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<title>if 条件判断</title>
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<h3><a href="http://caterpillar.onlyfun.net/GossipCN/index.html">From
Gossip@caterpillar</a></h3>
<h1><a href="CppGossip.html">C++ Gossip: if 条件判断</a></h1>
<br>
为了应付程式所可能遇到的各种状况，C++提供了if 条件判断陈述，它的语法如下：<br>
<div style="margin-left: 40px;"><span style="font-weight: bold; font-family: Courier New,Courier,monospace;">if(条件式)
</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">
陈述句一; </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">else </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">
陈述句二;</span><br>
</div>
<br>
这个语法的意思，白话来说，就是当条件式成立时（true），则执行陈述句一，要不然就执行陈述句二；如果条件式不成立时并不想作任何事，则else可以
省略。 <br>
<br>
在 if 后如果有两个以上陈述句，称之为“复合陈述句”（compound statement），此时您必须使用{
}将复合陈述句包括起来，例如： <br>
<div style="margin-left: 40px;"><span style="font-weight: bold; font-family: Courier New,Courier,monospace;">if(条件式)
{ </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">
陈述句一; </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">
陈述句二; </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">} </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">else {
</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">
陈述句三; </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">
陈述句四; </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">}</span><br>
</div>
<br>
下面来写个简单的程式，判断使用者的输入是奇数还是偶数： <br>
<br>
<pre>#include <iostream><br>using namespace std;<br><br>int main() { <br> int input = 0; <br> int remain = 0; <br><br> cout << "输入整数："; <br> cin >> input; <br><br> remain = input % 2; <br> if(remain == 1) <br> cout << input << "为奇数" << endl; <br> else <br> cout << input << "为偶数" << endl; <br><br> return 0;<br>}</pre>
<span class="postbody"><br>
执行结果：</span><br>
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<td style="background-color: rgb(0, 0, 0);"><small><span style="color: rgb(255, 255, 255);">输入整数：10<br>
10为偶数</span></small><span style="color: rgb(255, 255, 255);"><br>
</span></td>
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</tbody>
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<br>
这个程式很简单，使用余数运算来计算除以2后的余数，偶数的余数会是0，而奇数会是1，由此判断输入为奇数还是偶数；事实上如果您C++了解true与
false的意义的话，这个程式也可以这么写： <br>
<br>
<pre>#include <iostream><br>using namespace std;<br><br>int main() { <br> int input = 0; <br><br> cout << "输入整数："; <br> cin >> input; <br><br> if(input % 2) <br> cout << input << "为奇数" << endl; <br> else <br> cout << input << "为偶数" << endl; <br> <br> return 0;<br>}</pre>
<span class="postbody"><br>
</span>这只是一点程式写作的技巧，因为在C++中0在条件式中相当于false，而非0值都被视为true，所以上面的程式可以直接将余数运算的
结果当作条件式来使用。 <br>
<br>
在 if 中您也可以再设定执行的条件，例如： <br>
<div style="margin-left: 40px;"><span style="font-weight: bold; font-family: Courier New,Courier,monospace;">if(条件式一) { </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">
陈述句一; </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">
if(条件式二) </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">
陈述句二; </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">
陈述句三; </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">}</span><br>
</div>
<br>
这只个简单的例子，其中陈述句二要执行，必须同时满足条件式一与二才行；再来看个例子： <br>
<div style="margin-left: 40px;"><span style="font-weight: bold; font-family: Courier New,Courier,monospace;">if(条件式一) { </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">
陈述句一; </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> //
其它陈述句 </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">} </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">else </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">
if(条件式二) </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">
陈述句二;</span><br>
</div>
<br>
如果条件式一不满足，就会执行else中的陈述，而您在这边进行条件式二的测试，如果满足就执行条件式二，由于C++是个自由格式语言，您可以适当的排列
这个程式，这会比较好懂一些： <br>
<div style="margin-left: 40px;"><span style="font-weight: bold; font-family: Courier New,Courier,monospace;">if(条件式一) { </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">
陈述句一; </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> //
其它陈述句 </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">} </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">else if(条件式二) </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> 陈述句二;</span><br>
</div>
<br>
基于这个方式，您可以如下设定多个条件，且易读易懂： <br>
<div style="margin-left: 40px;"><span style="font-weight: bold; font-family: Courier New,Courier,monospace;">if(条件式一) </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> 陈述一;
</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">else if(条件式二) </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">
陈述句二; </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">else if(条件式三） </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">
陈述句三; </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">else </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> 陈述句四;</span><br>
</div>
<br>
陈述句四会在条件式一、二、三都不成立时执行；下面这个程式是个简单的例子，处理学生的成绩等级问题： <br>
<br>
<pre>#include <iostream><br>using namespace std;<br><br>int main() { <br> int score = 0; <br><br> cout << "输入分数："; <br> cin >> score; <br><br> if(score >= 90) <br> cout << "得A" << endl; <br> else if(score >= 80 && score < 90) <br> cout << "得B" << endl; <br> else if(score >= 70 && score < 80) <br> cout << "得C" << endl; <br> else if(score >= 60 && score < 70) <br> cout << "得D" << endl; <br> else <br> cout << "得E(不及格)" << endl; <br> <br> return 0;<br>}<br></pre>
<span class="postbody"><br>
</span>执行结果：<span class="postbody"></span><br>
<table style="text-align: left; width: 100%;" border="0" cellpadding="2" cellspacing="2">
<tbody>
<tr>
<td style="background-color: rgb(0, 0, 0);"><small><span style="color: rgb(255, 255, 255);">输入分数：60<br>
得D</span></small><span style="color: rgb(255, 255, 255);"><br>
</span></td>
</tr>
</tbody>
</table>
<br>
在这边要注意的是，if与else是就最近的一组来配对，例如下面这个程式依缩排来看，您觉得有无问题存在？<br>
<div style="margin-left: 40px;"><span style="font-weight: bold; font-family: Courier New,Courier,monospace;">if(条件式一) </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">
if(条件式二) </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">
陈述句一; </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">else </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> 陈述句二;</span><br>
</div>
<br>
很多人都会以为条件式一的if会与else配对，但事实上是条件式二的if与else配对，加上括号的话就可以一目了然了： <br>
<div style="margin-left: 40px;"><span style="font-weight: bold; font-family: Courier New,Courier,monospace;">if(条件式一) { </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">
if(条件式二) </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">
陈述句一; </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;"> else
</span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">
陈述句二; </span><br style="font-weight: bold; font-family: Courier New,Courier,monospace;">
<span style="font-weight: bold; font-family: Courier New,Courier,monospace;">}</span><br>
</div>
<br>
如果想避免这种错误，在程式中多使用括号是必要的，多写一些总是比少写一些来得保险一点。
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