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<HTML><HEAD><TITLE>Recipe 11.9. Constructing Records (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="1999-07-02T01:40:48Z"><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="ch11_01.htm"TITLE="11. References and Records"><LINKREL="prev"HREF="ch11_09.htm"TITLE="11.8. Creating References to Methods"><LINKREL="next"HREF="ch11_11.htm"TITLE="11.10. Reading and Writing Hash Records to Text 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="ch11_09.htm"TITLE="11.8. Creating References to Methods"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 11.8. Creating References to Methods"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1"><ACLASS="chapter"REL="up"HREF="ch11_01.htm"TITLE="11. References and Records"></A></FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch11_11.htm"TITLE="11.10. Reading and Writing Hash Records to Text Files"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 11.10. Reading and Writing Hash Records to Text Files"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch11-13425">11.9. Constructing Records</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch11-pgfId-928">Problem<ACLASS="indexterm"NAME="ch11-idx-1000004843-0"></A></A></H3><PCLASS="para">You want to create a record data type.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch11-pgfId-934">Solution</A></H3><PCLASS="para">Use a reference to an anonymous hash.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch11-pgfId-940">Discussion</A></H3><PCLASS="para">Suppose you wanted to create a data type that contained various data fields, akin to a C <CODECLASS="literal">struct</CODE> or a Pascal RECORD. The easiest way is to use an anonymous hash. For example, here's how to initialize and use that record:</P><PRECLASS="programlisting">$record = { NAME => "Jason", EMPNO => 132, TITLE => "deputy peon", AGE => 23, SALARY => 37_000, PALS => [ "Norbert", "Rhys", "Phineas"],};printf "I am %s, and my pals are %s.\n", $record->{NAME}, join(", ", @{$record->{PALS}});</PRE><PCLASS="para">Just having one of these records isn't much fun - you'd like to build larger structures. For example, you might want to create a <CODECLASS="literal">%ByName</CODE> hash that you could initialize and use this way:</P><PRECLASS="programlisting"># store record$byname{ $record->{NAME} } = $record;# later on, look up by nameif ($rp = $byname{"Aron"}) { # false if missing printf "Aron is employee %d.\n", $rp->{EMPNO};}# give jason a new palpush @{$byname{"Jason"}->{PALS}}, "Theodore";printf "Jason now has %d pals\n", scalar @{$byname{"Jason"}->{PALS}};</PRE><PCLASS="para">That makes <CODECLASS="literal">%byname</CODE> a hash of hashes, because its values are hash references. Looking up employees by name would be easy using such a structure. If we find a value in the hash, we store a reference to the record in a temporary variable, <CODECLASS="literal">$rp</CODE>, which we then use to get any field we want.</P><PCLASS="para">We can use our existing hash tools to manipulate <CODECLASS="literal">%byname</CODE>. For instance, we could use the <CODECLASS="literal">each</CODE> iterator to loop through it in an arbitrary order:</P><PRECLASS="programlisting"># Go through all recordswhile (($name, $record) = each %byname) { printf "%s is employee number %d\n", $name, $record->{EMPNO};}</PRE><PCLASS="para">What about looking employees up by employee number? Just build and use another data structure, an array of hashes called <CODECLASS="literal">@employees</CODE>. If your employee numbers aren't consecutive (for instance, they jump from 1 to 159997) an array would be a bad choice. Instead, you should use a hash mapping employee number to record. For consecutive employee numbers, use an array:</P><PRECLASS="programlisting"># store record$employees[ $record->{EMPNO} ] = $record;# lookup by idif ($rp = $employee[132]) { printf "employee number 132 is %s\n", $rp->{NAME};}</PRE><PCLASS="para">With a data structure like this, updating a record in one place effectively updates it everywhere. For example, this gives Jason a 3.5% raise:</P><PRECLASS="programlisting">$byname{"Jason"}->{SALARY} *= 1.035;</PRE><PCLASS="para">This change is reflected in all views of these records. Remember that both <CODECLASS="literal">$byname{"Jason"}</CODE> and <CODECLASS="literal">$employees[132]</CODE> refer to the same record because the references they contain refer to the same anonymous hash.</P><PCLASS="para">How would you select all records matching a particular criterion? This is what <CODECLASS="literal">grep</CODE> is for. Here's how to get everyone with "peon" in their titles or all the 27-year-olds:</P><PRECLASS="programlisting">@peons = grep { $_->{TITLE} =~ /peon/i } @employees;@tsevens = grep { $_->{AGE} == 27 } @employees;</PRE><PCLASS="para">Each element of <CODECLASS="literal">@peons</CODE> and <CODECLASS="literal">@tsevens</CODE> is itself a reference to a record, making them arrays of hashes, like <CODECLASS="literal">@employees</CODE>.</P><PCLASS="para">Here's how to print all records sorted in a particular order, say by age:</P><PRECLASS="programlisting"># Go through all recordsforeach $rp (sort { $a->{AGE} <=> $b->{AGE} } values %byname) { printf "%s is age %d.\n", $rp->{NAME}, $rp->{AGE}; # or with a hash slice on the reference printf "%s is employee number %d.\n", @$rp{'NAME','EMPNO'};}</PRE><PCLASS="para">Rather than take time to sort them by age, you could just create another view of these records, <CODECLASS="literal">@byage</CODE>. Each element in this array, <CODECLASS="literal">$byage[27]</CODE> for instance, would be an array of all the records with that age. In effect, this is an array of arrays of hashes. You would build it this way:</P><PRECLASS="programlisting"># use @byage, an array of arrays of recordspush @{ $byage[ $record->{AGE} ] }, $record;</PRE><PCLASS="para">Then you could find them all this way:</P><PRECLASS="programlisting">for ($age = 0; $age <= $#byage; $age++) { next unless $byage[$age]; print "Age $age: "; foreach $rp (@{$byage[$age]}) { print $rp->{NAME}, " "; } print "\n";}</PRE><PCLASS="para">A similar approach is to use <CODECLASS="literal">map</CODE> to avoid the <CODECLASS="literal">foreach</CODE> loop:</P><PRECLASS="programlisting">for ($age = 0; $age <= $#byage; $age++) { next unless $byage[$age]; printf "Age %d: %s\n", $age, join(", ", map {$_->{NAME}} @{$byage[$age]});<ACLASS="indexterm"NAME="ch11-idx-1000004845-0"></A>}</PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch11-pgfId-1000005735">See Also</A></H3><PCLASS="para"><ACLASS="xref"HREF="ch04_14.htm"TITLE="Finding All Elements in an Array Matching Certain Criteria">Recipe 4.13</A>; <ACLASS="xref"HREF="ch11_04.htm"TITLE="Taking References to Hashes">Recipe 11.3</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="ch11_09.htm"TITLE="11.8. 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