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<HTML><HEAD><TITLE>Recipe 14.3. Converting Between DBM Files (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="1999-07-02T01:42:48Z"><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="ch14_01.htm"TITLE="14. Database Access"><LINKREL="prev"HREF="ch14_03.htm"TITLE="14.2. Emptying a DBM File"><LINKREL="next"HREF="ch14_05.htm"TITLE="14.4. Merging DBM Files"></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="ch14_03.htm"TITLE="14.2. Emptying a DBM File"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 14.2. Emptying a DBM File"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1"><ACLASS="chapter"REL="up"HREF="ch14_01.htm"TITLE="14. Database Access"></A></FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch14_05.htm"TITLE="14.4. Merging DBM Files"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 14.4. Merging DBM Files"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch14-chap14_converting_0">14.3. Converting Between DBM Files</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch14-pgfId-308">Problem</A></H3><PCLASS="para"><ACLASS="indexterm"NAME="ch14-idx-1000004942-0"></A><ACLASS="indexterm"NAME="ch14-idx-1000004942-1"></A><ACLASS="indexterm"NAME="ch14-idx-1000004942-2"></A>You have a file in one DBM format, but another program expects input in a different DBM format.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch14-pgfId-314">Solution</A></H3><PCLASS="para">Reads the keys and values from the initial DBM file and writes them to a new file in the different DBM format as <ACLASS="indexterm"NAME="ch14-idx-1000004943-0"></A><ACLASS="indexterm"NAME="ch14-idx-1000004943-1"></A><ACLASS="indexterm"NAME="ch14-idx-1000004943-2"></A>in <ACLASS="xref"HREF="ch14_04.htm#ch14-12080"TITLE="db2gdbm">Example 14.2</A>.</P><DIVCLASS="example"><H4CLASS="example"><ACLASS="title"NAME="ch14-12080">Example 14.2: db2gdbm</A></H4><PRECLASS="programlisting">#!/usr/bin/perl -w# db2gdbm: converts DB to GDBMuse strict;use DB_File;use GDBM_File;unless (@ARGV == 2) { die "usage: db2gdbm infile outfile\n";}my ($infile, $outfile) = @ARGV; my (%db_in, %db_out); # open the filestie(%db_in, 'DB_File', $infile) or die "Can't tie $infile: $!";tie(%db_out, 'GDBM_File', $outfile, GDBM_WRCREAT, 0666) or die "Can't tie $outfile: $!";# copy (don't use %db_out = %db_in because it's slow on big databases)while (my($k, $v) = each %db_in) { $db_out{$k} = $v;}# these unties happen automatically at program exituntie %db_in;untie %db_out;</PRE></DIV><PCLASS="para">Call the program as:</P><PRECLASS="programlisting">% db2gdbm /tmp/users.db /tmp/users.gdbm</PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch14-pgfId-384">Discussion</A></H3><PCLASS="para">When multiple types of DBM file are used in the same program, you have to use <CODECLASS="literal">tie</CODE>, not the <CODECLASS="literal">dbmopen</CODE> interface. That's because with <CODECLASS="literal">dbmopen</CODE> you can only use one database format, which is why its use is deprecated.</P><PCLASS="para">Copying hashes by simple assignment, as in <CODECLASS="literal">%new</CODE> <CODECLASS="literal">=</CODE> <CODECLASS="literal">%old</CODE> , works on DBM files. However, it loads everything into memory first as a list, which doesn't matter with small hashes, but can be prohibitively expensive in the case of DBM files. For database hashes, use <CODECLASS="literal">each</CODE> to iterate through them instead. <ACLASS="indexterm"NAME="ch14-idx-1000004945-0"></A><ACLASS="indexterm"NAME="ch14-idx-1000004945-1"></A><ACLASS="indexterm"NAME="ch14-idx-1000004945-2"></A></P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch14-pgfId-392">See Also</A></H3><PCLASS="para">The documentation for the standard modules GDBM_File, NDBM_File, SDBM_File, DB_File, also in <ACLASS="olink"HREF="../prog/ch07_01.htm">Chapter 7</A> of <ACLASS="citetitle"HREF="../prog/index.htm"TITLE="Programming Perl"><CITECLASS="citetitle">Programming Perl</CITE></A>; <ACLASS="xref"HREF="ch14_02.htm"TITLE="Making and Using a DBM File">Recipe 14.1</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="ch14_03.htm"TITLE="14.2. 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Merging DBM Files</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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