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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!-- but may not be published or retransmitted without explicit permission.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/ary2dfunc3.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 6.20</title><link href="ary2dfunc2.html" rev=precedes><link href="aryparmsize.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="ary2dfunc2.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="aryparmsize.html" rel=precedes><img src="../images/buttonright.gif" alt="next"></a>&nbsp;<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>&middot;</font><!-- qtag -->Question 6.20</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>How can I use statically- and dynamically-allocatedmultidimensional arrays interchangeably when passing them to functions?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>There is no single perfect method.Given thedeclarations<pre>	int array[NROWS][NCOLUMNS];	int **array1;			/* ragged */	int **array2;			/* contiguous */	int *array3;			/* "flattened" */	int (*array4)[NCOLUMNS];<br>	int (*array5)[NROWS][NCOLUMNS];</pre>with the pointersinitializedasin the code fragments in question<a href="dynmuldimary.html">6.16</a>,and functions declared as<pre>	void f1a(int a[][NCOLUMNS], int nrows, int ncolumns);	void f1b(int (*a)[NCOLUMNS], int nrows, int ncolumns);	void f2(int *aryp, int nrows, int ncolumns);	void f3(int **pp, int nrows, int ncolumns);</pre>where<TT>f1a</TT> and <TT>f1b</TT> accept conventional two-dimensional arrays,<TT>f2</TT> accepts a ``flattened'' two-dimensional array,and<TT>f3</TT> accepts a pointer-to-pointer, simulated array(see also questions<a href="pass2dary.html">6.18</a> and<a href="ary2dfunc2.html">6.19</a>),the following callsshouldwork as expected:<pre>	f1a(array, NROWS, NCOLUMNS);	f1b(array, NROWS, NCOLUMNS);	f1a(array4, nrows, NCOLUMNS);	f1b(array4, nrows, NCOLUMNS);<br>	f1(*array5, NROWS, NCOLUMNS);<br>	f2(&amp;array[0][0], NROWS, NCOLUMNS);	f2(*array, NROWS, NCOLUMNS);	f2(*array2, nrows, ncolumns);	f2(array3, nrows, ncolumns);	f2(*array4, nrows, NCOLUMNS);<br>	f2(**array5, NROWS, NCOLUMNS);<br>	f3(array1, nrows, ncolumns);	f3(array2, nrows, ncolumns);</pre>The followingcalls would probably workon most systems,but involve questionable casts,and workonly if the dynamic <TT>ncolumns</TT>matches the static <TT>NCOLUMNS</TT>:<pre>	f1a((int (*)[NCOLUMNS])(*array2), nrows, ncolumns);	f1a((int (*)[NCOLUMNS])(*array2), nrows, ncolumns);	f1b((int (*)[NCOLUMNS])array3, nrows, ncolumns);	f1b((int (*)[NCOLUMNS])array3, nrows, ncolumns);</pre></p><p>It will be noticed that only <TT>f2</TT> can conveniently be made to workwith both statically- and dynamically-allocated arrays,thoughit will<em>not</em>work with the traditional``ragged'' array implementation,<TT>array1</TT>.However,it must alsobe noted thatpassing <TT>&amp;array[0][0]</TT>(or, equivalently, <TT>*array</TT>)to <TT>f2</TT>is not strictly conforming;see question<a href="ary2dfunc2.html">6.19</a>.</p><p>If you can understand why all of the above calls work and arewritten as they are,and if you understand why the combinations that are not listedwould not work,then you have a<em>very</em>good understanding of arrays and pointersin C.</p><p>Rather than worrying about all of this,one approach to using multidimensional arrays of various sizesis to make them <em>all</em> dynamic,as in question <a href="dynmuldimary.html">6.16</a>.If there are no static multidimensional arrays--ifall arrays are allocated like <TT>array1</TT> or <TT>array2</TT>in question <a href="dynmuldimary.html">6.16</a>--thenall functions can be written like <TT>f3</TT>.</p><p>Additional links:<a href="aryptr.c">Sample source code</a>(``torture test'')illustrating all these techniques</p><!-- aend --><p><hr><a href="ary2dfunc2.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="aryparmsize.html" rel=precedes><img src="../images/buttonright.gif" alt="next"></a>&nbsp;<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>&nbsp;<a href="../eskimo.html">about eskimo</a>&nbsp;<a href="../search.html">search</a>&nbsp;<a href="../feedback.html">feedback</a>&nbsp;<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/ary2dfunc3.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:49 GMT --></html>
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