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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!-- but may not be published or retransmitted without explicit permission.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 21:47:47 2005 by faqproc version 2.7 --><!-- from source file misctechn.sgml dated Sat Sep 28 22:23:36 2002 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/misc/ragged.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:59:04 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 20.2</title><link href="multretval.html" rev=precedes><link href="argv.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="multretval.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="argv.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 20.2</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>What's a good data structure to usefor storing lines of text?I started to usefixed-sizearrays of arrays of <TT>char</TT>,but they're just too restrictive.</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>One good way of doing this is with apointer(simulating an array)to a set of pointers(each simulating an array)of <TT>char</TT>.Thisdata structureis sometimes called a``ragged array,''and looks something like this:<blockquote>[FIGURE GOES HERE]</blockquote></p><p>You could set up the tiny array in the figure above with these simple declarations:<pre>char *a[4] = {"this", "is", "a", "test"};char **p = a;</pre>(where <TT>p</TT> is the pointer-to-pointer-to-<TT>char</TT>and <TT>a</TT> is an intermediate array used to allocate the four pointers-to-<TT>char</TT>).</p><p>To really do dynamic allocation,you'd of course have to call <TT>malloc</TT>:<pre>#include <stdlib.h>char **p = malloc(4 * sizeof(char *));if(p != NULL) { p[0] = malloc(5); p[1] = malloc(3); p[2] = malloc(2); p[3] = malloc(5); if(p[0] && p[1] && p[2] && p[3]) { strcpy(p[0], "this"); strcpy(p[1], "is"); strcpy(p[2], "a"); strcpy(p[3], "test"); }}</pre>(Some libraries have a <TT>strdup</TT> functionwhich would streamline the inner<TT>malloc</TT> and <TT>strcpy</TT> calls.It's not Standard,but it's obviously trivial to implement something like it.)</p><p>Here is a code fragmentwhich reads an entire file into memory,using the same kind of ragged array.This code is written in terms of the<TT>agetline</TT> functionfrom question <a href="../malloc/reallocnull.html">7.30</a>.<pre>#include <stdio.h>#include <stdlib.h>extern char *agetline(FILE *);FILE *ifp;/* assume ifp is open on input file */char **lines = NULL;size_t nalloc = 0;size_t nlines = 0;char *p;while((p = agetline(ifp)) != NULL) { if(nlines >= nalloc) { nalloc += 50;#ifdef SAFEREALLOC lines = realloc(lines, nalloc * sizeof(char *));#else if(lines == NULL) /* in case pre-ANSI realloc */ lines = malloc(nalloc * sizeof(char *)); else lines = realloc(lines, nalloc * sizeof(char *));#endif if(lines == NULL) { fprintf(stderr, "out of memory"); exit(1); } } lines[nlines++] = p;}</pre>(See the comments on reallocation strategyin question <a href="../malloc/reallocnull.html">7.30</a>.)</p><p>See also question <a href="../aryptr/dynmuldimary.html">6.16</a>.</p><!-- aend --><p><hr><a href="multretval.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="argv.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/misc/ragged.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:59:04 GMT --></html>⌨️ 快捷键说明
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