ch02_08.htm

编程珍珠,里面很多好用的代码,大家可以参考学习呵呵,

<pre class="programlisting">@froots = qw(
    apple       banana      carambola
    coconut     guava       kumquat
    mandarin    nectarine   peach
    pear        persimmon   plum
);</pre>
</blockquote>

(Note that those parentheses are behaving as quote characters, not
ordinary parentheses.  We could just as easily have picked angle
brackets or braces or slashes.  But parens are pretty.)
<a name="INDEX-689"></a>
</p>

<p>A list value may also be subscripted like a normal array.  You must put
the list in parentheses (real ones) to avoid ambiguity.  Though it's
often used to fetch a single value out of a list, it's really
a slice of the list, so the syntax is:
<blockquote>
<pre class="programlisting">(LIST)[LIST]</pre>
</blockquote>

Examples:
<blockquote>
<pre class="programlisting"># Stat returns list value.
$modification_time = (stat($file))[9];

# SYNTAX ERROR HERE.
$modification_time = stat($file)[9];  # OOPS, FORGOT PARENS

# Find a hex digit.
$hexdigit = ('a','b','c','d','e','f')[$digit-10];

# A "reverse comma operator".
return (pop(@foo),pop(@foo))[0];

# Get multiple values as a slice.
($day, $month, $year) = (localtime)[3,4,5];</pre>
</blockquote>
</p>


<h3 class="sect2">2.8.1. List Assignment</h3>

<a name="INDEX-690"></a><a name="INDEX-691"></a>
<p>
<a name="INDEX-692"></a>
A list may be assigned to only if each element of the list is
itself legal to assign to:
<blockquote>
<pre class="programlisting">($a, $b, $c) = (1, 2, 3);

($map{red}, $map{green}, $map{blue}) = (0xff0000, 0x00ff00, 0x0000ff);</pre>
</blockquote>

You may assign to <tt class="literal">undef</tt> in a list.  This is useful for throwing
away some of the return values of a function:
<blockquote>
<pre class="programlisting">($dev, $ino, undef, undef, $uid, $gid) = stat($file);</pre>
</blockquote>

The final list element may be an array or a hash:
<blockquote>
<pre class="programlisting">($a, $b, @rest) = split;
my ($a, $b, %rest) = @arg_list;</pre>
</blockquote>

You can actually put an array or hash anywhere in the list you assign
to, but the first array or hash in the list will soak up all the
remaining values, and anything after it will be set to the undefined
value.  This may be useful in a <tt class="literal">local</tt> or
<tt class="literal">my</tt>, where you probably want the arrays initialized
to be empty anyway.</p>

<p>You can even assign to the empty list:
<blockquote>
<pre class="programlisting">() = funkshun();</pre>
</blockquote>

That ends up calling your function in list context, but discarding
the return values.  If you had just called the function without an
assignment, it would have instead been called in void context, which
is a kind of scalar context, and might have caused the function to
behave completely differently.
<a name="INDEX-693"></a>
</p>

<p>
<a name="INDEX-694"></a>
List assignment in scalar context returns the number of elements
produced by the expression on the <em class="emphasis">right</em> side of the assignment:
<blockquote>
<pre class="programlisting">$x = ( ($a, $b) = (7,7,7) );    # set $x to 3, not 2
$x = ( ($a, $b) = funk() );     # set $x to funk()'s return count
$x = ( () = funk() );           # also set $x to funk()'s return count</pre>
</blockquote>

This is handy when you want to do a list assignment in a Boolean
context, because most list functions return a null list when finished,
which when assigned produces a 0, which is interpreted as false.
Here's how you might use it in a <tt class="literal">while</tt> statement:
<blockquote>
<pre class="programlisting">while (($login, $password) = getpwent) {
    if (crypt($login, $password) eq $password) {
        print "$login has an insecure password!\n";
    }
}</pre>
</blockquote>
<a name="INDEX-695"></a>
</p>






<h3 class="sect2">2.8.2. Array Length</h3>

<p>
<a name="INDEX-696"></a><a name="INDEX-697"></a><a name="INDEX-698"></a><a name="INDEX-699"></a>
You may find the number of elements in the array <tt class="literal">@days</tt> by evaluating
<tt class="literal">@days</tt> in a scalar context, such as:
<blockquote>
<pre class="programlisting">@days + 0;      # implicitly force @days into a scalar context
scalar(@days)   # explicitly force @days into a scalar context</pre>
</blockquote>

Note that this only works for arrays.  It does not work for list values
in general.  As we mentioned earlier, a comma-separated list evaluated
in scalar context returns the last value, like the C comma operator.
But because you almost never actually need to know the length of a list
in Perl, this is not a problem.</p>

<p>
<a name="INDEX-700"></a><a name="INDEX-701"></a><a name="INDEX-702"></a><a name="INDEX-703"></a>
Closely related to the scalar evaluation of
<tt class="literal">@days</tt> is <tt class="literal">$#days</tt>.  This will
return the subscript of the last element of the array, or one less
than the length, since there is (ordinarily) a 0th element.  Assigning
to <tt class="literal">$#days</tt> changes the length of the array.
Shortening an array by this method destroys intervening values.  You
can gain some measure of efficiency by pre-extending an array that is
going to get big. (You can also extend an array by assigning to an
element beyond the end of the array.) You can truncate an array down
to nothing by assigning the null list <tt class="literal">()</tt> to it.
The following two statements are equivalent:
<blockquote>
<pre class="programlisting">@whatever = ();
$#whatever = -1;</pre>
</blockquote>

And the following is always true:
<blockquote>
<pre class="programlisting">scalar(@whatever) == $#whatever + 1;</pre>
</blockquote>

Truncating an array does not recover its memory.  You have to
<tt class="literal">undef(@whatever)</tt> to free its memory back to your process's memory
pool.  You probably can't free it all the way back to your system's
memory pool, because few operating systems support this.
<a name="INDEX-704"></a>
</p>

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