ch01_06.htm

编程珍珠,里面很多好用的代码,大家可以参考学习呵呵,

<tt class="literal">until</tt>, <tt class="literal">for</tt>, and
<tt class="literal">foreach</tt>. These statements allow a Perl program to
repeatedly execute the same code.</p>


<h3 class="sect3">1.6.2.1. The while and until statements</h3>

<p>
<a name="INDEX-305"></a><a name="INDEX-306"></a><a name="INDEX-307"></a><a name="INDEX-308"></a>
 The <tt class="literal">while</tt> and
<tt class="literal">until</tt> statements behave just like the
<tt class="literal">if</tt> and <tt class="literal">unless</tt> statements, except
that they'll execute the block repeatedly.  That is, they loop.
First, the conditional part of the statement is checked.  If the
condition is met (if it is true for a <tt class="literal">while</tt> or
false for an <tt class="literal">until</tt>), the block of the statement is
executed.
<blockquote>
<pre class="programlisting">while ($tickets_sold &lt; 10000) {
    $available = 10000 - $tickets_sold;
    print "$available tickets are available.  How many would you like: ";
    $purchase = &lt;STDIN&gt;;
    chomp($purchase);
    $tickets_sold += $purchase;
}</pre>
</blockquote>

Note that if the original condition is never met, the loop will never be
entered at all.  For example, if we've already sold 10,000 tickets, we
might want to have the next line of the program say something like:
<blockquote>
<pre class="programlisting">print "This show is sold out, please come back later.\n";</pre>
</blockquote>

In our Average Example earlier, line 4 reads:
<blockquote>
<pre class="programlisting">while ($line = &lt;GRADES&gt;) {</pre>
</blockquote>
<a name="INDEX-309"></a>
This assigns the next line to the variable <tt class="literal">$line</tt> and, as we explained
earlier, returns the value of <tt class="literal">$line</tt> so that the condition of the
<tt class="literal">while</tt> statement can evaluate <tt class="literal">$line</tt> for truth.  You might wonder
whether Perl will get a false negative on blank lines and exit the loop
prematurely.  The answer is that it won't.  The reason is clear if you
think about everything we've said.  The line input operator leaves the
newline on the end of the string, so a blank line has the value <tt class="literal">"\n"</tt>.
And you know that <tt class="literal">"\n"</tt> is not one of the canonical false values.  So
the condition is true, and the loop continues even on blank lines.
<a name="INDEX-310"></a><a name="INDEX-311"></a>
</p>

<p>
<a name="INDEX-312"></a>
On the other hand, when we finally do reach the end of the file, the
line input operator returns the undefined value, which always evaluates
to false.  And the loop terminates, just when we wanted it to.  There's
no need for an explicit test of the <tt class="literal">eof</tt> function in Perl,
because the input operators are designed to work smoothly in a
conditional context.</p>

<p>In fact, almost everything is designed to work smoothly in a
conditional (Boolean) context.  If you mention an array in a scalar
context, the length of the array is returned.  So you often see command-line
arguments processed like this:
<blockquote>
<pre class="programlisting">while (@ARGV) {
    process(shift @ARGV);
}</pre>
</blockquote>
<a name="INDEX-313"></a> The
<tt class="literal">shift</tt> operator removes one element from the
argument list each time through the loop (and returns that element).
The loop automatically exits when array <tt class="literal">@ARGV</tt> is
exhausted, that is, when its length goes to 0.  And 0 is already false
in Perl.  In a sense, the array itself has become
"false".<a href="#FOOTNOTE-21">[21]</a>
</p>
<blockquote class="footnote">

<a name="FOOTNOTE-21"></a>
<p>[21] This is how Perl programmers think.  So there's
no need to compare 0 to 0 to see if it's false.  Despite the fact that
other languages force you to, don't go out of your way to write explicit
comparisons like <tt class="literal">while (@ARGV != 0)</tt>.  That's just
inefficient for both you and the computer.  And anyone who
has to maintain your code.</p>

</blockquote>




<h3 class="sect3">1.6.2.2. The for statement</h3>

<p>
<a name="INDEX-314"></a><a name="INDEX-315"></a><a name="INDEX-316"></a> Another iterative
statement is the <tt class="literal">for</tt> loop.  The
<tt class="literal">for</tt> loop runs exactly like the
<tt class="literal">while</tt> loop, but looks a good deal different. (C
programmers will find it very familiar though.)
<blockquote>
<pre class="programlisting">for ($sold = 0; $sold &lt; 10000; $sold += $purchase) {
    $available = 10000 - $sold;
    print "$available tickets are available.  How many would you like: ";
    $purchase = &lt;STDIN&gt;;
    chomp($purchase);
}</pre>
</blockquote>

This <tt class="literal">for</tt> loop takes three expressions within the
loop's parentheses: an expression to set the initial state of the loop
variable, a condition to test the loop variable, and an expression to
modify the state of the loop variable.  When a <tt class="literal">for</tt>
loop starts, the initial state is set and the truth condition is
checked.  If the condition is true, the block is executed.  When the
block finishes, the modification expression is executed, the truth
condition is again checked, and if true, the block is rerun with the
next value.  As long as the truth condition remains true, the block
and the modification expression will continue to be executed.  (Note
that only the middle expression is evaluated for its value.  The first
and third expressions are evaluated only for their side effects, and
the resulting values are thrown away!)</p>




<h3 class="sect3">1.6.2.3. The foreach statement</h3>

<p>
<a name="INDEX-317"></a><a name="INDEX-318"></a><a name="INDEX-319"></a> The last of
Perl's iterative statements is the <tt class="literal">foreach</tt>
statement, which is used to execute the same code for each of a known
set of scalars, such as an array:
<blockquote>
<pre class="programlisting">foreach $user (@users) {
    if (-f "$home{$user}/.nexrc") {
        print "$user is cool... they use a perl-aware vi!\n";
    }
}</pre>
</blockquote>
<a name="INDEX-320"></a>
Unlike the <tt class="literal">if</tt> and <tt class="literal">while</tt>
statements, which provide scalar context to a conditional expression,
the <tt class="literal">foreach</tt> statement provides a list context to
the expression in parentheses.  So the expression is evaluated to
produce a list (not a scalar, even if there's only one scalar in the
list).  Then each element of the list is aliased to the loop variable
in turn, and the block of code is executed once for each list
element.  Note that the loop variable refers to the element itself,
rather than a copy of the element.  Hence, modifying the loop variable
also modifies the original array.</p>

<p>You'll find many more <tt class="literal">foreach</tt> loops in the typical
Perl program than <tt class="literal">for</tt> loops, because it's very easy
in Perl to generate the kinds of lists that <tt class="literal">foreach</tt>
wants to iterate over.  One idiom you'll often see is a loop to
iterate over the sorted keys of a hash:
<blockquote>
<pre class="programlisting">foreach $key (sort keys %hash) {</pre>
</blockquote>

In fact, line 9 of our Average Example does precisely that.</p>




<h3 class="sect3">1.6.2.4. Breaking out: next and last</h3>

<p>
<a name="INDEX-321"></a><a name="INDEX-322"></a><a name="INDEX-323"></a>
The <tt class="literal">next</tt> and <tt class="literal">last</tt> operators allow you to modify the flow of your
loop.  It is not at all uncommon to have a special case; you may want to
skip it, or you may want to quit when you encounter it. For example, if
you are dealing with Unix accounts, you may want to skip the system
accounts (like <em class="emphasis">root</em> or <em class="emphasis">lp</em>).  The <tt class="literal">next</tt> operator would allow you to
skip to the end of your current loop iteration, and start the next
iteration.  The <tt class="literal">last</tt> operator would allow you to skip to the end of
your block, as if your loop's test condition had returned false.  This might be
useful if, for example, you are looking for a specific account and want
to quit as soon as you find it.
<blockquote>
<pre class="programlisting">foreach $user (@users) {
    if ($user eq "root" or $user eq "lp") {
        next;
    }
    if ($user eq "special") {
        print "Found the special account.\n";
        # do some processing
        last;
    }
}</pre>
</blockquote>

It's possible to break out of multilevel loops by labeling your loops
and specifying which loop you want to break out of.  Together with
statement modifiers (another form of conditional which we'll talk
about later), this can make for extremely readable loop exits (if you
happen to think English is readable):
<blockquote>
<pre class="programlisting">LINE: while ($line = &lt;ARTICLE&gt;) {
    last LINE if $line eq "\n"; # stop on first blank line
    next LINE if $line =~ /^#/; # skip comment lines
    # your ad here
}</pre>
</blockquote>
</p>

<p>You may be saying, "Wait a minute, what's that funny
<tt class="literal">^#</tt> thing there inside the leaning toothpicks?  That
doesn't look much like English."  And you're right.  That's a pattern
match containing a regular expression (albeit a rather simple one). And 
that's what the next section is about.  Perl is the best text
processing language in the world, and regular expressions are at the
heart of Perl's text processing.</p>

<a name="INDEX-324"></a>




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