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<p align="center"><strong>第二章 簡(jiǎn)單變量 </strong><br></p>
<a href="#1、整型">一、整型</a>
<br>
<a href="#二、浮點(diǎn)數(shù)">二、浮點(diǎn)數(shù)</a>
<br>
<a href="#三、字符串">三、字符串</a></p>
<p> 基本上，簡(jiǎn)單變量就是一個(gè)數(shù)據(jù)單元，這個(gè)單元可以是數(shù)字或字符串。<br>
<a name="1、整型">一、整型</a>
<br>
1、整型 <br>
PERL最常用的簡(jiǎn)單變量，由于其與其它語(yǔ)言基本相同，不再贅述。<br>
例：<br>
$x = 12345;<br>
if (1217 + 116 == 1333) {<br>
# statement block goes here<br>
}<br>
整型的限制：<br>
PERL實(shí)際上把整數(shù)存在你的計(jì)算機(jī)中的浮點(diǎn)寄存器中，所以實(shí)際上被當(dāng)作浮點(diǎn)數(shù)看待。在多數(shù)計(jì)算機(jī)中，浮點(diǎn)寄存器可以存貯約16位數(shù)字，長(zhǎng)于此的被丟棄。整數(shù)實(shí)為浮點(diǎn)數(shù)的特例。<br>
2、8進(jìn)制和16進(jìn)制數(shù)<br>
8進(jìn)制以0打頭，16進(jìn)制以0x打頭。<br>
例：$var1 = 047; (等于十進(jìn)制的39)<br>
$var2 = 0x1f; (等于十進(jìn)制的31)<br>
<a name="二、浮點(diǎn)數(shù)">二、浮點(diǎn)數(shù)</a>
<br>
如 11.4 、 -0.3 、.3 、
3. 、 54.1e+02 、 5.41e03<br>
浮點(diǎn)寄存器通常不能精確地存貯浮點(diǎn)數(shù)，從而產(chǎn)生誤差，在運(yùn)算和比較中要特別注意。指數(shù)的范圍通常為-309到+308。<br>
例：<br>
<br>
#!/usr/local/bin/perl<br>
$value = 9.01e+21 + 0.01 - 9.01e+21;<br>
print ("first value is ", $value,
"\n");<br>
$value = 9.01e+21 - 9.01e+21 + 0.01;<br>
print ("second value is ", $value,
"\n");<br>
<br>
---------------------------------------------------------<br>
<br>
$ program3_3<br>
first value is 0<br>
second value is 0.01<br>
<a name="三、字符串">三、字符串</a>
<br>
慣用C的程序員要注意，在PERL中，字符串的末尾并不含有隱含的NULL字符，NULL字符可以出現(xiàn)在串的任何位置。<br>
.
雙引號(hào)內(nèi)的字符串中支持簡(jiǎn)單變量替換，例如：<br>
$number = 11;<br>
$text = "This text contains the number
$number.";<br>
則$text的內(nèi)容為："This text contains the number
11."<br>
<br>
.雙引號(hào)內(nèi)的字符串中支持轉(zhuǎn)義字符<br>
<b>Table 3.1. Escape sequences in strings.</b> </p>
<div align="center"><center>
<table border="1" width="60%">
<tr>
<td valign="top" width="145"> <b>Escape
Sequence</b></td>
<td valign="top" width="335"> <b>Description</b></td>
</tr>
<tr>
<td valign="top" width="145"> <tt>\a</tt></td>
<td valign="top" width="335"> Bell
(beep) </td>
</tr>
<tr>
<td valign="top" width="145"> <tt>\b</tt></td>
<td valign="top" width="335"> Backspace
</td>
</tr>
<tr>
<td valign="top" width="145"> <tt>\cn</tt></td>
<td valign="top" width="335"> The
Ctrl+<tt>n</tt> character </td>
</tr>
<tr>
<td valign="top" width="145"> <tt>\e</tt></td>
<td valign="top" width="335"> Escape
</td>
</tr>
<tr>
<td valign="top" width="145"> <tt>\E</tt></td>
<td valign="top" width="335"> Ends
the effect of <tt>\L</tt>, <tt>\U</tt> or <tt>\Q</tt> </td>
</tr>
<tr>
<td valign="top" width="145"> <tt>\f</tt></td>
<td valign="top" width="335"> Form
feed </td>
</tr>
<tr>
<td valign="top" width="145"> <tt>\l</tt></td>
<td valign="top" width="335"> Forces
the next letter into lowercase </td>
</tr>
<tr>
<td valign="top" width="145"> <tt>\L</tt></td>
<td valign="top" width="335"> All
following letters are lowercase </td>
</tr>
<tr>
<td valign="top" width="145"> <tt>\n</tt></td>
<td valign="top" width="335"> Newline
</td>
</tr>
<tr>
<td valign="top" width="145"> <tt>\r</tt></td>
<td valign="top" width="335"> Carriage
return </td>
</tr>
<tr>
<td valign="top" width="145"> <tt>\Q</tt></td>
<td valign="top" width="335"> Do not
look for special pattern characters </td>
</tr>
<tr>
<td valign="top" width="145"> <tt>\t</tt></td>
<td valign="top" width="335"> Tab </td>
</tr>
<tr>
<td valign="top" width="145"> <tt>\u</tt></td>
<td valign="top" width="335"> Force
next letter into uppercase </td>
</tr>
<tr>
<td valign="top" width="145"> <tt>\U</tt></td>
<td valign="top" width="335"> All
following letters are uppercase </td>
</tr>
<tr>
<td valign="top" width="145"> <tt>\v</tt></td>
<td valign="top" width="335"> Vertical
tab </td>
</tr>
</table>
</center></div>
<p align="left"> \L、\U、\Q功能可以由\E關(guān)閉掉，如：<br>
$a = "T\LHIS IS A \ESTRING"; # same as "This
is a STRING"<br>
<br>
.要在字符串中包含雙引號(hào)或反斜線，則在其前加一個(gè)反斜線，反斜線還可以取消變量替換，如：<br>
$res = "A quote \" and A backslash
\\";<br>
$result = 14;<br>
print ("The value of \$result is
$result.\n")的結(jié)果為：<br>
The value of $result is 14.<br>
<br>
.可用\nnn(8進(jìn)制)或\xnn(16進(jìn)制)來(lái)表示ASCII字符，如：<br>
$result = "\377"; # this is the character
255,or EOF<br>
$result = "\xff"; # this is also 255<br>
<br>
.單引號(hào)字符串<br>
單引號(hào)字符串與雙引號(hào)字符串有兩個(gè)區(qū)別，一是沒(méi)有變量替換功能，二是反斜線不支持轉(zhuǎn)義字符，而只在包含單引號(hào)和反斜線時(shí)起作用。單引號(hào)另一個(gè)特性是可以跨多行，如：<br>
$text = 'This is two<br>
lines of text<br>
'; <br>
與下句等效：<br>
$text = "This is two\nlines of text\n"; <br>
<br>
.字符串和數(shù)值的互相轉(zhuǎn)換<br>
例1：<br>
$string = "43";<br>
$number = 28;<br>
$result = $string + $number; # $result = 71<br>
若字符串中含有非數(shù)字的字符，則從左起至第一個(gè)非數(shù)字的字符，如：<br>
$result = "hello" * 5; # $result = 0<br>
$result = "12a34" +1; # $result = 13<br>
<br>
.變量初始值<br>
在PERL中，所有的簡(jiǎn)單變量都有缺省初始值：""，即空字符。但是建議給所有變量賦初值，否則當(dāng)程序變得大而復(fù)雜后，很容易出現(xiàn)不可預(yù)料且很難調(diào)試的錯(cuò)誤。<br>
</p>
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