B> <A NAME=tex2html6133 HREF="node354.html"> Collectors</A>
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<HR> <P>
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<H2><A NAME=SECTION003426000000000000000>A.2.6. Alteration of Series</A></H2>
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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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Series that come from scanning data structures such as lists and vectors
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are closely linked to these structures. The function
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<tt>alter</tt> can be used to modify the underlying data structure with
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reference to the series derived from it. (Conversely, it is possible to
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modify a series by destructively modifying the data structure it is derived
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from. However, given the lazy evaluation nature of series, the effects of
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such modifications can be very hard to predict. As a result, this kind of
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modification is inadvisable.)
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<P>
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<BR><b>[Function]</b><BR>
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<tt>alter <i>destinations</i> <i>items</i></tt><P><tt>alter</tt> changes the series <i>destinations</i> so that it contains
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the elements in the series <i>items</i>. More importantly, in the
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manner of <tt>setf</tt>, the data structure that underlies <i>destinations</i> is changed so that if the series <i>destinations</i> were
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to be regenerated, the new values would be obtained. The alteration
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process stops as soon as either input runs out of elements. The value
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<tt>nil</tt> is always returned. In the example below each negative element in
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a list is replaced with its square.
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<P><pre>
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(let* ((data (list 1 -2 3 4 -5 6))
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(x (choose-if #'minusp (scan data))))
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(alter x (#M* x x))
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data)
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=> (1 4 3 4 25 6)
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</pre><P>
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<P>
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<tt>alter</tt> can be applied only to series that are <i>alterable</i>.
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<tt>scan</tt>, <tt>scan-alist</tt>, <tt>scan-multiple</tt>, <tt>scan-plist</tt>, and
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<tt>scan-lists-of-lists-fringe</tt> produce alterable series.
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However, the alterability of
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the output of
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<tt>scan-lists-of-lists-fringe</tt>
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is incomplete. If
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<tt>scan-lists-of-lists-fringe</tt>
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is applied to an object that is a leaf,
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altering the output series does not change the object.
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<P>
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In general, the output of a transducer is alterable as long as the elements
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of the output come directly from the elements of an input that is
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alterable. In particular, the outputs of <tt>choose</tt>, <tt>choose-if</tt>,
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<tt>split</tt>, <tt>split-if</tt>, <tt>cotruncate</tt>, <tt>until</tt>, <tt>until-if</tt>,
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and <tt>subseries</tt> are alterable as long as the corresponding inputs are
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alterable.
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<P>
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<BR><b>[Function]</b><BR>
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<tt>to-alter <i>items</i> <i>alter-fn</i> &rest <em>args</em></tt><P>Given a series <i>items</i>, <tt>to-alter</tt> returns an alterable series <i>A</i> containing the same elements. The argument <i>alter-fn</i> is a
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function. The remaining arguments are all series. Let these series be
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<i>S1</i>, <b>...</b>, <i>Sn</i>. If there are <b><i>n</i></b> arguments after <i>alter-fn</i>,
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<i>alter-fn</i> must accept <b><i>n</i>+1</b> inputs. If <tt>(alter <i>A</i> <i>B</i>)</tt> is
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later encountered, the expression
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<tt>(map-fn t <i>alter-fn</i> <i>B</i> <i>S1</i> ... <i>Sn</i>)</tt> is implicitly
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evaluated. For each
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element in <i>B</i>, <i>alter-fn</i> should make appropriate changes in the
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data structure underlying <i>A</i>.
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<P>
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As an example, consider the following definition of a series function
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that scans the elements of a list. Alteration is performed by
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changing cons cells in the list being scanned.
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<P><pre>
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(defun scan-list (list)
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(declare (optimizable-series-function))
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(let ((sublists (scan-sublists list)))
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(to-alter (#Mcar sublists)
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#'(lambda (new parent) (setf (car parent) new))
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sublists)))
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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> <HR><A NAME=tex2html6138 HREF="node356.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html6136 HREF="node349.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html6132 HREF="node354.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html6140 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html6141 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html6139 HREF="node356.html"> Optimization</A>
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<B>Up:</B> <A NAME=tex2html6137 HREF="node349.html"> Series Functions</A>
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<B> Previous:</B> <A NAME=tex2html6133 HREF="node354.html"> Collectors</A>
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<HR> <P>
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<P><ADDRESS>
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AI.Repository@cs.cmu.edu
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