ch01_05.htm

编程珍珠,里面很多好用的代码,大家可以参考学习呵呵,

If you place one of these "auto" operators before the variable, it is known
as a pre-incremented (pre-decremented) variable.  Its value will be
changed before it is referenced.  If it is placed after the variable,
it is known as a post-incremented (post-decremented) variable, and its
value is changed after it is used.  For example:
<blockquote>
<pre class="programlisting">$a = 5;        # $a is assigned 5
$b = ++$a;     # $b is assigned the incremented value of $a, 6
$c = $a--;     # $c is assigned 6, then $a is decremented to 5</pre>
</blockquote>

Line 15 of our Average Example increments the number of scores by one,
so that we'll know how many scores we're averaging.  It uses a
post-increment operator (<tt class="literal">$scores++</tt>), but in this
case it doesn't matter, since the expression is in a void context,
which is just a funny way of saying that the expression is being
evaluated only for the side effect of incrementing the variable.  The
value returned is being thrown away.<a href="#FOOTNOTE-18">[18]</a><a name="INDEX-235"></a>
</p>
<blockquote class="footnote">

<a name="FOOTNOTE-18"></a>
<p>[18] The optimizer
will notice this and optimize the post-increment into a pre-increment,
because that's a bit faster to execute.  (You didn't need to know
that, but we hoped it would cheer you up.)</p>

</blockquote>






<h3 class="sect2">1.5.5. Logical Operators</h3>

<p>
<a name="INDEX-236"></a><a name="INDEX-237"></a><a name="INDEX-238"></a><a name="INDEX-239"></a>
Logical operators, also known as "short-circuit" operators, allow the
program to make decisions based on multiple criteria without using
nested <tt class="literal">if</tt> statements.  They are known as
short-circuit operators because they skip (short circuit) the
evaluation of their right argument if they decide the left argument
has already supplied enough information to decide the overall
value. This is not just for efficiency.  You are explicitly allowed to
depend on this short-circuiting behavior to avoid evaluating code in
the right argument that you know would blow up if the left argument
were not "guarding" it.  You can say "California or bust!" in Perl
without busting (presuming you do get to California).</p>

<p>
<a name="INDEX-240"></a><a name="INDEX-241"></a>
Perl actually has two sets of logical operators, a traditional set
borrowed from C and a newer (but even more traditional) set of
ultralow-precedence operators borrowed from BASIC.  Both sets contribute
to readability when used appropriately.  C's punctuational operators
work well when you want your logical operators to bind more tightly than
commas, while BASIC's word-based operators work well when you want your
commas to bind more tightly than your logical operators.  Often they work
the same, and which set you use is a matter of personal preference.  (For
contrastive examples, see <a href="ch03_20.htm#ch03-sect-logic">Section 1.20, "Logical and, or, not, and xor"</a> in <a href="ch03_01.htm">Chapter 3, "Unary and Binary Operators"</a>.)  Although the
two sets of operators are not interchangeable due to precedence, once
they're parsed, the operators themselves behave identically;
precedence merely governs the extent of their arguments.  
<a href="ch01_05.htm#perl3-tab-over-logical">Table 1-1</a> lists logical operators.</p>


<a name="perl3-tab-over-logical"></a>
<h4 class="objtitle">Table 1.1. Logical Operators</h4>
<table border="1">

<tr>
<th>Example</th>
<th>Name</th>
<th>Result</th>
</tr>




<tr>
<td><tt class="literal">$a &amp;&amp; $b</tt></td>
<td>And</td>
<td><tt class="literal">$a</tt> if <tt class="literal">$a</tt> is false,  <tt class="literal">$b</tt> otherwise<a name="INDEX-242"></a><a name="INDEX-243"></a></td>
</tr>

<tr>
<td><tt class="literal">$a || $b</tt></td>
<td>Or</td>
<td><tt class="literal">$a</tt> if <tt class="literal">$a</tt>  is true, <tt class="literal">$b</tt> otherwise<a name="INDEX-244"></a><a name="INDEX-245"></a></td>
</tr>

<tr>
<td><tt class="literal">! $a</tt></td>
<td>Not</td>
<td>True if <tt class="literal">$a</tt> is not true</td>
</tr>

<tr>
<td><tt class="literal">$a and $b</tt></td>
<td>And</td>
<td><tt class="literal">$a</tt> if <tt class="literal">$a</tt> is false, <tt class="literal">$b</tt> otherwise</td>
</tr>

<tr>
<td><tt class="literal">$a or $b</tt></td>
<td>Or</td>
<td><tt class="literal">$a</tt> if <tt class="literal">$a</tt>  is true, <tt class="literal">$b</tt> otherwise<a name="INDEX-246"></a></td>
</tr>

<tr>
<td><tt class="literal">not $a</tt></td>
<td>Not</td>
<td>True if <tt class="literal">$a</tt> is not true</td>
</tr>

<tr>
<td><tt class="literal">$a xor $b</tt></td>
<td>Xor</td>
<td>True if <tt class="literal">$a</tt> or <tt class="literal">$b</tt> is true, but not both<a name="INDEX-247"></a></td>
</tr>


</table>

<p>
<a name="INDEX-248"></a><a name="INDEX-249"></a>
Since the logical operators "short-circuit" the way they do, they're
often used in Perl to conditionally execute code.  The following line
(line 3 from our Average Example) tries to open the file
<em class="emphasis">grades</em>:
<blockquote>
<pre class="programlisting">open(GRADES, "grades") or die "Can't open file grades: $!\n";</pre>
</blockquote>

If it opens the file, it will jump to the next line of the program.  If
it can't open the file, it will provide us with an error message and
then stop execution.</p>

<p>
<a name="INDEX-250"></a>
Literally, this line means "Open <em class="emphasis">grades</em> or bust!"
Besides being another example of natural language, the short-circuit
operators preserve the visual flow. Important actions are listed down
the left side of the screen, and secondary actions are hidden off to
the right.  (The <tt class="literal">$!</tt> variable contains the error
message returned by the operating system--see
<a href="ch28_01.htm">Chapter 28, "Special Names"</a>.)  Of course, these logical
operators can also be used within the more traditional kinds of
conditional constructs, such as the <tt class="literal">if</tt> and
<tt class="literal">while</tt> statements.
<a name="INDEX-251"></a><a name="INDEX-252"></a>
</p>






<h3 class="sect2">1.5.6. Some Numeric and String Comparison Operators</h3>

<p>
<a name="INDEX-253"></a><a name="INDEX-254"></a><a name="INDEX-255"></a><a name="INDEX-256"></a>
</p>

<p>Comparison, or relational, operators tell us how two scalar values
(numbers or strings) relate to each other.  There are two sets of
operators; one does numeric comparison and the other does string
comparison.  (In either case, the arguments will be "coerced" to have
the appropriate type first.)  Assuming left and right arguments of
<tt class="literal">$a</tt> and <tt class="literal">$b</tt>, we have:</p>

<a name="perl3-tab-over-compare"></a>
<table border="1">

<tr>
<th>Comparison</th>
<th>Numeric</th>
<th>String</th>
<th>Return Value</th>
</tr>




<tr>
<td>Equal</td>
<td><tt class="literal">==</tt></td>
<td><tt class="literal">eq</tt></td>
<td>True if <tt class="literal">$a</tt> is equal to <tt class="literal">$b</tt><a name="INDEX-257"></a></td>
</tr>

<tr>
<td>Not equal</td>
<td><tt class="literal">!=</tt></td>
<td><tt class="literal">ne</tt></td>
<td>True if <tt class="literal">$a</tt> is not equal to <tt class="literal">$b</tt><a name="INDEX-258"></a></td>
</tr>

<tr>
<td>Less than</td>
<td><tt class="literal">&lt;</tt></td>
<td><tt class="literal">lt</tt></td>
<td>True if <tt class="literal">$a</tt> is less than <tt class="literal">$b</tt><a name="INDEX-259"></a></td>
</tr>

<tr>
<td>Greater than</td>
<td><tt class="literal">&gt;</tt></td>
<td><tt class="literal">gt</tt></td>
<td>True if <tt class="literal">$a</tt> is greater than <tt class="literal">$b</tt><a name="INDEX-260"></a></td>
</tr>

<tr>
<td>Less than or equal</td>
<td><tt class="literal">&lt;=</tt></td>
<td><tt class="literal">le</tt></td>
<td>True if <tt class="literal">$a</tt> not greater than <tt class="literal">$b</tt><a name="INDEX-261"></a></td>
</tr>

<tr>
<td>Comparison</td>
<td><tt class="literal">&lt;=&gt;</tt></td>
<td><tt class="literal">cmp</tt></td>
<td>0 if equal, 1 if <tt class="literal">$a</tt> greater, -1 if <tt class="literal">$b</tt> greater<a name="INDEX-262"></a><a name="INDEX-263"></a><a name="INDEX-264"></a></td>
</tr>


</table>
<p>
<a name="INDEX-265"></a><a name="INDEX-266"></a>
The last pair of operators (<tt class="literal">&lt;=&gt;</tt> and
<tt class="literal">cmp</tt>) are entirely redundant. However, they're
incredibly useful in <tt class="literal">sort</tt> subroutines (see
<a href="ch29_01.htm">Chapter 29, "Functions"</a>).<a href="#FOOTNOTE-19">[19]</a>
</p>
<blockquote class="footnote">

<a name="FOOTNOTE-19"></a>
<p>[19] Some folks feel
that such redundancy is evil because it keeps a language from being
minimalistic, or orthogonal.  But Perl isn't an orthogonal language;
it's a diagonal language.  By this we mean that Perl doesn't force you
to always go at right angles.  Sometimes you just want to follow the
hypotenuse of the triangle to get where you're going.  TMTOWTDI is
about shortcuts.  Shortcuts are about efficiency.</p>

</blockquote>






<h3 class="sect2">1.5.7. Some File Test Operators</h3>

<p>
<a name="INDEX-267"></a><a name="INDEX-268"></a><a name="INDEX-269"></a>
The file test operators allow you to test whether certain file
attributes are set before you go and blindly muck about with the files.
The most basic file attribute is, of course, whether the file exists.
For example, it would be very nice to know whether your mail aliases
file already exists before you go and open it as a new file, wiping out
everything that was in there before.  Here are a few of the
file test operators:</p>

<a name="perl3-tab-over-file"></a>
<table border="1">

<tr>
<th>Example</th>
<th>Name</th>
<th>Result</th>
</tr>




<tr>
<td><tt class="literal">-e $a</tt></td>
<td>Exists</td>
<td>True if file named in <tt class="literal">$a</tt> exists<a name="INDEX-270"></a></td>
</tr>

<tr>
<td><tt class="literal">-r $a</tt></td>
<td>Readable</td>
<td>True if file named in <tt class="literal">$a</tt> is readable<a name="INDEX-271"></a><a name="INDEX-272"></a></td>
</tr>

<tr>
<td><tt class="literal">-w $a</tt></td>
<td>Writable</td>
<td>True if file named in <tt class="literal">$a</tt> is writable<a name="INDEX-273"></a><a name="INDEX-274"></a></td>
</tr>

<tr>
<td><tt class="literal">-d $a</tt></td>
<td>Directory</td>
<td>True if file named in <tt class="literal">$a</tt> is a directory<a name="INDEX-275"></a><a name="INDEX-276"></a></td>
</tr>

<tr>
<td><tt class="literal">-f $a</tt></td>
<td>File</td>
<td>True if file named in <tt class="literal">$a</tt> is a regular file<a name="INDEX-277"></a><a name="INDEX-278"></a></td>
</tr>

<tr>
<td><tt class="literal">-T $a</tt></td>
<td>Text File</td>
<td>True if file named in <tt class="literal">$a</tt> is a text file<a name="INDEX-279"></a><a name="INDEX-280"></a></td>
</tr>


</table>
<p>You might use them like this:
<blockquote>
<pre class="programlisting">-e "/usr/bin/perl" or warn "Perl is improperly installed\n";
-f "/vmlinuz" and print "I see you are a friend of Linus\n";</pre>
</blockquote>
Note that a regular file is not the same thing as a text file.  Binary
files like <em class="emphasis">/vmlinuz</em> are regular files, but they
aren't text files.  Text files are the opposite of binary files, while
regular files are the opposite of "irregular"
files like directories
and devices.  <a name="INDEX-281"></a><a name="INDEX-282"></a>
</p>

<p>
<a name="INDEX-283"></a><a name="INDEX-284"></a>
There are a lot of file test operators, many of which we didn't list.
Most of the file tests are unary Boolean operators, which is to say
they take only one operand (a scalar that evaluates to a filename or a
filehandle), and they return either a true or false value.  A few of
them return something fancier, like the file's size or age, but you can
look those up when you need them in the section <a href="ch03_10.htm#ch03-sect-nu">Section 1.10, "Named Unary and File Test Operators"</a> in <a href="ch03_01.htm">Chapter 3, "Unary and Binary Operators"</a>.</p>



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