📄 perl 语言－perl 中文教程（第七章）.htm
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class=myFont><SPAN class=myFont><FONT face=宋体>翻译：</FONT></SPAN></SPAN>
<SPAN class=myFont><SPAN class=myFont>flamephoenix</SPAN></SPAN> <BR>
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<P align=center><B>第七章 控制结构</B></P>
<P><A
href="http://www.sun126.com/perl5/perl5-7.htm#一、条件判断">一、条件判断</A><BR><A
href="http://www.sun126.com/perl5/perl5-7.htm#二、循环：">二、循环：</A><BR> <A
href="http://www.sun126.com/perl5/perl5-7.htm#1、while循环">1、while循环</A>
<BR> <A
href="http://www.sun126.com/perl5/perl5-7.htm#2、until循环">2、until循环</A>
<BR> <A
href="http://www.sun126.com/perl5/perl5-7.htm#3、类C的for循环">3、for循环</A>
<BR> <A
href="http://www.sun126.com/perl5/perl5-7.htm#4、针对列表(数组)每个元素的循环">4、针对列表(数组)每个元素的foreach循环</A><BR>
<A
href="http://www.sun126.com/perl5/perl5-7.htm#5、do循环">5、do循环</A><BR> <A
href="http://www.sun126.com/perl5/perl5-7.htm#6、循环控制">6、循环控制</A>
<BR> <A
href="http://www.sun126.com/perl5/perl5-7.htm#7、传统的goto label;语句。">7、传统的goto语句</A><BR><A
href="http://www.sun126.com/perl5/perl5-7.htm#三、单行条件">三、单行条件</A><BR><BR><A
name=一、条件判断>一、条件判断</A> <BR> if ( <expression>)
{<BR> <statement_block_1><BR> }<BR> elsif
( <expression> )
{<BR> <statement_block_2><BR> }<BR> ...<BR> else{<BR> <statement_block_3><BR> }<BR><BR><A
name=二、循环：>二、循环：</A> <BR><A name=1、while循环>1、while循环</A>
<BR> while ( <expression> )
{<BR> <statement_block><BR> }<BR><A
name=2、until循环>2、until循环</A> <BR> until ( <expression> )
{<BR> <statement_block><BR> }<BR><A
name=3、类C的for循环>3、类C的for循环</A> ，如<BR> for ($count=1; $count
<= 5; $count++) {<BR> # statements inside the
loop go here<BR> }<BR>下面是在for循环中使用逗号操作符的例子：<BR> for
($line = <STDIN>, $count = 1; $count <= 3; $line =
<STDIN>, $count++) {<BR> print
($line);<BR> }<BR>它等价于下列语句：<BR> $line =
<STDIN>;<BR> $count = 1;<BR> while ($count
<= 3) { <BR> print
($line);<BR> $line =
<STDIN>;<BR> $count++;<BR> }<BR><A
name=4、针对列表(数组)每个元素的循环>4、针对列表(数组)每个元素的循环：foreach，语法为：</A><BR> foreach
localvar (listexpr)
{<BR> statement_block;<BR> }<BR>例：<BR> foreach
$word (@words) {<BR> if ($word eq "the")
{<BR> print ("found the word 'the'\n");
<BR> }<BR> }<BR>注：<BR>(1)此处的循环变量localvar是个局部变量，如果在此之前它已有值，则循环后仍恢复该值。<BR>(2)在循环中改变局部变量，相应的数组变量也会改变，如：<BR> @list
= (1, 2, 3, 4, 5);<BR> foreach $temp (@list)
{<BR> if ($temp == 2)
{<BR> $temp =
20;<BR> }<BR> }<BR>此时@list已变成了(1, 20, 3,
4, 5)。<BR><A name=5、do循环>5、do循环</A> <BR> do
{<BR> statement_block<BR> }
while_or_until (condexpr);<BR> do循环至少执行一次循环。<BR><A
name=6、循环控制>6、循环控制</A>
<BR> 退出循环为last，与C中的break作用相同；执行下一个循环为next，与C中的continue作用相同；PERL特有的一个命令是redo，其含义是重复此次循环，即循环变量不变，回到循环起始点，但要注意，redo命令在do循环中不起作用。<BR><A
name="7、传统的goto label;语句。">7、传统的goto label;语句。</A> <BR><BR><A
name=三、单行条件>三、单行条件</A> <BR> 语法为statement keyword
condexpr。其中keyword可为if、unless、while或until，如：<BR> print
("This is zero.\n") if ($var == 0);<BR> print
("This is zero.\n") unless ($var != 0);<BR> print
("Not zero yet.\n") while ($var-- >
0);<BR> print ("Not zero yet.\n") until ($var-- ==
0);<BR> 虽然条件判断写在后面，但却是先执行的。</P></TD></TR></TBODY></TABLE>
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