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<td bgcolor="#ffff99" width="451">JavaScript教程 - 第四课</td>
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<td bgcolor="#FF6600" width="451"><a href="mailto:thau@wired.com">Thau</a></td>
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<td width="458" valign="top" align="left"><strong><small><small><br>
</small></small><font face="宋体" size="3">第四页：</font><font SIZE="3">再谈<font
face="宋体">WHILE</font>循环</font></strong> <p><font SIZE="3">我们已见到了很多所请求的<font
face="宋体">X`S</font>，现在复习一下：</font></p>
<font SIZE="3"><p>第一步：请求<font face="宋体">X`S</font>的数目</p>
</font><p><font face="宋体">var width = prompt("How many x's would you like? (1-10
is<br>
good)","5");<br>
</font></p>
<p><font face="宋体" size="3">下一步，申报一些变量：</font> <ul>
<pre><font face="宋体">
var a_line="";
var loop = 0;
</font></pre>
</ul>
<p><font face="宋体" size="3">现在，关键之处：</font> <ul>
<pre><font face="宋体">
while (loop < width)
{
a_line = a_line + "x";
loop=loop+1;
}
</font></pre>
</ul>
<p><font face="宋体" size="3">即说：“当变量循环小于所请求的X`S行宽时，在该行加入另一<br>
X然后在循环值上加1。”该循环将继续加一X到该行并在循环值<br>
上加1直到循环不在小于请求的行宽。请看下列源码分析：</font> <dl>
<dt><font face="宋体" size="3">第一次</font></dt>
<dd><ul>
<li><font face="宋体">a_line = "" (because we initialized it to be "")
</font></li>
<li><font face="宋体">loop=0 (because we initialized it to be 0) </font></li>
<li><font face="宋体">width=2 (because that's what the user asked for) </font></li>
<li><font face="宋体">0 is less than 2 so </font></li>
<li><font face="宋体">a_line = a_line + "x", so now a_line = "x" </font></li>
<li><font face="宋体">loop=loop+1, so now loop = 1 </font></li>
</ul>
</dd>
<dt><font face="宋体" size="3">返回循环：第二次</font></dt>
<dd><ul>
<li><font face="宋体">loop=1 </font></li>
<li><font face="宋体">width=2 </font></li>
<li><font face="宋体">a_line = "x" </font></li>
<li><font face="宋体">1 is less than 2 so </font></li>
<li><font face="宋体">a_line = a_line + "x", so now a_line = "xx" </font></li>
<li><font face="宋体">loop=loop+1, so now loop = 2 </font></li>
</ul>
</dd>
<dt><font face="宋体" size="3">返回循环：第3次</font></dt>
<font size="2">
<dd><ul>
<li><font face="宋体">loop=2 </font></li>
<li><font face="宋体">width=2 </font></li>
<li><font face="宋体">a_line = "xx" </font></li>
<li><font face="宋体">2 is NOT less than 2 so </font></li>
<li><font face="宋体">fall out of the loop and do what follows </font></li>
</ul>
</dd>
</font>
</dl>
<p ALIGN="JUSTIFY"><font SIZE="3">随后是：</font></p>
<ul>
<pre>
<font SIZE="3">
<font face="宋体">alert(a_line);
</font></font></pre>
<p ALIGN="JUSTIFY"><font SIZE="3">启动一个警告对话框。</font></p>
<p ALIGN="JUSTIFY"><font SIZE="3">这种循环运用的如此普遍以至于程序员们开发了一些简便<br>
方式，条件循环可写成这样：</font></p>
</ul>
<ul>
<pre>
<font SIZE="3">
<font face="宋体">while (loop < width)
{
a_line += "x"; //this was a_line = a_line + "x";
loop++; //this was loop=loop+1;
}
</font></font></pre>
</ul>
<p><font face="宋体" size="3">首行，“x”，表示“本身加x”。若已有a_number=5，那么可<br>
写成，a_number+=3，即是说：a_number=a_number+3。程序员<br>
就是这么偷懒的。</font></p>
<p ALIGN="JUSTIFY"><font face="宋体" size="2">下行，</font><font size="3"
face="宋体">loop++，意味着“自身加1”。于是loop++即为：<br>
loop=loop=1。它也可写成loop+=1。这样偷懒行为非常有效。</font></p>
<p ALIGN="JUSTIFY"><font size="3" face="宋体">就象不止一种方式在一个数上加</font><font
SIZE="3"><font face="宋体">1</font>一样，也有不止一种方法来写<br>
一个循环。<font face="宋体">While</font>循环不是唯一的循环模式，另一流行的循环<br>
是<font face="宋体"><a href="day4_5.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day4_5.html">for循环</a>。</font></font></p>
<p align="left"><font face="宋体" size="3" color="#000000"><strong>JavaScript教程</strong></font><font
color="#FF0000" face="宋体" size="3"><br>
</font><font size="3" color="#FF0000">第一页</font><font size="2" face="宋体"> </font><a
href="day4_4.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day4_4.html"><font SIZE="3">第四课介绍</font></a><font size="2"><br>
</font><font SIZE="3"><font color="#FF0000">第二页</font> <a href="day4_2.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day4_2.html">循环介绍</a></font><font
size="2"><br>
</font><font SIZE="3"><font color="#FF0000">第三页</font> <a href="day4_3.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day4_3.html">循环的密码</a></font><font
size="2"><br>
</font><font SIZE="3"><font color="#FF0000">第四页</font> 再谈<font face="宋体">WHILE</font>循环</font><font
size="2"><br>
</font><font SIZE="3"><font color="#FF0000">第五页</font> <a href="day4_5.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day4_5.html">For循环</a></font><font
size="2"><br>
</font><font SIZE="3"><font color="#FF0000">第六页</font> <a href="day4_6.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day4_6.html">嵌套循环</a></font><font
size="2"><br>
</font><font SIZE="3"><font color="#FF0000">第七页</font> <a href="day4_7.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day4_7.html">循环练习</a></font><font
size="2"><br>
</font><font SIZE="3"><font color="#FF0000">第八页</font> <a href="day4_8.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day4_8.html">数组</a></font><font
size="2"><br>
</font><font SIZE="3"><font color="#FF0000">第九页</font> <a href="day4_9.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day4_9.html">数组和循环</a></font><font
size="2"><br>
</font><font SIZE="3"><font color="#FF0000">第十页</font> <a href="day4_10.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day4_10.html">文件目标模块中的数组</a></font><font
size="2"><br>
</font><font SIZE="3"><font color="#FF0000">第十一页</font> <a href="day4_11.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day4_11.html">函数</a></font><font
size="2"><br>
</font><font SIZE="3"><font color="#FF0000">第十二页</font> <a href="day4_12.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day4_12.html">无参数函数</a></font><font
size="2"><br>
</font><font SIZE="3"><font color="#FF0000">第十三页</font> <a href="day4_13.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day4_13.html">参数及返回值</a></font><font
size="2"><br>
</font><font SIZE="3"><font color="#FF0000">第十四页</font> <a href="day4_14.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day4_14.html">多于一个参数的函数</a></font></p>
<p align="left"><font face="宋体" size="3">[<a href="index.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/index.html">第1课</a>][<a
href="day2_1.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day2_1.html">第2课</a>][<a href="day3_1.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day3_1.html">第3课</a>][第4课][<a
href="day5_1.html" tppabs="http://www.pchome.net/ch/tur/pstext/teacher/javascript/day5_1.html">第5课</a>]</font></p>
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