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<HTML><HEAD><TITLE>Recipe 4.10. Reversing an Array (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="1999-07-02T01:31:56Z"><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="ch04_01.htm"TITLE="4. Arrays"><LINKREL="prev"HREF="ch04_10.htm"TITLE="4.9. Appending One Array to Another"><LINKREL="next"HREF="ch04_12.htm"TITLE="4.11. Processing Multiple Elements of an Array"></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="ch04_10.htm"TITLE="4.9. Appending One Array to Another"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 4.9. Appending One Array to Another"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1"><ACLASS="chapter"REL="up"HREF="ch04_01.htm"TITLE="4. Arrays"></A></FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch04_12.htm"TITLE="4.11. Processing Multiple Elements of an Array"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 4.11. Processing Multiple Elements of an Array"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch04-62375">4.10. Reversing an Array</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch04-pgfId-875">Problem <ACLASS="indexterm"NAME="ch04-idx-1000006688-0"></A><ACLASS="indexterm"NAME="ch04-idx-1000006688-1"></A><ACLASS="indexterm"NAME="ch04-idx-1000006688-2"></A><ACLASS="indexterm"NAME="ch04-idx-1000006688-3"></A></A></H3><PCLASS="para">You want to reverse an array.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch04-pgfId-881">Solution</A></H3><PCLASS="para">Use the <CODECLASS="literal">reverse</CODE> function:</P><PRECLASS="programlisting"># reverse @ARRAY into @REVERSED@REVERSED = reverse @ARRAY;</PRE><PCLASS="para">Or use a <CODECLASS="literal">for</CODE> loop:</P><PRECLASS="programlisting">for ($i = $#ARRAY; $i >= 0; $i--) { # do something with $ARRAY[$i]}</PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch04-pgfId-899">Discussion</A></H3><PCLASS="para">The <CODECLASS="literal">reverse</CODE> function actually reverses a list; the <CODECLASS="literal">for</CODE> loop simply processes the list in reverse order. If you don't need a reversed copy of the list, <CODECLASS="literal">for</CODE> saves memory and time.</P><PCLASS="para">If you're using <CODECLASS="literal">reverse</CODE> to reverse a list that you just sorted, you should have sorted it in the correct order to begin with. For example:</P><PRECLASS="programlisting"># two-step: sort then reverse@ascending = sort { $a cmp $b } @users;@descending = reverse @ascending;# one-step: sort with reverse comparison@descending = sort { $b cmp $a } @users;</PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch04-pgfId-919">See Also</A></H3><PCLASS="para">The <CODECLASS="literal">reverse</CODE> function in <EMCLASS="emphasis">perlfunc </EM>(1) and <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>; we use <CODECLASS="literal">reverse</CODE> in <ACLASS="xref"HREF="ch01_07.htm"TITLE="Reversing a String by Word or Character">Recipe 1.6</A> <ACLASS="indexterm"NAME="ch04-idx-1000006690-0"></A><ACLASS="indexterm"NAME="ch04-idx-1000006690-1"></A><ACLASS="indexterm"NAME="ch04-idx-1000006690-2"></A><ACLASS="indexterm"NAME="ch04-idx-1000006690-3"></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="ch04_10.htm"TITLE="4.9. 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