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4.5 KiB
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82 lines
4.5 KiB
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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>2.5.1. Vectors</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Vectors">
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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=tex2html1881 HREF="node31.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html1879 HREF="node29.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html1873 HREF="node29.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html1883 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html1884 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html1882 HREF="node31.html"> Strings</A>
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<B>Up:</B> <A NAME=tex2html1880 HREF="node29.html"> Arrays</A>
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<B> Previous:</B> <A NAME=tex2html1874 HREF="node29.html"> Arrays</A>
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<HR> <P>
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<H2><A NAME=SECTION00651000000000000000>2.5.1. Vectors</A></H2>
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<P>
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One-dimensional arrays are called <i>vectors</i> in Common Lisp
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and constitute the type <tt>vector</tt> (which is therefore a subtype of <tt>array</tt>).
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Vectors and lists are collectively considered to be
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<i>sequences</i>. They differ in that any component of a one-dimensional array
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can be accessed in constant time,
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whereas the average component access time for a
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list is linear in the length of the list; on the other hand, adding a new
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element to the front of a list takes constant time, whereas the same
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operation on an array takes time linear in the length of the array.
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<P>
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A general vector (a one-dimensional array
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that can have any data object as an element but that has
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no additional paraphernalia) can be notated by notating the
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components in order, separated by whitespace and surrounded by <tt>#(</tt>
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and <tt>)</tt>.
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For example:
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<P><pre>
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#(a b c) ;A vector of length 3
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#() ;An empty vector
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#(2 3 5 7 11 13 17 19 23 29 31 37 41 43 47)
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;A vector containing the primes below 50
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</pre><P>
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Note that when the function <tt>read</tt> parses this syntax, it always constructs
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a <i>simple</i> general vector.
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<P>
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<hr>
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<b>Rationale:</b> Many people have suggested that brackets be used
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to notate vectors, as <tt>[a b c]</tt>
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instead of <tt>#(a b c)</tt>. This notation
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would be shorter, perhaps more readable, and certainly in accord with
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cultural conventions in other parts of computer science and mathematics.
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However, to preserve the usefulness of the user-definable macro-character
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feature of the function <tt>read</tt>, it is necessary to leave some
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characters to the user for this purpose. Experience in MacLisp has
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shown that users, especially implementors of languages for use
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in artificial intelligence research, often want
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to define special kinds of brackets. Therefore Common Lisp avoids using
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brackets and braces for any syntactic purpose.
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<hr>
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<P>
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Implementations may provide certain specialized representations of
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arrays for efficiency in the case where all the components are of
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the same specialized (typically numeric) type. All implementations
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provide specialized arrays for the cases when the components
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are characters (or rather, a special subset of the characters);
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the one-dimensional instances of
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this specialization are called <i>strings</i>.
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All implementations are also required to provide specialized arrays
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of bits, that is, arrays of type <tt>(array bit)</tt>;
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the one-dimensional instances of
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this specialization are called <i>bit-vectors</i>.
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<P>
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<BR> <HR><A NAME=tex2html1881 HREF="node31.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html1879 HREF="node29.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html1873 HREF="node29.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html1883 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html1884 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html1882 HREF="node31.html"> Strings</A>
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<B>Up:</B> <A NAME=tex2html1880 HREF="node29.html"> Arrays</A>
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<B> Previous:</B> <A NAME=tex2html1874 HREF="node29.html"> Arrays</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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