ch10_08.htm

by Randal L. Schwartz and Tom Phoenix ISBN 0-596-00132-0 Third Edition, published July 2001. (See

<tt class="literal">fred</tt>. In that case, we count
<tt class="literal">$valid</tt> up by one, and also
<tt class="literal">$count{$_}</tt>, keeping a count for each different
word. So, when we finish the two loops, we've counted every
word in every line of input from every file the user wanted us to
use.
</p>

<p>We're not going to explain the last few lines. By now, we hope
you've got stuff like that down already.
</p>

<p>Like <tt class="literal">last</tt>, <tt class="literal">next</tt> may be used in
any of the five kinds of <a name="INDEX-699" />loop blocks: <tt class="literal">for</tt>,
<tt class="literal">foreach</tt>, <tt class="literal">while</tt>,
<tt class="literal">until</tt>, or the naked block. Also, if loop blocks
are nested, <tt class="literal">next</tt> works with the innermost one.
We'll see how to change that at the end of this section.
</p>

</div>
<a name="lperl3-CHP-10-SECT-8.3" /><div class="sect2">
<h3 class="sect2">10.8.3. The redo Operator</h3>

<p>The third member of the loop control triad is
<tt class="literal">redo</tt><a name="INDEX-700" />. It says to go back to the top of the
current loop block, without testing any conditional expression or
advancing to the next iteration. (If you've used C or a similar
language, you've never seen this one before. Those languages
don't have this kind of operator.) Here's an example:
</p>

<blockquote><pre class="code"># Typing test
my @words = qw{ fred barney pebbles dino wilma betty };
my $errors = 0;

foreach (@words) {
  ## redo comes here ##
  print "Type the word '$_': ";
  chomp(my $try = &lt;STDIN&gt;);
  if ($try ne $_) {
    print "Sorry - That's not right.\n\n";
    $errors++;
    redo;  # jump back up to the top of the loop
  }
}
print "You've completed the test, with $errors errors.\n";</pre></blockquote>

<p>Like the other two operators, <tt class="literal">redo</tt> will work with
any of the five kinds of loop blocks, and it will work with the
innermost loop block when they're nested.
</p>

<p>The big difference between <tt class="literal">next</tt> and
<tt class="literal">redo</tt> is that <tt class="literal">next</tt> will advance
to the next iteration, but <tt class="literal">redo</tt> will redo the
current iteration. Here's an example program that you can play
with to get a feel for how these three operators work:<a href="#FOOTNOTE-233">[233]</a>
</p><blockquote class="footnote">
<a name="FOOTNOTE-233" /><p>[233]If you've downloaded the example files from the
O'Reilly website (as described in the Preface), you'll
find this program called <em class="filename">lnr-example.</em></p>
</blockquote>

<blockquote><pre class="code">foreach (1..10) {
  print "Iteration number $_.\n\n";
  print "Please choose: last, next, redo, or none of the above? ";
  chomp(my $choice = &lt;STDIN&gt;);
  print "\n";
  last if $choice =~ /last/i;
  next if $choice =~ /next/i;
  redo if $choice =~ /redo/i;
  print "That wasn't any of the choices... onward!\n\n";
}
print "That's all, folks!\n";</pre></blockquote>

<p>If you just press return without typing anything (try it two or three
times), the loop counts along from one number to the next. If you
choose <tt class="literal">last</tt> when you get to number four, the loop
is done, and you won't go on to number five. If you choose
<tt class="literal">next</tt> when you're on four, you're on to
number five without printing the "onward" message. And if
you choose <tt class="literal">redo</tt> when you're on four,
you're back to doing number four all over again.<a name="INDEX-701" />
</p>

</div>
<a name="lperl3-CHP-10-SECT-8.4" /><div class="sect2">
<h3 class="sect2">10.8.4. Labeled Blocks</h3>

<p>When you need to work with a <a name="INDEX-702" />
<a name="INDEX-703" />loop block
that's not the innermost one, use a label. Labels in Perl are
like other identifiers -- made of letters, digits, and
underscores, but they can't start with a digit -- however,
since they have no prefix character, labels could be confused with
the names of builtin function names, or even with your own
subroutines' names. So, it would be a poor choice to make a
label called <tt class="literal">print</tt> or <tt class="literal">if</tt>.
Because of that, Larry recommends that they be all uppercase. That
not only ensures that the label won't conflict with another
identifier but it also makes it easy to spot the label in the code.
In any case, labels are rare, only showing up in a small percentage
of Perl programs.
</p>

<p>To label a loop block, just put the label and a colon in front of the
loop. Then, inside the loop, you may use the label after
<tt class="literal">last</tt>, <tt class="literal">next</tt>, or
<tt class="literal">redo</tt> as needed:
</p>

<blockquote><pre class="code">LINE: while (&lt;&gt;) {
  foreach (split) {
    last LINE if /_ _END_  _/;  # bail out of the LINE loop
    ...;
  }
}</pre></blockquote>

<p>For readability, it's generally nice to put the label at the
left margin, even if the current code is at a higher indentation.
Notice that the label names the entire block; it's not marking
a target point in the code.<a href="#FOOTNOTE-234">[234]</a>
</p><blockquote class="footnote"> <a name="FOOTNOTE-234" /><p>[234]This isn't
<tt class="literal">goto</tt>, after all.</p> </blockquote>

<p>In that previous snippet of sample code, the special <tt class="literal">__END__</tt> token marks the end of all input. Once that token shows
up, the program will ignore any remaining lines (even from other
files).
</p>

<p>It often makes sense to choose a noun as the name of the
loop.<a href="#FOOTNOTE-235">[235]</a> That
is, the outer loop is processing a line at a time, so we called it
<tt class="literal">LINE</tt>. If we had to name the inner loop, we would
have called it <tt class="literal">WORD</tt>, since it processes a word at
a time. That makes it convenient to say things like "(move on
to the) <tt class="literal">next</tt> <tt class="literal">WORD</tt>" or
"<tt class="literal">redo</tt> (the current)
<tt class="literal">LINE</tt>".<a name="INDEX-704" /> <a name="INDEX-705" /> 
</p><blockquote class="footnote"> <a name="FOOTNOTE-235" /><p>[235]That is, it makes more sense to do that than
not to do that. Perl doesn't care if you call your loop labels
things like <tt class="literal">XYZZY</tt> or <tt class="literal">PLUGH</tt>.
However, unless you were friendly with the Colossal Cave in the
70's, you might not get the reference.</p> </blockquote>

</div>


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