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<h2 align="center"><font face="楷体_GB2312"><a name="_top"></a>2.1 语句和运算符</font></h2>
<div align="center"><center>
<table border="0" width="100%" cellspacing="0" cellpadding="0">
<tr>
<td width="176" align="center"><p align="center"><a href="c21.htm#c21.html#c21">语句和复合语句</a></td>
<td width="233" align="center"><p align="center"><a href="c21.htm#c22.html#c22">关系运算符和逻辑运算符</a></td>
<td width="296" align="center"><p align="center"><a href="c21.htm#c23.html#c23">if_else 语句和
else_if语句</a></td>
<td width="169" align="center"><p align="center"><a href="c21.htm#c24.html#c24">switch 语句</a></td>
</tr>
</table>
</center></div>
<hr>
<blockquote>
<h3>1.语句和复合语句 </h3>
<p> a.语句。语句是一个程序的主要构成部分
。一个程序是一系列由标点分隔的语句组成的。一个语句是给计算机的一条完整的指令。在
C 语言里,
一个语句是在结尾处用分号结束的。我们到现在已经学习了五种语句。这里有一个使用了所有这些语句的例子。</p>
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<p> b.复合语句。复合语句,
是由包含在大括号里的一个或多个语句组成的程序块。语法上,
复合语句被当作单个语句对待。请看例子：</p>
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<p>一般, 一个程序可以有更复杂的结构。</p>
<p align="right"><a href="c21.htm#_top.html#_top">返回页首</a></p>
</blockquote>
<hr>
<h3><a name="c22"></a>2.关系运算符和逻辑运算符</h3>
<blockquote>
<p>a.关系运算符。C 语言提供了六种关系运算符:</p>
</blockquote>
<blockquote>
<div align="center"><center><table border="5" width="576" bordercolor="#FF9933"
bgcolor="#CCFFFF" cellspacing="0" cellpadding="0">
<tr>
<td width="57" align="center"><</td>
<td width="95" align="center">小于</td>
<td width="29" align="center" rowspan="2"> </td>
<td width="371" align="center" rowspan="2" colspan="2" bgcolor="#FF9933">它们的优先级低于算术运算符的优先级,但高于赋值运算符的优先级。它们的优先级和结合性如下所示</td>
</tr>
<tr>
<td width="57" align="center"><=</td>
<td width="95" align="center">小于等于</td>
</tr>
<tr>
<td width="57" align="center">></td>
<td width="95" align="center">大于</td>
<td width="29" align="center" rowspan="4"> </td>
<td width="184" align="center" rowspan="2">优先级(从高到低)</td>
<td width="181" align="center" rowspan="2">结合性</td>
</tr>
<tr>
<td width="57" align="center">>=</td>
<td width="95" align="center">大于等于</td>
</tr>
<tr>
<td width="57" align="center">==</td>
<td width="95" align="center">等于</td>
<td width="184" align="center">< <= > >=</td>
<td width="181" align="center">L-R</td>
</tr>
<tr>
<td width="57" align="center">!=</td>
<td width="95" align="center">不等于</td>
<td width="184" align="center">== !=</td>
<td width="181" align="center">L-R</td>
</tr>
</table>
</center></div><p>让我们看一下,
表达式是怎样计算的。无论它是否包含一个关系运算符,
每一个表达式都有一个逻辑值。如果它的算术值是0, 它的逻辑值是假,
否则, 它是真。假设 a=1, b=0, a+b 的逻 辑值是真。但是, a*b
的逻辑值是假。</p>
<p> b.逻辑运算符。通常用逻辑运算符来连接条件表达式中两个或多个表达式。<br>
exp1 <font color="#800080">&&</font>
exp2 : 逻辑<font color="#800080"> 与</font> 运算符<br>
exp2 <font color="#800080">||</font> exp2 :
逻辑<font color="#800080"> 或</font> 运算符</p>
<p> C语言是怎样得到逻辑表达式 的值的?
让我们来看一下吧!</p>
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<p align="center"> </p>
<p> 在结束本课前, 让我们看一下
更复杂的例子。它查看某一年是否是闰年。<br>
条件表达式: <font size="4" color="#800080"><em><strong>( year%4 == 0 &&
year%100 != 0 ) || year%400 == 0 )</strong></em></font></p>
<p align="right"><a href="c21.htm#_top.html#_top">返回页首</a></p>
</blockquote>
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