ch09_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,

    $row = $AoA[$i];
    for $j ( 0 .. $#{$row} ) {
        print "element $i $j is $row->[$j]\n";
    }
}</pre>
</blockquote>
</p>






<h3 class="sect2">9.1.4. Slices</h3>

<a name="INDEX-2097"></a>
<p>
<a name="INDEX-2098"></a><a name="INDEX-2099"></a>
If you want to access a slice (part of a row) of a multidimensional
array, you're going to have to do some fancy subscripting.  The
pointer arrows give us a nice way to access a single element, but no
such convenience exists for slices.  You can always extract the
elements of your slice one-by-one with a loop:
<blockquote>
<pre class="programlisting">@part = ();
for ($y = 7; $y &lt; 13; $y++) {
    push @part, $AoA[4][$y];
}</pre>
</blockquote>

This particular loop could be replaced with an array slice:
<blockquote>
<pre class="programlisting">@part = @{ $AoA[4] } [ 7..12 ];</pre>
</blockquote>

If you want a <em class="emphasis">two-dimensional slice</em>, say, with <tt class="literal">$x</tt> running from
<tt class="literal">4..8</tt> and <tt class="literal">$y</tt> from <tt class="literal">7..12</tt>, here's one way to do it:
<blockquote>
<pre class="programlisting">@newAoA = ();
for ($startx = $x = 4; $x &lt;= 8; $x++) {
    for ($starty = $y = 7; $y &lt;= 12; $y++) {
        $newAoA[$x - $startx][$y - $starty] = $AoA[$x][$y];
    }
}</pre>
</blockquote>
<a name="INDEX-2100"></a>
In this example, the individual values within our destination
two-dimensional array, <tt class="literal">@newAoA</tt>, are assigned one by one, taken from a
two-dimensional subarray of <tt class="literal">@AoA</tt>.  An alternative is to create
anonymous arrays, each consisting of a desired slice of an <tt class="literal">@AoA</tt>
subarray, and then put references to these anonymous arrays into
<tt class="literal">@newAoA</tt>.  We would then be writing references into <tt class="literal">@newAoA</tt>
(subscripted once, so to speak) instead of subarray values into a
twice-subscripted <tt class="literal">@newAoA</tt>.  This method eliminates the innermost
loop:
<blockquote>
<pre class="programlisting">for ($x = 4; $x &lt;= 8; $x++) {
    push @newAoA, [ @{ $AoA[$x] } [ 7..12 ] ];
}</pre>
</blockquote>
<a name="INDEX-2101"></a>
Of course, if you do this often, you should probably write a
subroutine called something like <tt class="literal">extract_rectangle</tt>.
And if you do it very often with large collections of multidimensional
data, you should probably use the <tt class="literal">PDL</tt> (Perl Data
Language) module, available from CPAN.</p>






<h3 class="sect2">9.1.5. Common Mistakes</h3>

<a name="INDEX-2102"></a><a name="INDEX-2103"></a><a name="INDEX-2104"></a>
<p>As mentioned earlier, Perl arrays and hashes are one-dimensional.  In
Perl, even "multidimensional" arrays are actually one-dimensional, but
the values along that dimension are references to other arrays, which
collapse many elements into one.  If you print these values out
without dereferencing them, you will get the

stringified references rather than the data you want.  For example,
these two lines:
<blockquote>
<pre class="programlisting">@AoA = ( [2, 3], [4, 5, 7], [0] );
print "@AoA";</pre>
</blockquote>

result in something like:
<blockquote>
<pre class="programlisting">ARRAY(0x83c38) ARRAY(0x8b194) ARRAY(0x8b1d0)</pre>
</blockquote>

On the other hand, this line displays <tt class="literal">7</tt>:
<blockquote>
<pre class="programlisting">print $AoA[1][2];</pre>
</blockquote>
</p>

<p>When constructing an array of arrays, remember to compose new
references for the subarrays.  Otherwise, you will just create
an array containing the element counts of the subarrays, like
this:
<blockquote>
<pre class="programlisting">for $i (1..10) {
    @array = somefunc($i);
    $AoA[$i] = @array;       # WRONG!
}</pre>
</blockquote>

Here <tt class="literal">@array</tt> is being accessed in a scalar context,
and therefore yields the count of its elements, which is dutifully
assigned to <tt class="literal">$AoA[$i]</tt>.  The proper way to assign the
reference will be shown in a moment.</p>

<p>After making the previous mistake, people realize they need to assign a
reference, so the next mistake people naturally make involves taking a
reference to the same memory location over and over again:
<blockquote>
<pre class="programlisting">for $i (1..10) {
    @array = somefunc($i);
    $AoA[$i] = \@array;      # WRONG AGAIN!
}</pre>
</blockquote>

Every reference generated by the second line of the <tt class="literal">for</tt> loop is the
same, namely, a reference to the single array <tt class="literal">@array</tt>.  Yes, this
array changes on each pass through the loop, but when everything is
said and done, <tt class="literal">$AoA</tt> contains 10 references to the same array,
which now holds the last set of values assigned to it.  <tt class="literal">print
@{$AoA[1]}</tt> will reveal the same values as <tt class="literal">print @{$AoA[2]}</tt>.</p>

<p>Here's a more successful approach:
<blockquote>
<pre class="programlisting">for $i (1..10) {
    @array = somefunc($i);
    $AoA[$i] = [ @array ];   # RIGHT!
}</pre>
</blockquote>

The brackets around <tt class="literal">@array</tt> create a new anonymous
array, into which the elements of <tt class="literal">@array</tt> are
copied.  We then store a reference to that new array.</p>

<p>A similar result--though more difficult to read--would be produced by:
<blockquote>
<pre class="programlisting">for $i (1..10) {
    @array = somefunc($i);
    @{$AoA[$i]} = @array;
}</pre>
</blockquote>

Since <tt class="literal">$AoA[$i]</tt> needs to be a new reference, the reference springs into
existence.  Then, the preceding <tt class="literal">@</tt> dereferences this new reference,
with the result that the values of <tt class="literal">@array</tt> are assigned (in list
context) to the array referenced by <tt class="literal">$AoA[$i]</tt>.  You might wish
to avoid this construct for clarity's sake.</p>

<p>But there <em class="emphasis">is</em> a situation in which you might use it. Suppose <tt class="literal">@AoA</tt>
is already an array of references to arrays. That is, you've made
assignments like:
<blockquote>
<pre class="programlisting">$AoA[3] = \@original_array;</pre>
</blockquote>

And now suppose that you want to change <tt class="literal">@original_array</tt> (that is, you
want to change the fourth row of <tt class="literal">$AoA</tt>) so that it refers to the
elements of <tt class="literal">@array</tt>.  This code will work:
<blockquote>
<pre class="programlisting">@{$AoA[3]} = @array;</pre>
</blockquote>

In this case, the reference itself does not change, but the elements
of the referenced array do.  This overwrites the values of
<tt class="literal">@original_array</tt>.</p>

<p>Finally, the following dangerous-looking code actually works fine:
<blockquote>
<pre class="programlisting">for $i (1..10) {
    my @array = somefunc($i);
    $AoA[$i] = \@array;
}</pre>
</blockquote>

That's because the lexically scoped <tt class="literal">my @array</tt> variable is created
afresh on each pass through the loop.  So even though it looks as
though you've stored the same variable reference each time, you haven't.
This is a subtle distinction, but the technique can produce more
efficient code, at the risk of misleading less-enlightened programmers.
(It's more efficient because there's no copy in the final assignment.)
On the other hand, if you have to copy the values anyway (which the
first assignment in the loop is doing), then you might as well use the
copy implied by the brackets and avoid the temporary variable:
<blockquote>
<pre class="programlisting">for $i (1..10) {
    $AoA[$i] = [ somefunc($i) ];
}</pre>
</blockquote>

In summary:
<blockquote>
<pre class="programlisting">$AoA[$i] = [ @array ];   # Safest, sometimes fastest
$AoA[$i] = \@array;      # Fast but risky, depends on my-ness of array
@{ $AoA[$i] } = @array;  # A bit tricky</pre>
</blockquote>

Once you've mastered arrays of arrays, you'll want to tackle more
complex data structures.  If you're looking for C structures or Pascal
records, you won't find any special reserved words in Perl to set
these up for you.  What you get instead is a more flexible system.  If
your idea of a record structure is less flexible than this, or if
you'd like to provide your users with something more opaque and rigid,
then you can use the object-oriented features detailed in <a href="ch12_01.htm">Chapter 12, "Objects"</a>.</p>

<p>
<a name="INDEX-2105"></a><a name="INDEX-2106"></a><a name="INDEX-2107"></a><a name="INDEX-2108"></a><a name="INDEX-2109"></a>
Perl has just two ways of organizing data: as ordered lists stored in
arrays and accessed by position, or as unordered key/value pairs
stored in hashes and accessed by name.  The best way to represent a
record in Perl is with a hash reference, but how you choose to
organize such records will vary.  You might want to keep an ordered
list of these records that you can look up by number, in which case
you'd use an array of hash references to store the records.  Or, you
might wish to look the records up by name, in which case you'd
maintain a hash of hash references. You could even do both at once,
with pseudohashes.</p>

<p>In the following sections, you will find code examples detailing how
to compose (from scratch), generate (from other sources), access, and
display several different data structures.  We first demonstrate three
straightforward combinations of arrays and hashes, followed by a hash
of functions and more irregular data structures.  We end with a
demonstration of how these data structures can be saved.  These
examples assume that you have already familiarized yourself with the
explanations set forth earlier in this chapter.
<a name="INDEX-2110"></a><a name="INDEX-2111"></a><a name="INDEX-2112"></a>
</p>


























































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