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<title>第三章：VB语言</title>
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<td width="100%"><font size="3" color="#8000FF"><strong><em>第三章：VB语言（2）</em></strong></font></td>
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<td width="100%" style="background-color: rgb(232,255,254)"><span style="line-height: 25px">四、条件语句</span><blockquote>
<p ALIGN="JUSTIFY"><span style="line-height: 25px">如果你学过任何一种编程的话，都会知道在程序里控制其流程的有两种语句，条件与循环，VB自然也不例外，其实不管哪种语言，条件和循环的样子都差不多，只不过各有各的规矩罢了。</span></p>
<p ALIGN="JUSTIFY"><span style="line-height: 25px">在条件语句里，我们先来学习“If……Then……”语句。</span></p>
<p ALIGN="JUSTIFY"><span style="line-height: 25px">它的一般语法是：</span></p>
<p ALIGN="JUSTIFY"><span style="line-height: 25px"><font color="#004080"><strong>If <条件>
Then </strong></font></span></p>
<blockquote>
<p ALIGN="JUSTIFY"><span style="line-height: 25px"><strong><font color="#004080"><语句><br>
<语句></font><br>
<font color="#004080">………</font></strong></span></p>
</blockquote>
<p ALIGN="JUSTIFY"><span style="line-height: 25px"><font color="#004080"><strong>End If</strong></font></span></p>
<p ALIGN="JUSTIFY"><span style="line-height: 25px">与C 语言不同，VB的条件语句更符合英语的语法，如果Then后面所跟的语句只有一条，可以写成：</span></p>
<p ALIGN="JUSTIFY"><span style="line-height: 25px"><font color="#004080"><strong>If <条件>
Then <语句></strong></font></span></p>
<p ALIGN="JUSTIFY"><span style="line-height: 25px">而不用End If。</span></p>
<p ALIGN="JUSTIFY"><span style="line-height: 25px">有时我们还会用到</span></p>
<p ALIGN="JUSTIFY"><span style="line-height: 25px"><font color="#004080"><strong>If <条件>
Then</strong></font></span></p>
<blockquote>
<p ALIGN="JUSTIFY"><span style="line-height: 25px"><strong><font color="#004080">
<语句></font><br>
<font color="#004080">Else </font><br>
<font color="#004080"> <语句></font></strong></span></p>
</blockquote>
<p ALIGN="JUSTIFY"><span style="line-height: 25px"><font color="#004080"><strong>End If</strong></font></span></p>
<p ALIGN="JUSTIFY"><span style="line-height: 25px">更可以在Else里在嵌套If语句，如：</span></p>
<p ALIGN="JUSTIFY"><span style="line-height: 25px"><font color="#004080"><strong>If <条件>
Then</strong></font></span></p>
<blockquote>
<p ALIGN="JUSTIFY"><span style="line-height: 25px"><strong><font color="#004080">
<语句></font><br>
<font color="#004080">Else If </font><br>
<font color="#004080"> <语句></font><br>
<font color="#004080">Else</font><br>
<font color="#004080"> <语句></font></strong></span></p>
</blockquote>
<p ALIGN="JUSTIFY"><span style="line-height: 25px"><font color="#004080"><strong>End If</strong></font></span></p>
<p ALIGN="JUSTIFY"><span style="line-height: 25px">所有这些可以用一个表格表示：</span></p>
<table BORDER="1" CELLSPACING="1" CELLPADDING="7" WIDTH="568" bordercolor="#000080" bordercolorlight="#000080" bordercolordark="#000080">
<tr>
<td VALIGN="TOP" COLSPAN="5"><p ALIGN="JUSTIFY"><span style="line-height: 25px">条件1</span></td>
</tr>
<tr>
<td WIDTH="20%" VALIGN="TOP"><p ALIGN="JUSTIFY"><span style="line-height: 25px">True</span></td>
<td WIDTH="80%" VALIGN="TOP" COLSPAN="4"><p ALIGN="JUSTIFY"><span style="line-height: 25px">False</span></td>
</tr>
<tr>
<td WIDTH="20%" VALIGN="TOP" ROWSPAN="8"><p ALIGN="JUSTIFY"><span style="line-height: 25px">语</span></p>
<p ALIGN="JUSTIFY"><span style="line-height: 25px">句</span></p>
<p ALIGN="JUSTIFY"><span style="line-height: 25px">1</span></td>
<td WIDTH="80%" VALIGN="TOP" COLSPAN="4"><p ALIGN="JUSTIFY"><span style="line-height: 25px">条件2</span></td>
</tr>
<tr>
<td WIDTH="20%" VALIGN="TOP"><p ALIGN="JUSTIFY"><span style="line-height: 25px">True</span></td>
<td WIDTH="60%" VALIGN="TOP" COLSPAN="3"><p ALIGN="JUSTIFY"><span style="line-height: 25px">False</span></td>
</tr>
<tr>
<td WIDTH="20%" VALIGN="TOP" ROWSPAN="6"><p ALIGN="JUSTIFY"><span style="line-height: 25px">语</span></p>
<p ALIGN="JUSTIFY"><span style="line-height: 25px">句</span></p>
<p ALIGN="JUSTIFY"><span style="line-height: 25px">2</span></td>
<td WIDTH="60%" VALIGN="TOP" COLSPAN="3"><p ALIGN="JUSTIFY"><span style="line-height: 25px">条件3</span></td>
</tr>
<tr>
<td WIDTH="20%" VALIGN="TOP"><p ALIGN="JUSTIFY"><span style="line-height: 25px">True</span></td>
<td WIDTH="40%" VALIGN="TOP" COLSPAN="2"><p ALIGN="JUSTIFY"><span style="line-height: 25px">False</span></td>
</tr>
<tr>
<td WIDTH="20%" VALIGN="TOP" ROWSPAN="4"><p ALIGN="JUSTIFY"><span style="line-height: 25px">…</span></p>
<p ALIGN="JUSTIFY"><span style="line-height: 25px">…</span></p>
<p ALIGN="JUSTIFY"><span style="line-height: 25px">…</span></td>
<td WIDTH="40%" VALIGN="TOP" COLSPAN="2"><p ALIGN="JUSTIFY"><span style="line-height: 25px">………………………</span></td>
</tr>
<tr>
<td WIDTH="40%" VALIGN="TOP" COLSPAN="2"><p ALIGN="JUSTIFY"><span style="line-height: 25px">条件N</span></td>
</tr>
<tr>
<td WIDTH="20%" VALIGN="TOP"><p ALIGN="JUSTIFY"><span style="line-height: 25px">True</span></td>
<td WIDTH="20%" VALIGN="TOP"><p ALIGN="JUSTIFY"><span style="line-height: 25px">False</span></td>
</tr>
<tr>
<td WIDTH="20%" VALIGN="TOP"><p ALIGN="JUSTIFY"><span style="line-height: 25px">语句N</span></td>
<td WIDTH="20%" VALIGN="TOP"><p ALIGN="JUSTIFY"><span style="line-height: 25px">语句N＋1</span></td>
</tr>
</table>
<p> </p>
<p ALIGN="JUSTIFY"> </p>
<p ALIGN="JUSTIFY"><span style="line-height: 25px">用了过多的Else后你会感到很不方便，而且程序结构也不清晰，所以大多数的语言还有Select语句帮助解决这个问题。</span></p>
<p ALIGN="JUSTIFY"><span style="line-height: 25px">VB里的Select语句的格式是这样的：</span></p>
<p ALIGN="JUSTIFY"><span style="line-height: 25px"><font color="#004080"><strong>Select
Case <变量名></strong></font></span></p>
<blockquote>
<p ALIGN="JUSTIFY"><span style="line-height: 25px"><strong><font color="#004080">Case <情况1></font><br>
<font color="#004080"> ……</font><br>
<font color="#004080">Case <情况2></font><br>
<font color="#004080"> ……</font><br>
<font color="#004080">Case <情况3></font><br>
<font color="#004080"> ……<br>
……<br>
……</font><br>
<font color="#004080">Case Else</font><br>
<font color="#004080"> ……</font></strong></span></p>
</blockquote>
<p ALIGN="JUSTIFY"><span style="line-height: 25px"><font color="#004080"><strong>End
Select</strong></font></span></p>
<p ALIGN="JUSTIFY"><span style="line-height: 25px">例如：</span></p>
<p ALIGN="JUSTIFY"><span style="line-height: 25px"><font color="#004080"><strong>Select
Case a%</strong></font></span></p>
<p ALIGN="JUSTIFY"><span style="line-height: 25px"><font color="#004080"><strong>Case 1<br>
Print “a=1”<br>
Case 2<br>
Print “a=2”<br>
Case Else<br>
Print “a does not equal to 1 or 2.”</strong></font></span></p>
<p><span style="line-height: 25px"><font color="#004080"><strong>End Select</strong></font></span></p>
</blockquote>
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