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<HTML><HEAD><TITLE>Recipe 8.12. Using Random-Access I/O (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="1999-07-02T01:38:49Z"><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="ch08_01.htm"TITLE="8. File Contents"><LINKREL="prev"HREF="ch08_12.htm"TITLE="8.11. Processing Binary Files"><LINKREL="next"HREF="ch08_14.htm"TITLE="8.13. Updating a Random-Access File"></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="ch08_12.htm"TITLE="8.11. Processing Binary Files"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 8.11. Processing Binary Files"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1"><ACLASS="chapter"REL="up"HREF="ch08_01.htm"TITLE="8. File Contents"></A></FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch08_14.htm"TITLE="8.13. Updating a Random-Access File"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 8.13. Updating a Random-Access File"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch08-24750">8.12. Using Random-Access I/O</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch08-pgfId-1120">Problem<ACLASS="indexterm"NAME="ch08-idx-1000004680-0"></A><ACLASS="indexterm"NAME="ch08-idx-1000004680-1"></A><ACLASS="indexterm"NAME="ch08-idx-1000004680-2"></A><ACLASS="indexterm"NAME="ch08-idx-1000004680-3"></A><ACLASS="indexterm"NAME="ch08-idx-1000004680-4"></A><ACLASS="indexterm"NAME="ch08-idx-1000004680-5"></A></A></H3><PCLASS="para">You have to read a binary record from the middle of a large file but don't want to read a record at a time to get there.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch08-pgfId-1126">Solution</A></H3><PCLASS="para">Once you know the record's size, multiply it by the record number to get the byte address, and then seek to that byte address and read the record:</P><PRECLASS="programlisting">$ADDRESS = $RECSIZE * $RECNO;seek(FH, $ADDRESS, 0) or die "seek:$!";read(FH, $BUFFER, $RECSIZE);</PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch08-pgfId-1138">Discussion</A></H3><PCLASS="para">The Solution assumes the first record has a <CODECLASS="literal">RECNO</CODE> of 0. If you're counting from one, use:</P><PRECLASS="programlisting">$ADDRESS = $RECSIZE * ($RECNO-1);</PRE><PCLASS="para">This won't work on a text file unless all lines are the same length. This is rarely the case.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch08-pgfId-1146">See Also</A></H3><PCLASS="para">The <CODECLASS="literal">seek</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>; <ACLASS="xref"HREF="ch08_14.htm"TITLE="Updating a Random-Access File">Recipe 8.13</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="ch08_12.htm"TITLE="8.11. Processing Binary Files"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 8.11. 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Updating a Random-Access File</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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