ch03_01.htm

编程珍珠,里面很多好用的代码,大家可以参考学习呵呵,

defined usually follow your intuition, presuming you're not psychotic.
And second, if you're merely neurotic, you can always put in extra
parentheses to relieve your anxiety.</p>

<p>
<a name="INDEX-788"></a><a name="INDEX-789"></a>
Another helpful hint is that any operators borrowed from C keep the
same precedence relationship with each other, even where C's precedence
is slightly screwy. (This makes learning Perl easier for C folks and
C++ folks.  Maybe even Java folks.)</p>

<p>
<a name="INDEX-790"></a>
The following sections cover these operators in precedence
order.  With very few exceptions, these all operate on scalar values
only, not list values.  We'll mention the exceptions as they come up.</p>

<p>
<a name="INDEX-791"></a><a name="INDEX-792"></a>
Although references are scalar values, using most of these operators
on references doesn't make much sense, because the numeric value of
a reference is only meaningful to the internals of Perl.  Nevertheless,
if a reference points to an object of a class that allows overloading,
you can call these operators on such objects, and if the class has
defined an overloading for that particular operator, it will define
how the object is to be treated under that operator.  This is how
complex numbers are implemented in Perl, for instance.  For more on
overloading, see <a href="ch13_01.htm">Chapter 13, "Overloading"</a>.</p>


<h2 class="sect1">3.1. Terms and List Operators (Leftward)</h2>

<a name="INDEX-793"></a><a name="INDEX-794"></a><a name="INDEX-795"></a><a name="INDEX-796"></a><a name="INDEX-797"></a><a name="INDEX-798"></a><a name="INDEX-799"></a><a name="INDEX-800"></a>
<p>Any <em class="emphasis">term</em> is of highest precedence in Perl.  Terms
include variables, quote and quotelike operators, most expressions in
parentheses, or brackets or braces, and any function whose arguments
are parenthesized.  Actually, there aren't really any functions in
this sense, just list operators and unary operators behaving as
functions because you put parentheses around their
arguments. Nevertheless, the name of Chapter 29 is
<em class="emphasis">Functions</em>.</p>

<p>
<a name="INDEX-801"></a>
Now listen carefully.  Here are a couple of rules that are very
important and simplify things greatly, but may occasionally produce
counterintuitive results for the unwary.  If any list operator (such
as <tt class="literal">print</tt>) or any named unary operator (such as
<tt class="literal">chdir</tt>) is followed by a left parenthesis as the
next token (ignoring whitespace), the operator and its parenthesized
arguments are given highest precedence, as if it were a normal
function call.  The rule is this: If it <em class="emphasis">looks</em>
like a function call, it <em class="emphasis">is</em> a function call. You
can make it look like a nonfunction by prefixing the parentheses with
a unary plus, which does absolutely nothing, semantically speaking--it
doesn't even coerce the argument to be numeric.
<a name="INDEX-802"></a>
</p>

<p>
<a name="INDEX-803"></a><a name="INDEX-804"></a><a name="INDEX-805"></a><a name="INDEX-806"></a><a name="INDEX-807"></a>
For example, since || has lower precedence than <tt class="literal">chdir</tt>, we get:
<blockquote>
<pre class="programlisting">chdir $foo    || die;       # (chdir $foo) || die
chdir($foo)   || die;       # (chdir $foo) || die
chdir ($foo)  || die;       # (chdir $foo) || die
chdir +($foo) || die;       # (chdir $foo) || die</pre>
</blockquote>

but, because <tt class="literal">*</tt> has higher precedence than <tt class="literal">chdir</tt>, we get:
<blockquote>
<pre class="programlisting">chdir $foo * 20;            # chdir ($foo * 20)
chdir($foo) * 20;           # (chdir $foo) * 20
chdir ($foo) * 20;          # (chdir $foo) * 20
chdir +($foo) * 20;         # chdir ($foo * 20)</pre>
</blockquote>

Likewise for any numeric operator that happens to be a named unary operator,
such as <tt class="literal">rand</tt>:
<blockquote>
<pre class="programlisting">rand 10 * 20;               # rand (10 * 20)
rand(10) * 20;              # (rand 10) * 20
rand (10) * 20;             # (rand 10) * 20
rand +(10) * 20;            # rand (10 * 20)</pre>
</blockquote>

In the absence of parentheses, the precedence of list operators such as
<tt class="literal">print</tt>, <tt class="literal">sort</tt>, or <tt class="literal">chmod</tt> is either very high or very low depending
on whether you look at the left side or the right side of the operator.
(That's what the "Leftward" is doing in the title of this
section.)  For example, in:
<blockquote>
<pre class="programlisting">@ary = (1, 3, sort 4, 2);
print @ary;         # prints 1324</pre>
</blockquote>
the commas on the right of the <tt class="literal">sort</tt> are evaluated
before the <tt class="literal">sort</tt>, but the commas on the left are
evaluated after.  In other words, a list operator tends to gobble up
all the arguments that follow it, and then act like a simple term with
regard to the preceding expression.  You still have to be careful with
parentheses:
<blockquote>
<pre class="programlisting"># These evaluate exit before doing the print:
print($foo, exit);  # Obviously not what you want.
print $foo, exit;   # Nor this.

# These do the print before evaluating exit:
(print $foo), exit; # This is what you want.
print($foo), exit;  # Or this.
print ($foo), exit; # Or even this.</pre>
</blockquote>

The easiest place to get burned is where you're using parentheses to
group mathematical arguments, and you forget that parentheses are also
used to group function arguments:
<blockquote>
<pre class="programlisting">print ($foo &amp; 255) + 1, "\n";   # prints ($foo &amp; 255)</pre>
</blockquote>

That probably doesn't do what you expect at first glance.  Fortunately,
mistakes of this nature generally produce warnings like "<tt class="literal">Useless use of addition in a void context</tt>" when warnings are enabled.</p>

<p>
<a name="INDEX-808"></a><a name="INDEX-809"></a>

Also parsed as terms are the <tt class="literal">do {}</tt> and <tt class="literal">eval {}</tt> constructs, as well
as subroutine and method calls, the anonymous array and hash composers
<tt class="literal">[]</tt> and <tt class="literal">{}</tt>, and the anonymous subroutine composer <tt class="literal">sub {}</tt>.</p>





























































































































































































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