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<HTML><HEAD><TITLE>Recipe 11.3. Taking References to Hashes (Perl Cookbook)</TITLE><METANAME="DC.title"CONTENT="Perl Cookbook"><METANAME="DC.creator"CONTENT="Tom Christiansen & Nathan Torkington"><METANAME="DC.publisher"CONTENT="O'Reilly & Associates, Inc."><METANAME="DC.date"CONTENT="1999-07-02T01:40:33Z"><METANAME="DC.type"CONTENT="Text.Monograph"><METANAME="DC.format"CONTENT="text/html"SCHEME="MIME"><METANAME="DC.source"CONTENT="1-56592-243-3"SCHEME="ISBN"><METANAME="DC.language"CONTENT="en-US"><METANAME="generator"CONTENT="Jade 1.1/O'Reilly DocBook 3.0 to HTML 4.0"><LINKREV="made"HREF="mailto:online-books@oreilly.com"TITLE="Online Books Comments"><LINKREL="up"HREF="ch11_01.htm"TITLE="11. References and Records"><LINKREL="prev"HREF="ch11_03.htm"TITLE="11.2. Making Hashes of Arrays"><LINKREL="next"HREF="ch11_05.htm"TITLE="11.4. Taking References to Functions"></HEAD><BODYBGCOLOR="#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="Perl Cookbook"><area shape="rect" coords="629,-11,726,25" href="jobjects/fsearch.htm" alt="Search this book" /></map><div class="navbar"><p><TABLEWIDTH="684"BORDER="0"CELLSPACING="0"CELLPADDING="0"><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch11_03.htm"TITLE="11.2. Making Hashes of Arrays"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 11.2. Making Hashes of Arrays"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1"><ACLASS="chapter"REL="up"HREF="ch11_01.htm"TITLE="11. References and Records"></A></FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch11_05.htm"TITLE="11.4. Taking References to Functions"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 11.4. Taking References to Functions"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch11-33633">11.3. Taking References to Hashes</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch11-pgfId-402">Problem</A></H3><PCLASS="para"><ACLASS="indexterm"NAME="ch11-idx-1000004809-0"></A><ACLASS="indexterm"NAME="ch11-idx-1000004809-1"></A><ACLASS="indexterm"NAME="ch11-idx-1000004809-2"></A>You need to manipulate a hash by reference. This might be because it was passed into a function that way or because it's part of a larger data structure.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch11-pgfId-408">Solution</A></H3><PCLASS="para">To get a hash reference:</P><PRECLASS="programlisting">$href = \%hash;$anon_hash = { "key1" => "value1", "key2" => "value2", ... };$anon_hash_copy = { %hash };</PRE><PCLASS="para">To dereference a hash reference:</P><PRECLASS="programlisting">%hash = %$href;$value = $href->{$key};@slice = @$href{$key1, $key2, $key3}; # note: no arrow!@keys = keys %$href;</PRE><PCLASS="para">To check whether something is a hash reference:</P><PRECLASS="programlisting">if (ref($someref) ne 'HASH') { die "Expected a hash reference, not $someref\n";}</PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch11-pgfId-438">Discussion</A></H3><PCLASS="para">This example prints out all the keys and values in two predefined hashes:</P><PRECLASS="programlisting">foreach $href ( \%ENV, \%INC ) { # OR: for $href ( \(%ENV,%INC) ) { foreach $key ( keys %$href ) { print "$key => $href->{$key}\n"; }}</PRE><PCLASS="para">Accessing slices of hashes by reference works just as it does with slices of arrays by reference. For example:</P><PRECLASS="programlisting">@values = @$hash_ref{"key1", "key2", "key3"};for $val (@$hash_ref{"key1", "key2", "key3"}) { $val += 7; # add 7 to each value in hash slice} </PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch11-pgfId-1000005653">See Also</A></H3><PCLASS="para"><ACLASS="xref"HREF="ch05_01.htm#ch05-22072"TITLE="Introduction">Recipe 5.0</A>; <ACLASS="olink"HREF="../prog/ch04_01.htm">Chapter 4</A> of <ACLASS="citetitle"HREF="../prog/index.htm"TITLE="Programming Perl"><CITECLASS="citetitle">Programming Perl</CITE></A>; <ICLASS="filename">perlref </I>(1); <ACLASS="xref"HREF="ch11_10.htm"TITLE="Constructing Records">Recipe 11.9</A></P></DIV></DIV><DIVCLASS="htmlnav"><P></P><HRALIGN="LEFT"WIDTH="684"TITLE="footer"><TABLEWIDTH="684"BORDER="0"CELLSPACING="0"CELLPADDING="0"><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch11_03.htm"TITLE="11.2. Making Hashes of Arrays"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 11.2. 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Taking References to Functions</TD></TR></TABLE><HRALIGN="LEFT"WIDTH="684"TITLE="footer"><FONTSIZE="-1"></DIV<!-- LIBRARY NAV BAR --> <img src="../gifs/smnavbar.gif" usemap="#library-map" border="0" alt="Library Navigation Links"><p> <a href="copyrght.htm">Copyright © 2002</a> O'Reilly & Associates. All rights reserved.</font> </p> <map name="library-map"> <area shape="rect" coords="1,0,85,94" href="../index.htm"><area shape="rect" coords="86,1,178,103" href="../lwp/index.htm"><area shape="rect" coords="180,0,265,103" href="../lperl/index.htm"><area shape="rect" coords="267,0,353,105" href="../perlnut/index.htm"><area shape="rect" coords="354,1,446,115" href="../prog/index.htm"><area shape="rect" coords="448,0,526,132" href="../tk/index.htm"><area shape="rect" coords="528,1,615,119" href="../cookbook/index.htm"><area shape="rect" coords="617,0,690,135" href="../pxml/index.htm"></map> </BODY></HTML>⌨️ 快捷键说明
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