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<HTML><HEAD><TITLE>Recipe 5.10. Merging 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="1999-07-02T01:32:50Z"><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="ch05_01.htm"TITLE="5. Hashes"><LINKREL="prev"HREF="ch05_10.htm"TITLE="5.9. Sorting a Hash"><LINKREL="next"HREF="ch05_12.htm"TITLE="5.11. Finding Common or Different Keys in Two Hashes"></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="ch05_10.htm"TITLE="5.9. Sorting a Hash"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 5.9. Sorting a Hash"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1"><ACLASS="chapter"REL="up"HREF="ch05_01.htm"TITLE="5. Hashes"></A></FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch05_12.htm"TITLE="5.11. Finding Common or Different Keys in Two Hashes"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 5.11. Finding Common or Different Keys in Two Hashes"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch05-30040">5.10. Merging Hashes</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch05-pgfId-972">Problem <ACLASS="indexterm"NAME="ch05-idx-1000006496-0"></A><ACLASS="indexterm"NAME="ch05-idx-1000006496-1"></A></A></H3><PCLASS="para">You need to make a new hash with the entries of two existing hashes.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch05-pgfId-978">Solution</A></H3><PCLASS="para">Treat them as lists, and join them as you would lists.</P><PRECLASS="programlisting">%merged = (%A, %B);</PRE><PCLASS="para">To save memory, loop over the hashes' elements and build a new hash that way:</P><PRECLASS="programlisting">%merged = ();while ( ($k,$v) = each(%A) ) { $merged{$k} = $v;}while ( ($k,$v) = each(%B) ) { $merged{$k} = $v;}</PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch05-pgfId-1002">Discussion</A></H3><PCLASS="para">The first method, like the earlier recipe on inverting a hash, uses the hash-list equivalence explained in the introduction. <CODECLASS="literal">(%A,</CODE> <CODECLASS="literal">%B)</CODE> evaluates to a list of paired keys and values. When we assign it to <CODECLASS="literal">%merged</CODE>, Perl turns that list of pairs back into a hash.</P><PCLASS="para">Here's an example of that technique:</P><PRECLASS="programlisting"># %food_color as per the introduction%drink_color = ( Galliano => "yellow", "Mai Tai" => "blue" );%ingested_color = (%drink_color, %food_color);</PRE><PCLASS="para">Keys in both input hashes appear only once in the output hash. If a food and a drink shared the same name, for instance, then the last one seen by the first merging technique would be the one that showed up in the resultant hash.</P><PCLASS="para">This style of direct assignment, as in the first example, is easier to read and write, but requires a lot of memory if the hashes are large. That's because Perl has to unroll both hashes into a temporary list before the assignment to the merged hash is done. Step-by-step merging using <CODECLASS="literal">each</CODE>, as in the second technique, spares you that cost and lets you decide what to do with duplicate keys.</P><PCLASS="para">The first example could be rewritten to use the <CODECLASS="literal">each</CODE><ACLASS="indexterm"NAME="ch05-idx-1000006497-0"></A> technique:</P><PRECLASS="programlisting"># %food_color per the introduction, then%drink_color = ( Galliano => "yellow", "Mai Tai" => "blue" );%substance_color = ();while (($k, $v) = each %food_color) { $substance_color{$k} = $v;} while (($k, $v) = each %drink_color) { $substance_color{$k} = $v;} </PRE><PCLASS="para">That technique duplicated the <CODECLASS="literal">while</CODE> and assignment code. Here's a sneaky way to get around that:</P><PRECLASS="programlisting">foreach $substanceref ( \%food_color, \%drink_color ) { while (($k, $v) = each %$substanceref) { $substance_color{$k} = $v; }}</PRE><PCLASS="para">If we were merging hashes with duplicates, we can insert our own code to decide what to do with those duplicates:</P><PRECLASS="programlisting">foreach $substanceref ( \%food_color, \%drink_color ) { while (($k, $v) = each %$substanceref) { if (exists $substance_color{$k}) { print "Warning: $k seen twice. Using the first definition.\n"; next; } $substance_color{$k} = $v; }}</PRE><PCLASS="para">In the special case of appending one hash to another, we can use the hash slice notation to give an elegant shorthand:</P><PRECLASS="programlisting">@all_colors{keys %new_colors} = values %new_colors;</PRE><PCLASS="para">This requires enough memory for lists of the keys and values of <CODECLASS="literal">%new_colors</CODE>. As with the first technique, the memory requirement might make this technique infeasible when such lists would be large.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch05-pgfId-1086">See Also</A></H3><PCLASS="para">This is a variation on <ACLASS="xref"HREF="ch04_10.htm"TITLE="Appending One Array to Another">Recipe 4.9</A>; the <CODECLASS="literal">each</CODE> function in <ICLASS="filename">perlfunc </I>(1) and in <ACLASS="olink"HREF="../prog/ch03_01.htm">Chapter 3</A> of <ACLASS="citetitle"HREF="../prog/index.htm"TITLE="Programming Perl"><CITECLASS="citetitle">Programming Perl</CITE></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="ch05_10.htm"TITLE="5.9. Sorting a Hash"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 5.9. Sorting a Hash"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><ACLASS="book"HREF="index.htm"TITLE="Perl Cookbook"><IMGSRC="../gifs/txthome.gif"ALT="Perl Cookbook"BORDER="0"></A></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch05_12.htm"TITLE="5.11. Finding Common or Different Keys in Two Hashes"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 5.11. Finding Common or Different Keys in Two Hashes"BORDER="0"></A></TD></TR><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="228">5.9. Sorting a Hash</TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><ACLASS="index"HREF="index/index.htm"TITLE="Book Index"><IMGSRC="../gifs/index.gif"ALT="Book Index"BORDER="0"></A></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228">5.11. Finding Common or Different Keys in Two Hashes</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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