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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN"><!-- This collection of hypertext pages is Copyright 1995-2005 by Steve Summit. --><!-- Content from the book "C Programming FAQs: Frequently Asked Questions" --><!-- (Addison-Wesley, 1995, ISBN 0-201-84519-9) is made available here by --><!-- permission of the author and the publisher as a service to the community. --><!-- It is intended to complement the use of the published text --><!-- and is protected by international copyright laws. --><!-- The on-line content may be accessed freely for personal use --><!-- but may not be published or retransmitted without explicit permission. --><!-- --><!-- this page built Sat Dec 24 21:47:45 2005 by faqproc version 2.7 --><!-- from source file aryptr.sgml dated Wed Dec 21 12:50:38 2005 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/aryptr/dynmuldimary.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:49 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 6.16</title><link href="dynlocarys.html" rev=precedes><link href="non0based.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="dynlocarys.html" rev=precedes><img src="../images/buttonleft.gif" alt="prev"></a><a href="index.html" rev=subdocument><img src="../images/buttonup.gif" alt="up"></a><a href="non0based.html" rel=precedes><img src="../images/buttonright.gif" alt="next"></a> <a href="../index-2.html"><img src="../images/buttontop.gif" alt="top/contents"></a><a href="../search.html"><img src="../images/buttonsrch.gif" alt="search"></a><hr><p><!-- qbegin --><h1>comp.lang.c FAQ list<font color=blue>·</font><!-- qtag -->Question 6.16</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>How can I dynamically allocate a multidimensional array?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>The traditional solution isto allocatean array<a href="arraysim.html" rel=subdocument>[footnote]</a>ofpointers topointers,and theninitializeeach pointertoa dynamically-allocated``row.''Here is a two-dimensional example:<pre> #include <stdlib.h> int **array1 = malloc(nrows * sizeof(int *)); for(i = 0; i < nrows; i++) array1[i] = malloc(ncolumns * sizeof(int));</pre>(In real code, of course,all of <TT>malloc</TT>'s return values would be checked.You can also use<TT>sizeof(*array1)</TT> and <TT>sizeof(**array1)</TT>instead of<TT>sizeof(int *)</TT> and <TT>sizeof(int)</TT>;see <a href="fn33.html" rel=subdocument>[footnote]</a>.)</p><p>You can keep the array's contents contiguous,at the cost ofmaking later reallocation of individual rows more difficult,with a bit of explicit pointer arithmetic:<pre> int **array2 = malloc(nrows * sizeof(int *)); array2[0] = malloc(nrows * ncolumns * sizeof(int)); for(i = 1; i < nrows; i++) array2[i] = array2[0] + i * ncolumns;</pre>In either case(i.e for <TT>array1</TT> or <TT>array2</TT>),the elements of the dynamic array can be accessedwith normal-looking array subscripts:<TT>arrayx[i][j]</TT>(for 0 <= <TT>i</TT> < <TT>nrows</TT> and0 <= <TT>j</TT> < <TT>ncolumns</TT>).Here isa schematic illustrationof the layout of <TT>array1</TT> and <TT>array2</TT>:<br><br><img src="array1.gif"><br><br><img src="array2.gif"><p>If the double indirection implied by the above schemes is forsome reason unacceptable,<a href="doubleindir.html" rel=subdocument>[footnote]</a>you can simulate a two-dimensionalarray with a single, dynamically-allocated one-dimensional array:<pre> int *array3 = malloc(nrows * ncolumns * sizeof(int));</pre>However, you must now perform subscript calculations manually,accessing the <TT>i</TT>,<TT>j</TT>th element withthe expression<pre><TT>array3[i * ncolumns + j]</TT></pre>and this array cannotnecessarilybepassed to functionswhich expectmultidimensional arrays.(A macrosuch as<pre> #define Arrayaccess(a, i, j) ((a)[(i) * ncolumns + (j)])</pre>couldhide the explicit calculation,but invoking itwould requireparentheses and commaswhich wouldn't look exactly likeconventional Cmultidimensional array syntax,andthe macrowould needaccess to at least one of the dimensions,as well.See also question <a href="ary2dfunc2.html">6.19</a>.)</p><p>Yet another optionis touse pointers to arrays:<pre> int (*array4)[NCOLUMNS] = malloc(nrows * sizeof(*array4));</pre>or even<pre> int (*array5)[NROWS][NCOLUMNS] = malloc(sizeof(*array5));</pre>but the syntax starts getting horrific(accesses to <TT>array5</TT> look like <TT>(*array5)[i][j]</TT>),andat most one dimension may be specified at run time.</p><p>With all of these techniques,you may of course need to remember to free the arrayswhen they are no longer needed;in the case of <TT>array1</TT> and <TT>array2</TT>this takes several steps(see also question <a href="../malloc/freeforall.html">7.23</a>):<pre> for(i = 0; i < nrows; i++) free((void *)array1[i]); free((void *)array1); free((void *)array2[0]); free((void *)array2);</pre>Also,you cannot necessarily intermix dynamically-allocatedarrays with conventional, statically-allocated ones(see question<a href="ary2dfunc3.html">6.20</a>,and also question<a href="pass2dary.html">6.18</a>).</p><p>Finally, in C99 you can use a variable-length array.</p><p>All ofthese techniques can also be extended to three or moredimensions.Here is a three-dimensional version of the first technique(which, like the rest of the fragments presented here,requires error-checking before being used in a real program):<pre> int ***a3d = (int ***)malloc(xdim * sizeof(int **)); for(i = 0; i < xdim; i++) { a3d[i] = (int **)malloc(ydim * sizeof(int *)); for(j = 0; j < ydim; j++) a3d[i][j] = (int *)malloc(zdim * sizeof(int)); }</pre></p><p>See also question <a href="../misc/ragged.html">20.2</a>.</p><p>References:C9X Sec. 6.5.5.2<br></p><!-- aend --><p><hr><a href="dynlocarys.html" rev=precedes><img src="../images/buttonleft.gif" alt="prev"></a><a href="index.html" rev=subdocument><img src="../images/buttonup.gif" alt="up"></a><a href="non0based.html" rel=precedes><img src="../images/buttonright.gif" alt="next"></a> <a href="../questions.html"><img src="../images/buttontop.gif" alt="contents"></a><a href="../search.html"><img src="../images/buttonsrch.gif" alt="search"></a><br><!-- lastfooter --><a href="../about.html">about this FAQ list</a> <a href="../eskimo.html">about eskimo</a> <a href="../search.html">search</a> <a href="../feedback.html">feedback</a> <a href="copyright.html">copyright</a><p>Hosted by<a href="http://www.eskimo.com/"><img src="../../www.eskimo.com/img/link/eskitiny.gif" alt="Eskimo North"></a></body><!-- Mirrored from c-faq.com/aryptr/dynmuldimary.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:49 GMT --></html>⌨️ 快捷键说明
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