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<HTML><HEAD><TITLE>Recipe 4.16. Implementing a Circular List (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="1999-07-02T01:32:13Z"><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_16.htm"TITLE="4.15. Sorting a List by Computable Field"><LINKREL="next"HREF="ch04_18.htm"TITLE="4.17. Randomizing 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_16.htm"TITLE="4.15. Sorting a List by Computable Field"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 4.15. Sorting a List by Computable Field"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_18.htm"TITLE="4.17. Randomizing an Array"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 4.17. Randomizing an Array"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch04-chap04_implementing_0">4.16. Implementing a Circular List</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch04-pgfId-1581">Problem <ACLASS="indexterm"NAME="ch04-idx-1000006771-0"></A><ACLASS="indexterm"NAME="ch04-idx-1000006771-1"></A><ACLASS="indexterm"NAME="ch04-idx-1000006771-2"></A></A></H3><PCLASS="para">You want to create and manipulate a circular list.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch04-pgfId-1587">Solution</A></H3><PCLASS="para">Use <CODECLASS="literal">unshift</CODE> and <CODECLASS="literal">pop</CODE> (or <CODECLASS="literal">push</CODE> and <CODECLASS="literal">shift</CODE>) on a normal array. <ACLASS="indexterm"NAME="ch04-idx-1000006772-0"></A><ACLASS="indexterm"NAME="ch04-idx-1000006772-1"></A><ACLASS="indexterm"NAME="ch04-idx-1000006772-2"></A><ACLASS="indexterm"NAME="ch04-idx-1000006772-3"></A></P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch04-chap04_procedure_0">Procedure</A></H3><PRECLASS="programlisting">unshift(@circular, pop(@circular)); # the last shall be firstpush(@circular, shift(@circular)); # and vice versa</PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch04-pgfId-1601">Discussion</A></H3><PCLASS="para">Circular lists are commonly used to repeatedly process things in order; for example, connections to a server. The code shown above isn't a true computer science circular list, with pointers and true circularity. Instead, the operations provide for moving the last element to the first position, and vice versa.</P><PRECLASS="programlisting">sub grab_and_rotate ( \@ ) { my $listref = shift; my $element = $listref->[0]; push(@$listref, shift @$listref); return $element;}@processes = ( 1, 2, 3, 4, 5 );while (1) { $process = grab_and_rotate(@processes); print "Handling process $process\n"; sleep 1;}</PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch04-pgfId-1633">See Also</A></H3><PCLASS="para">The <CODECLASS="literal">unshift</CODE> and <CODECLASS="literal">push</CODE> functions 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>; <ACLASS="xref"HREF="ch13_14.htm"TITLE="Coping with Circular Data Structures">Recipe 13.13</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_16.htm"TITLE="4.15. 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