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<HTML><HEAD><TITLE>Recipe 5.11. Finding Common or Different Keys in Two 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="1999-07-02T01:32:52Z"><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_11.htm"TITLE="5.10. Merging Hashes"><LINKREL="next"HREF="ch05_13.htm"TITLE="5.12. Hashing References"></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_11.htm"TITLE="5.10. Merging Hashes"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 5.10. Merging Hashes"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_13.htm"TITLE="5.12. Hashing References"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 5.12. Hashing References"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch05-chap05_finding_0">5.11. Finding Common or Different Keys in Two Hashes</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch05-pgfId-1096">Problem<ACLASS="indexterm"NAME="ch05-idx-1000006503-0"></A><ACLASS="indexterm"NAME="ch05-idx-1000006503-1"></A><ACLASS="indexterm"NAME="ch05-idx-1000006503-2"></A></A></H3><PCLASS="para">You need to find keys in one hash that are present in another hash or keys in one hash that are not present in another.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch05-pgfId-1102">Solution</A></H3><PCLASS="para">Use <CODECLASS="literal">keys</CODE> to loop through the keys of one hash, checking whether each key is also in the other hash.</P><DIVCLASS="sect3"><H4CLASS="sect3"><ACLASS="title"NAME="ch05-pgfId-1000005736">Find common keys</A></H4><PRECLASS="programlisting">my @common = ();foreach (keys %hash1) { push(@common, $_) if exists $hash2{$_};}# @common now contains common keys</PRE></DIV><DIVCLASS="sect3"><H4CLASS="sect3"><ACLASS="title"NAME="ch05-pgfId-1000005753">Find keys from one hash that aren't in both</A></H4><PRECLASS="programlisting">my @this_not_that = ();foreach (keys %hash1) { push(@this_not_that, $_) unless exists $hash2{$_};}</PRE></DIV></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch05-pgfId-1108">Discussion</A></H3><PCLASS="para">Because we're finding common or different keys of the hashes, we can apply our earlier array recipes for finding common or different elements to arrays of the hashes' keys. For an explanation, see <ACLASS="xref"HREF="ch04_09.htm"TITLE="Computing Union, Intersection, or Difference of Unique Lists">Recipe 4.8</A>.</P><PCLASS="para">This code uses the difference technique to find non-citrus foods:</P><PRECLASS="programlisting"># %food_color per the introduction# %citrus_color is a hash mapping citrus food name to its color.%citrus_color = ( Lemon => "yellow", Orange => "orange", Lime => "green" );# build up a list of non-citrus foods@non_citrus = ();foreach (keys %food_color) { push (@non_citrus, $_) unless exists $citrus_color{$_};}</PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch05-pgfId-1144">See Also</A></H3><PCLASS="para">The "Variables" section of <ACLASS="olink"HREF="../prog/ch02_01.htm">Chapter 2</A> of <ACLASS="citetitle"HREF="../prog/index.htm"TITLE="Programming Perl"><CITECLASS="citetitle">Programming Perl</CITE></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_11.htm"TITLE="5.10. 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