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<td width="*" height="100%" style="font-size:9pt;line-height:12pt" valign="top"><img src="Skins/Default/topicface/face1.gif" border="0" alt="发贴心情" align="absmiddle"> <B>[第05篇]ACM/ICPC竞赛之STL--iterator简介</B><BR><H1 align=center>第05篇 ACM/ICPC竞赛之STL--iterator简介</H1><P>iterator(迭代器)是用于访问容器中元素的指示器，从这个意义上说，iterator(迭代器)相当于数据结构中所说的“遍历指针”，也可以把iterator(迭代器)看作是一种泛化的指针。</P><P>STL中关于iterator(迭代器)的实现是相当复杂的，这里我们暂时不去详细讨论关于iterator(迭代器)的实现和使用，而只对iterator(迭代器)做一点简单的介绍。</P><P>简单地说，STL中有以下几类iterator(迭代器)：</P><UL><LI>输入iterator(迭代器)，在容器的连续区间内向前移动，可以读取容器内任意值； </LI><LI>输出iterator(迭代器)，把值写进它所指向的容器中； </LI><LI>前向iterator(迭代器)，读取队列中的值，并可以向前移动到下一位置(++p,p++)；</LI><LI>双向iterator(迭代器)，读取队列中的值，并可以向前向后遍历容器； </LI><LI>随机访问iterator(迭代器), 可以直接以下标方式对容器进行访问，vector的iterator(迭代器)就是这种iterator(迭代器)； </LI><LI>流iterator(迭代器)，可以直接输出、输入流中的值； </LI></UL><P>每种STL容器都有自己的iterator(迭代器)子类，下面先来看一段简单的示例代码：</P><BLOCKQUOTE><P>#include <iostream><br>#include <vector><br>using namespace std;<br>main()<br>{<br> vector<int> s;<br> for (int i=0; i<10; i++) s.push_back(i);<br> for (<FONT color=#ff0000>vector<int>::iterator it=s.begin()</FONT>; <FONT color=#ff0000>it!=s.end()</FONT>; <FONT color=#ff0000>it++</FONT>) <br> cout << <FONT color=#ff0000>*it</FONT> << " ";<br> cout << endl;<br> return 1;<br>}</P></BLOCKQUOTE><P>vector的begin()和end()方法都会返回一个vector::iterator对象，分别指向vector的首元素位置和尾元素的下一个位置（我们可以称之为结束标志位置）。</P><P>对一个iterator(迭代器)对象的使用与一个指针变量的使用极为相似，或者可以这样说，指针就是一个非常标准的iterator(迭代器)。</P><P>再来看一段稍微特别一点的代码：</P><BLOCKQUOTE><P>#include <iostream><br>#include <vector><br>using namespace std;<br>main()<br>{<br> vector<int> s;<br> s.push_back(1);<br> s.push_back(2);<br> s.push_back(3);<br><FONT color=#ff0000> copy(s.begin(), s.end(), ostream_iterator<int>(cout, " "));<br></FONT> cout <<endl;<br> return 1;<br>}</P></BLOCKQUOTE><P>这段代码中的<FONT color=#ff0000>copy</FONT>就是STL中定义的一个模板函数，<FONT color=#ff0000>copy(s.begin(), s.end(), ostream_iterator<int>(cout, " "));</FONT>的意思是将由s.begin()至s.end()(不含s.end())所指定的序列复制到标准输出流cout中，用" "作为每个元素的间隔。也就是说，这句话的作用其实就是将表中的所有内容依次输出。</P><P>iterator(迭代器)是STL容器和算法之间的“胶合剂”，几乎所有的STL算法都是通过容器的iterator(迭代器)来访问容器内容的。只有通过有效地运用iterator(迭代器)，才能够有效地运用STL强大的算法功能。</P></td>
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