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<html><head><title>The local Operator (Learning Perl, 3rd Edition)</title><link rel="stylesheet" type="text/css" href="../style/style1.css" /><meta name="DC.Creator" content="Randal L. Schwartz and Tom Phoenix" /><meta name="DC.Format" content="text/xml" scheme="MIME" /><meta name="DC.Language" content="en-US" /><meta name="DC.Publisher" content="O'Reilly & Associates, Inc." /><meta name="DC.Source" scheme="ISBN" content="0596001320L" /><meta name="DC.Subject.Keyword" content="stuff" /><meta name="DC.Title" content="Learning Perl, 3rd Edition" /><meta name="DC.Type" content="Text.Monograph" /></head><body bgcolor="#ffffff"><img alt="Book Home" border="0" src="gifs/smbanner.gif" usemap="#banner-map" /><map name="banner-map"><area shape="rect" coords="1,-2,616,66" href="index.htm" alt="Learning Perl, 3rd Edition" /><area shape="rect" coords="629,-11,726,25" href="jobjects/fsearch.htm" alt="Search this book" /></map><div class="navbar"><table width="684" border="0"><tr><td align="left" valign="top" width="228"><a href="ch04_06.htm"><img alt="Previous" border="0" src="../gifs/txtpreva.gif" /></a></td><td align="center" valign="top" width="228"><a href="index.htm"></a></td><td align="right" valign="top" width="228"><a href="ch04_08.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr></table></div><h2 class="sect1">4.7. The local Operator</h2><p>You might consider this next section a giant footnote, but then wecouldn't have footnotes on footnotes, so we decided to put itup in the main text. Skip over this text on first reading, and popright on down to <a href="ch04_08.htm#lperl3-CHP-4-SECT-8">Section 4.8, "Variable-length Parameter Lists"</a>.You won't need any of it to do the exercises or write Perl codefor a long time. But someone invariably asks us in class somethinglike "What is that <tt class="literal">local</tt> thing I see in someprograms?" so we're including what we normally say as anaside in class for your enjoyment and edification.</p><p>Occasionally, mostly in older code or older Perl books, you'llsee the <tt class="literal">local</tt><a name="INDEX-343" /> operator used instead of<tt class="literal">my</tt>. It often looks much the same as<tt class="literal">my</tt>:</p><blockquote><pre class="code">sub max { local($a, $b) = @_; # looks a lot like my if ($a > $b) { $a } else { $b }}</pre></blockquote><p>But <tt class="literal">local</tt> is misnamed, or at least<em class="emphasis">misleadingly</em> named. Our friend Chip Salzenbergsays that if he ever gets a chance to go back in a time machine to1986 and give Larry one piece of advice, he'd tell Larry tocall <tt class="literal">local</tt> by the name "save"instead.<a href="#FOOTNOTE-110">[110]</a>That's because <tt class="literal">local</tt> actually will save thegiven global variable's value away, so it will laterautomatically be restored to the global<a name="INDEX-344" /><a name="INDEX-345" />variable. (That's right: theseso-called "<tt class="literal">local</tt>" variables areactually globals!) This save-and-restore mechanism is the same onewe've already seen twice now, in the control variable of a<tt class="literal">foreach</tt> loop, and in the <tt class="literal">@_</tt>array of subroutine parameters.</p><blockquote class="footnote"> <a name="FOOTNOTE-110" /><p>[110]We would tell Larry to buy stock in Yahoo!,but Chip is more idealistic than we are.</p> </blockquote><p>What <tt class="literal">local</tt> actually does, then, is to save away acopy of the variable's value in a secret place (called the<a name="INDEX-346" />stack). That valuecan't be accessed, modified, or deleted<a href="#FOOTNOTE-111">[111]</a> while it is saved. Then<tt class="literal">local</tt> sets the variable to an empty value(<tt class="literal">undef</tt> for scalars, or empty list for arrays), orto whatever value is being assigned. When Perl returns from thesubroutine,<a href="#FOOTNOTE-112">[112]</a> the variable is automatically restored to its originalvalue. In effect, the variable was borrowed for a time and given back(hopefully) before anyone noticed that it was borrowed.</p><blockquote class="footnote"> <a name="FOOTNOTE-111" /><p>[111]Ordamaged, defiled, read, checked, touched, seen, changed, or printed,for that matter. There's no way from within Perl to get at thesaved value.</p> </blockquote><blockquote class="footnote"> <a name="FOOTNOTE-112" /><p>[112]Or when it finishes execution of thesmallest enclosing block or file, to be more precise.</p></blockquote><a name="lperl3-CHP-4-SECT-7.1" /><div class="sect2"><h3 class="sect2">4.7.1. The Difference Between local and my</h3><p>But what if the subroutine called another subroutine, one that<em class="emphasis">did</em> notice that the variable was being borrowedby <tt class="literal">local</tt>? For example:</p><blockquote><pre class="code">$office = "global"; # Global $office&say( ); # says "global", accessing $office directly&fred( ); # says "fred", dynamic scope, # because fred's local $office hides the global&barney( ); # says "global", lexical scope; # barney's $office is visible only in that blocksub say { print "$office\n"; } # print the currently visible $officesub fred { local($office) = "fred"; &say( ); }sub barney { my($office) = "barney"; &say( ); }</pre></blockquote><p>First, we call the subroutine <tt class="literal">&say</tt>, whichtells us which <tt class="literal">$office</tt> it sees -- the global<tt class="literal">$office</tt>. That's normal.</p><p>But then we call Fred's subroutine. Fred has made his own<tt class="literal">local $office</tt>, so he has actually changed thebehavior of the <tt class="literal">&say</tt> subroutine; now it tellsus what's in Fred's <tt class="literal">$office</tt>. Wecan't tell whether that's what Fred wanted to do or notwithout understanding the meaning of his code. But it's alittle odd.</p><p>Barney, however, is a little smarter, as well as being shorter, so heuses the shorter (and smarter) operator, <tt class="literal">my</tt>.Barney's variable <tt class="literal">$office</tt> is private, andBarney's private <tt class="literal">$office</tt> can't beaccessed from outside his subroutine, so the<tt class="literal">&say</tt> subroutine is back to normal; it can seeonly the global <tt class="literal">$office</tt>. Barney didn'tchange the way <tt class="literal">&say</tt> works, which is more likewhat most programmers would want and expect.</p><p>Now, if you're confused about these two operators at thispoint, that's to be expected. But any time that you see<tt class="literal">local</tt>, think "save," and that mayhelp. In any new code, just use <tt class="literal">my</tt>, since<tt class="literal">my</tt><a name="INDEX-347" /> variables (lexical variables) arefaster than globals -- remember, so-called<tt class="literal">local</tt> variables are really globals -- andthey'll work more like the traditional variables in othermodern programming languages. But when you're maintainingsomeone else's old code, you can't necessarily changeevery <tt class="literal">local</tt> to <tt class="literal">my</tt> withoutchecking upon whether the programmer was using that save-and-restorefunctionality.</p></div><hr width="684" align="left" /><div class="navbar"><table width="684" border="0"><tr><td align="left" valign="top" width="228"><a href="ch04_06.htm"><img alt="Previous" border="0" src="../gifs/txtpreva.gif" /></a></td><td align="center" valign="top" width="228"><a href="index.htm"><img alt="Home" border="0" src="../gifs/txthome.gif" /></a></td><td align="right" valign="top" width="228"><a href="ch04_08.htm"><img alt="Next" border="0" src="../gifs/txtnexta.gif" /></a></td></tr><tr><td align="left" valign="top" width="228">4.6. Private Variables in Subroutines</td><td align="center" valign="top" width="228"><a href="index/index.htm"><img alt="Book Index" border="0" src="../gifs/index.gif" /></a></td><td align="right" valign="top" width="228">4.8. Variable-length Parameter Lists</td></tr></table></div><hr width="684" align="left" /><img alt="Library Navigation Links" border="0" src="../gifs/navbar.gif" usemap="#library-map" /><p><p><font size="-1"><a href="copyrght.htm">Copyright © 2002</a> O'Reilly & Associates. 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