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110 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>2.5. Arrays</TITLE>
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<meta name="description" value=" Arrays">
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<meta name="keywords" value="clm">
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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=tex2html1866 HREF="node30.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html1864 HREF="node15.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html1858 HREF="node28.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html1868 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html1869 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html1867 HREF="node30.html"> Vectors</A>
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<B>Up:</B> <A NAME=tex2html1865 HREF="node15.html"> Data Types</A>
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<B> Previous:</B> <A NAME=tex2html1859 HREF="node28.html"> Lists and Conses</A>
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<HR> <P>
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<H1><A NAME=SECTION00650000000000000000>2.5. Arrays</A></H1>
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<P>
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<A NAME=ARRAYTYPESECTION>An</A>
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<A NAME=1214><tt>array</tt></A> is an object with components arranged according
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to a Cartesian coordinate system.
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In general, these components may be any Lisp data objects.
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<P>
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The number of dimensions of an array is called its <i>rank</i>
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(this terminology is borrowed from APL);
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the rank is a non-negative integer.
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Likewise, each dimension is itself a non-negative integer.
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The total number of elements in the array is the product of all the
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dimensions.
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<P>
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An implementation of Common Lisp may impose a limit on the rank of an array,
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but this limit may not be smaller than 7. Therefore, any Common Lisp
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program may assume the use of arrays of rank 7 or less.
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(A program may determine the actual limit on array ranks for
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a given implementation by examining the constant <tt>array-rank-limit</tt>.)
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<P>
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It is permissible for a dimension to be zero. In this case,
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the array has no elements, and any attempt to access an element
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is in error. However, other properties of the array, such as the
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dimensions themselves, may be used.
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If the rank is zero, then there are no dimensions, and the
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product of the dimensions is then by definition 1.
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A zero-rank array therefore has a single element.
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<P>
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An array element is specified by a sequence of indices.
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The length of the sequence must equal the rank of the array.
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Each index must be a non-negative integer strictly less than
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the corresponding array dimension. Array indexing is
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therefore zero-origin, not one-origin as in (the default case of)
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Fortran.
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<P>
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As an example, suppose that the variable <tt>foo</tt> names a 3-by-5 array.
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Then the first index may be 0, 1, or 2, and the second index
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may be 0, 1, 2, 3, or 4. One may refer to array elements using
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the function <tt>aref</tt>; for example, <tt>(aref foo 2 1)</tt>
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refers to element (2, 1) of the array. Note that <tt>aref</tt> takes
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a variable number of arguments: an array, and as many indices
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as the array has dimensions.
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A zero-rank array has no dimensions, and therefore
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<tt>aref</tt> would take such an array and no indices, and return the sole
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element of the array.
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<P>
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In general, arrays can be multidimensional,
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can share their contents with other array objects, and can have their
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size altered dynamically (either enlarging or shrinking) after creation.
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A one-dimensional array may also have a <i>fill pointer</i>.
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<P>
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Multidimensional arrays store their components in row-major order;
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that is, internally a multidimensional array is stored as a one-dimensional
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array, with the multidimensional index sets ordered lexicographically,
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last index varying fastest. This is important in two situations:
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(1) when arrays with different dimensions share their contents, and
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(2) when accessing very large arrays in a virtual-memory implementation.
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(The first situation is a matter of semantics; the second, a matter
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of efficiency.)
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<P>
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An array that is not displaced to another array, has no fill pointer, and
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is not to have its size adjusted dynamically after creation is called a
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<i>simple</i> array. The user may provide declarations that certain arrays
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will be simple. Some implementations can handle simple arrays in an
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especially efficient manner; for example, simple arrays may have a more
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compact representation than non-simple arrays.
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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 June 1989
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(ADJUST-ARRAY-NOT-ADJUSTABLE) <A NAME=1226> </A>
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to clarify that if one or more of the <tt>:adjustable</tt>, <tt>:fill-pointer</tt>,
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and <tt>:displaced-to</tt> arguments is true when <tt>make-array</tt>
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is called, then whether the resulting
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array is simple is unspecified; but if all three arguments are false,
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then the resulting array is guaranteed to be simple.
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<HR>
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<UL>
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<LI> <A NAME=tex2html1870 HREF="node30.html#SECTION00651000000000000000"> Vectors</A>
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<LI> <A NAME=tex2html1871 HREF="node31.html#SECTION00652000000000000000"> Strings</A>
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<LI> <A NAME=tex2html1872 HREF="node32.html#SECTION00653000000000000000"> Bit-Vectors</A>
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</UL>
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<BR> <HR><A NAME=tex2html1866 HREF="node30.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html1864 HREF="node15.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html1858 HREF="node28.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html1868 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html1869 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html1867 HREF="node30.html"> Vectors</A>
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<B>Up:</B> <A NAME=tex2html1865 HREF="node15.html"> Data Types</A>
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<B> Previous:</B> <A NAME=tex2html1859 HREF="node28.html"> Lists and Conses</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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