ch16.htm

《Perl 5 Unreleased》

<P>
will let the named <TT><FONT FACE="Courier">program</FONT></TT>
use the bell character (7 octal) as the end-of-line character
when it reads the input files that have a <TT><FONT FACE="Courier">.bin</FONT></TT>
extension. 
<P>
For example, the following header comment line will set the end-of-line
character to a space, (octal 40): 
<BLOCKQUOTE>
<TT><FONT FACE="Courier">#!perl -0 040</FONT></TT>
</BLOCKQUOTE>
<P>
To read one paragraph at a time, specify <TT><FONT FACE="Courier">00</FONT></TT>
as the input to the <TT><FONT FACE="Courier">-0</FONT></TT> option.
This will let the Perl interpreter read input until it sees two
newlines together, and thus you will be able to read in one paragraph
at a time. If you do not specify a value with the <TT><FONT FACE="Courier">-0</FONT></TT>
option, the Perl interpreter assumes the null character (ASCII
0).
<H3><A NAME="UsingthelOption">Using the <TT><FONT SIZE=4 FACE="Courier">-l</FONT></TT><FONT SIZE=4>
Option</FONT></A><FONT SIZE=4> </FONT></H3>
<P>
The <TT><FONT FACE="Courier">-l</FONT></TT> option lets you use
a new output end-of-line character for printing statements. Like
the <TT><FONT FACE="Courier">-0</FONT></TT> option, the <TT><FONT FACE="Courier">-l</FONT></TT>
option takes an octal number instead of an ASCII character for
use in place of the newline. This is a one, not the letter &quot;el.&quot;
When the <TT><FONT FACE="Courier">-l</FONT></TT> option is specified,
the Perl interpreter always replaces the end-of-line character
in print statements with the newer version. Also, in the case
of <TT><FONT FACE="Courier">-n</FONT></TT> or <TT><FONT FACE="Courier">-p</FONT></TT>
options, the end-of-line character is removed after reading the
input. 
<P>
The Perl interpreter uses the character specified by the <TT><FONT FACE="Courier">-0</FONT></TT>
option, if it is defined, in case you do not specify the <TT><FONT FACE="Courier">-I</FONT></TT>
option. If <TT><FONT FACE="Courier">-0</FONT></TT> also has not
been specified, the end-of-line character is set to the newline
character.
<P>
When using both the <TT><FONT FACE="Courier">-l</FONT></TT> and
the <TT><FONT FACE="Courier">-0</FONT></TT> option, specify the
<TT><FONT FACE="Courier">-1</FONT></TT> option first, then <TT><FONT FACE="Courier">-0</FONT></TT>
option. Recall that options are processed from left to right.
If the <TT><FONT FACE="Courier">-l</FONT></TT> option appears
first, the output end-of-line character is set to the newline
character. If the <TT><FONT FACE="Courier">-0</FONT></TT> option
appears first, the output end-of-line character (set by <TT><FONT FACE="Courier">-l</FONT></TT>)
becomes the same as the input end-of-line character (set by <TT><FONT FACE="Courier">-0</FONT></TT>).
<P>
<CENTER>
<TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%>
<TR VALIGN=TOP><TD ><B>Note</B></TD></TR>
<TR VALIGN=TOP><TD >
<BLOCKQUOTE>
It's probably easier to control the input and output end-of-line characters also by using the system variables <TT><FONT FACE="Courier">$/</FONT></TT> and <TT><FONT FACE="Courier">$\</FONT></TT>, respectively. 
</BLOCKQUOTE>

</TD></TR>
</TABLE></CENTER>
<P>
<H3><A NAME="UsingthexOptiontoGetaPerlProgra">Using the <TT><FONT SIZE=4 FACE="Courier">-x</FONT></TT><FONT SIZE=4>
Option to Get a Perl Program from Another File</FONT></A></H3>
<P>
The <TT><FONT FACE="Courier">-x</FONT></TT> option enables you
to process a Perl program that appears in the middle of a file.
When the <TT><FONT FACE="Courier">-x</FONT></TT> option is specified,
the Perl interpreter ignores every line in the program until it
sees a header comment. The Perl interpreter then processes the
program as usual until the bottom of the program file is reached
or the <TT><FONT FACE="Courier">__END__</FONT></TT> statement
is reached. Everything after the <TT><FONT FACE="Courier">__END__</FONT></TT>
statement is ignored by the Perl interpreter. 
<H3><A NAME="UsingtheSOption">Using the <TT><FONT SIZE=4 FACE="Courier">-S</FONT></TT><FONT SIZE=4>
Option</FONT></A></H3>
<P>
You need to use <TT><FONT FACE="Courier">-S</FONT></TT> only if
you run your Perl program using the <TT><FONT FACE="Courier">Perl
</FONT></TT>command. If you run a program directly using a script,
the <TT><FONT FACE="Courier">-S</FONT></TT> option is meaningless
because the shell will hunt for your program in the directories
specified in your <TT><FONT FACE="Courier">PATH</FONT></TT> environment
variable. The <TT><FONT FACE="Courier">-S</FONT></TT> option simply
tells the Perl interpreter that your program might be contained
in any of the directories specified by your <TT><FONT FACE="Courier">PATH</FONT></TT>
environment variable. 
<H3><A NAME="ThevOptionPrintingthePerlVersion">The <TT><FONT SIZE=4 FACE="Courier">-v</FONT></TT><FONT SIZE=4>
Option: Printing the Perl Version Number</FONT></A></H3>
<P>
You might be curious as to which version of Perl you are running.
The <TT><FONT FACE="Courier">-v</FONT></TT> option prints a string
with the version information for the Perl interpreter you are
running. The Perl interpreter will not run any scripts, nor will
it honor any other options when this <TT><FONT FACE="Courier">-v</FONT></TT>
option is specified. Here is sample output from the <TT><FONT FACE="Courier">-v</FONT></TT>
command:
<BLOCKQUOTE>
<TT><FONT FACE="Courier">$ <B>perl -v<BR>
</B>This is perl, version 5.002<BR>
<BR>
Copyright 1987-1996, Larry Wall<BR>
<BR>
Perl may be copied only under the terms of either the Artistic
License or the<BR>
GNU General Public License, which may be found in the Perl 5.0
source kit.</FONT></TT>
</BLOCKQUOTE>
<P>
Now that you've learned the command-line options for the Perl
interpreter, you're ready to learn how to process input in your
Perl applications.
<H3><A NAME="UsingConditionalCodewiththeCPrepro">Using Conditional
Code with the C Preprocessor</A></H3>
<P>
The C preprocessor also provided five statements, <TT><FONT FACE="Courier">#ifdef</FONT></TT>,
<TT><FONT FACE="Courier">#ifndef</FONT></TT>,<TT><FONT FACE="Courier">
#if</FONT></TT>, <TT><FONT FACE="Courier">#else</FONT></TT>, and
<TT><FONT FACE="Courier">#endif</FONT></TT>, for conditional statements
to include or exclude parts of your Perl program. The syntax for
the <TT><FONT FACE="Courier">#ifdef</FONT></TT> and <TT><FONT FACE="Courier">#endif</FONT></TT>
statements is
<BLOCKQUOTE>
<TT><FONT FACE="Courier">#ifdef <I>cond<BR>
<I>...code if cond is defined...<BR>
</I></I>#else <BR>
<I>...code if cond is NOT defined...<BR>
</I>#endif</FONT></TT>
</BLOCKQUOTE>
<P>
The <TT><I><FONT FACE="Courier">cond</FONT></I></TT> is a character
string that can be used in a <TT><FONT FACE="Courier">#define</FONT></TT>
statement. If the character string has been defined to a value,
the first set of code (above the <TT><FONT FACE="Courier">#else</FONT></TT>
clause) is inserted in your program; otherwise, the second part
of code (after the <TT><FONT FACE="Courier">#else</FONT></TT>
and before the <TT><FONT FACE="Courier">#endif</FONT></TT> clause)
is inserted in your program. Because the <TT><FONT FACE="Courier">#else</FONT></TT>
clause is optional, you can also have statements of the form
<BLOCKQUOTE>
<TT><FONT FACE="Courier">#ifdef <I>cond<BR>
<I>...code if cond is defined...<BR>
</I></I>#endif</FONT></TT>
</BLOCKQUOTE>
<P>
The <TT><FONT FACE="Courier">#ifndef</FONT></TT> lets you define
code that is to be executed when a particular string is not defined.
Thus, <TT><FONT FACE="Courier">#ifndef</FONT></TT> takes the opposite
action of the <TT><FONT FACE="Courier">#ifdef</FONT></TT> statement.
For example:
<BLOCKQUOTE>
<TT><FONT FACE="Courier">#ifdef SOMBER<BR>
print (&quot;Hello, Cruel world!\n&quot;);<BR>
#else<BR>
print (&quot;Hello, Beautiful world!\n&quot;);<BR>
#endif</FONT></TT>
</BLOCKQUOTE>
<P>
This code prints a sad message (<TT><FONT FACE="Courier">Hello,
Cruel world!</FONT></TT>) if <TT><FONT FACE="Courier">SOMBER</FONT></TT>
was defined earlier, or a happy message (<TT><FONT FACE="Courier">Hello,
Beautiful world!</FONT></TT>) if <TT><FONT FACE="Courier">SOMBER</FONT></TT>
was not defined earlier. 
<P>
Code enclosed by <TT><FONT FACE="Courier">#ifdef</FONT></TT> and
<TT><FONT FACE="Courier">#endif</FONT></TT> does not have to be
a complete Perl statement. For example, the following code will
set the value of <TT><FONT FACE="Courier">$result</FONT></TT>
to different settings based on the whether or not <TT><FONT FACE="Courier">METRIC</FONT></TT>
was defined:
<BLOCKQUOTE>
<TT><FONT FACE="Courier">$area =&nbsp;&nbsp;$radius * PI * 2<BR>
#ifdef METRIC<BR>
* 2.54<BR>
#endif<BR>
;</FONT></TT>
</BLOCKQUOTE>
<P>
Here, <TT><FONT FACE="Courier">$result</FONT></TT> is assigned
a value in centimeters if <TT><FONT FACE="Courier">METRIC</FONT></TT>
is defined or in inches if it's not.<P>
<CENTER>
<TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%>
<TR VALIGN=TOP><TD ><B>Tip</B></TD></TR>
<TR VALIGN=TOP><TD >
<BLOCKQUOTE>
Don't overuse the C preprocessor because it might make your program hard to read, especially by people who are not familiar with the C programming language.</BLOCKQUOTE>

</TD></TR>
</TABLE></CENTER>
<P>
<P>
The <TT><FONT FACE="Courier">#if</FONT></TT> statement in the
C preprocessor is similar to the <TT><FONT FACE="Courier">#ifdef</FONT></TT>
statement. The <TT><FONT FACE="Courier">#if</FONT></TT> statement
uses the value of a variable, whereas the <TT><FONT FACE="Courier">#ifdef</FONT></TT>
statement simply checks to see whether a variable is defined.
The syntax for the <TT><FONT FACE="Courier">#if</FONT></TT> statement
is as follows:
<BLOCKQUOTE>
<TT><FONT FACE="Courier">#if <I>expr<BR>
<I>...code...<BR>
</I></I>#endif</FONT></TT>
</BLOCKQUOTE>
<P>
The <TT><I><FONT FACE="Courier">expr</FONT></I></TT> is the expression
that is evaluated by the C preprocessor, and <TT><I><FONT FACE="Courier">code</FONT></I></TT>
is the code to be executed if <TT><I><FONT FACE="Courier">expr</FONT></I></TT>
is nonzero. For example, the following statements will set the
value of <TT><FONT FACE="Courier">$result</FONT></TT> to <TT><FONT FACE="Courier">&quot;hello&quot;</FONT></TT>
if the sum of <TT><FONT FACE="Courier">S1</FONT></TT> and <TT><FONT FACE="Courier">S2</FONT></TT>
is nonzero:
<BLOCKQUOTE>
<TT><FONT FACE="Courier">#if S1 + S2<BR>
$result = &quot;hello&quot;;<BR>
#endif</FONT></TT>
</BLOCKQUOTE>
<P>
If you want to set the value of <TT><FONT FACE="Courier">$result</FONT></TT>
if either <TT><FONT FACE="Courier">S1</FONT></TT> or <TT><FONT FACE="Courier">S2</FONT></TT>
is set to a nonzero value, you can use the following statement:
<BLOCKQUOTE>
<TT><FONT FACE="Courier">#if S1 || S2<BR>
$result = &quot;hello&quot;;<BR>
#endif</FONT></TT>
</BLOCKQUOTE>
<P>
By specifying <TT><FONT FACE="Courier">0</FONT></TT> to the <TT><FONT FACE="Courier">#if</FONT></TT>
statement, you can easily prevent lines of code from being interpreted
without having to put a hash (<TT><FONT FACE="Courier">#</FONT></TT>)
in front of each line:
<BLOCKQUOTE>
<TT><FONT FACE="Courier">#if 0<BR>
$result = &quot;hello&quot;;<BR>
print (&quot;I will not be printed if the -P option is used.\n&quot;);
<BR>
#endif</FONT></TT>
</BLOCKQUOTE>
<P>
You can also use <TT><FONT FACE="Courier">#else</FONT></TT> with
the <TT><FONT FACE="Courier">#if</FONT></TT> operator:
<BLOCKQUOTE>
<TT><FONT FACE="Courier">#if S1 || S2<BR>
$result = &quot;hello&quot;;<BR>
#else<BR>
$result = &quot;goodbye&quot;; <BR>
#endif</FONT></TT>
</BLOCKQUOTE>
<P>
In this case, the value of <TT><FONT FACE="Courier">$result</FONT></TT>
will be <TT><FONT FACE="Courier">&quot;hello&quot;</FONT></TT>
if either <TT><FONT FACE="Courier">S1</FONT></TT> or <TT><FONT FACE="Courier">S2</FONT></TT>
has a nonzero value; otherwise, the value will be <TT><FONT FACE="Courier">&quot;goodbye&quot;</FONT></TT>.
<P>
<CENTER>
<TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%>
<TR VALIGN=TOP><TD ><B>Note</B></TD></TR>
<TR VALIGN=TOP><TD >
<BLOCKQUOTE>
The C preprocessor does not support the exponent operator, so you cannot evaluate <BR>
<TT><FONT FACE="Courier">(x ** y)</FONT></TT> with the <TT><FONT FACE="Courier">#if</FONT></TT> statement.
</BLOCKQUOTE>

</TD></TR>
</TABLE></CENTER>
<P>
<P>
You can embed <TT><FONT FACE="Courier">#ifdef</FONT></TT>/<TT><FONT FACE="Courier">#else</FONT></TT>/<TT><FONT FACE="Courier">#endif</FONT></TT>
constructs inside one another. Just make sure that you match all
the <TT><FONT FACE="Courier">#ifdef</FONT></TT> and <TT><FONT FACE="Courier">#endif</FONT></TT>
statements so that there is one <TT><FONT FACE="Courier">#endif</FONT></TT>
for each <TT><FONT FACE="Courier">#ifdef</FONT></TT> and <TT><FONT FACE="Courier">#ifndef</FONT></TT>
statement. For example, here is a snippet of code that illustrates