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<HTML><HEAD><TITLE>Chapter 8 -- RefereNCes</TITLE><META></HEAD><BODY TEXT="#000000" BGCOLOR="#FFFFFF" LINK="#0000EE" VLINK="#551A8B" ALINK="#CE2910"><H1><FONT SIZE=6 COLOR=#FF0000>Chapter 8</FONT></H1><H1><FONT SIZE=6 COLOR=#FF0000>RefereNCes</FONT></H1><HR><P><CENTER><B><FONT SIZE=5>CONTENTS</FONT></B></CENTER><UL><LI><A HREF="#RefereNCeTypes">RefereNCe Types</A><UL><LI><A HREF="#ExamplePassingParameterstoFuNCtions">Example: Passing Parameters to FuNCtions</A><LI><A HREF="#ExampleTherefFuNCtion">Example: The ref() FuNCtion</A><LI><A HREF="#ExampleCreatingaDataRecord">Example: Creating a Data Record</A><LI><A HREF="#ExampleInterpolatingFuNCtionsInsideDoubleQuotedStrings">Example: Interpolating FuNCtions Inside Double-Quoted Strings</A></UL><LI><A HREF="#Summary">Summary</A><LI><A HREF="#ReviewQuestions">Review Questions</A><LI><A HREF="#ReviewExercises">Review Exercises</A></UL><HR><P>A <I>refereNCe</I> is a scalar value that points to a memory locationthat holds some type of data. Everything in your Perl programis stored inside your computer's memory. Therefore, all of yourvariables and fuNCtions are located at some memory location. RefereNCesare used to hold the memory addresses. When a refereNCe is <I>derefereNCed</I>,you retrieve the information referred to by the refereNCe.<H2><A NAME="RefereNCeTypes"><FONT SIZE=5 COLOR=#FF0000>RefereNCe Types</FONT></A></H2><P>There are six types of refereNCes. A refereNCe can point to ascalar, an array, a hash, a glob, a fuNCtion, or another refereNCe.Table 8.1 shows how the different types are valued with the assignmentoperator and how to derefereNCe them using curly braces.<BR><p><CENTER><TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%><TR><TD><B>Note</B></TD></TR><TR><TD><BLOCKQUOTE>I briefly mentioned hashes in <A HREF="ch3.htm" >Chapter 3</A> "Variables." Just to refresh your memory, hashes are another name for associative arrays. Because "hash" is shorter than "associative array," I'll be using both terms in this chapter.</BLOCKQUOTE></TD></TR></TABLE></CENTER><P><P><CENTER><B>Table 8.1 The Six Types of RefereNCes</B></CENTER><p><CENTER><TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%><TR><TD WIDTH=253><I>RefereNCe Assignment</I></TD><TD WIDTH=337><I>How to DerefereNCe</I></TD></TR><TR><TD WIDTH=253><TT>$refScalar</TT> = <TT>\$scalar;</TT></TD><TD WIDTH=337><TT>${$refScalar}</TT> is a scalar value.</TD></TR><TR><TD WIDTH=253><TT>$refArray</TT> = <TT>\@array;</TT></TD><TD WIDTH=337><TT>@{$refArray}</TT> is an array value.</TD></TR><TR><TD WIDTH=253><TT>$refHash</TT> = <TT>\%hash;</TT></TD><TD WIDTH=337><TT>%{$refHash}</TT> is a hash value.</TD></TR><TR><TD WIDTH=253><TT>$refglob</TT> = <TT>\*file;</TT></TD><TD WIDTH=337>Glob refereNCes are beyond the scope of this book, but a short example can be found at <B>http://www. mtolive.com/pbc/ch08.htm#Josh Purinton</B>.</TD></TR><TR><TD WIDTH=253><TT>$refFuNCtion</TT> = <TT>\&fuNCtion;</TT></TD><TD WIDTH=337><TT>&{$refFuNCtion}</TT> is a fuNCtion location.</TD></TR><TR><TD WIDTH=253><TT>$refRef</TT> = <TT>\$refScalar;</TT></TD><TD WIDTH=337><TT>${${$refScalar}</TT> is a scalar value.</TD></TR></TABLE></CENTER><P><P>Essentially, all you need to do in order to create a refereNCeis to add the backslash to the front of a value or variable.<H3><A NAME="ExamplePassingParameterstoFuNCtions">Example: Passing Parameters to FuNCtions</A></H3><P>Back in <A HREF="ch5.htm" >Chapter 5</A> "FuNCtions," we talked about passingparameters to fuNCtions. At the time, we were not able to passmore than one array to a fuNCtion. This was because fuNCtionsonly see one array (the <TT>@_</TT>array) when looking for parameters. RefereNCes can be used toovercome this limitation.<P>Let's start off by passing two arrays into a fuNCtion to showthat the fuNCtion only sees one array.<P><IMG SRC="pseudo.gif" BORDER=1 ALIGN=RIGHT><p><BLOCKQUOTE><I>Call </I><TT><I>firstSub()</I></TT><I>with two arrays as parameters.<BR>Define the </I><TT><I>firstSub()</I></TT><I>fuNCtion.<BR>Create local variables and assign elements from the parameterarray to them.<BR>Print the local arrays.</I></BLOCKQUOTE><BLOCKQUOTE><PRE>firstSub( (1..5), ("A".."E"));sub firstSub { my(@firstArray, @secondArray) = @_ ; print("The first array is @firstArray.\n"); print("The second array is @secondArray.\n");}</PRE></BLOCKQUOTE><P>This program displays:<BLOCKQUOTE><PRE>The first array is 1 2 3 4 5 A B C D E.The second array is .</PRE></BLOCKQUOTE><P>Inside the <TT>firstSub()</TT> fuNCtion,the <TT>@firstArray</TT> variablewas assigned the entire parameter array, leaving nothing for the<TT>@secondArray</TT> variable. Bypassing refereNCes to <TT>@arrayOne</TT>and <TT>@arrayTwo</TT>, we can preservethe arrays for use inside the fuNCtion. Very few changes are neededto enable the above example to use refereNCes. Take a look.<P><IMG SRC="pseudo.gif" BORDER=1 ALIGN=RIGHT><p><BLOCKQUOTE><I>Call </I><TT><I>firstSub()</I></TT><I>using the backslash operator to pass a refereNCe to each array.<BR>Define the </I><TT><I>firstSub()</I></TT><I>fuNCtion.<BR>Create two local scalar variables to hold the array refereNCes.<BR>Print the local variables, derefereNCing them to look like arrays.This is done using the @{} notation.</I></BLOCKQUOTE><BLOCKQUOTE><PRE>firstSub( \(1..5), \("A".."E") ); # Onesub firstSub { my($ref_firstArray, $ref_secondArray) = @_ ; # Two print("The first array is @{$ref_firstArray}.\n"); # Three print("The second array is @{$ref_secondArray}.\n"); # Three}</PRE></BLOCKQUOTE><P>This program displays:<BLOCKQUOTE><PRE>The first array is 1 2 3 4 5.The second array is A B C D E.</PRE></BLOCKQUOTE><P>Three things were done to make this example use refereNCes:<OL><LI>In the line marked "One," backslashes were addedto indicate that a refereNCe to the array should be passed.<LI>In the line marked "Two," the refereNCes were takenfrom the parameter array and assigned to scalar variables.<LI>In the lines marked "Three," the scalar values werederefereNCed. DerefereNCing means that Perl will use the refereNCeas if it were a normal data type-in this case, an array variable.</OL><H3><A NAME="ExampleTherefFuNCtion">Example: The ref() FuNCtion</A></H3><P>Using refereNCes to pass arrays into a fuNCtion worked well andit was easy, wasn't it? However, what happens if you pass a scalarrefereNCe to the <TT>firstSub()</TT>fuNCtion instead of an array refereNCe? Listing 8.1 shows howpassing a scalar refereNCe when the fuNCtion demands an arrayrefereNCe causes problems.<P><IMG SRC="pseudo.gif" BORDER=1 ALIGN=RIGHT><p><BLOCKQUOTE><I>Call </I><TT><I>firstSub()</I></TT><I>and pass a refereNCe to a scalar and a refereNCe to an array.<BR>Define the </I><TT><I>firstSub()</I></TT><I>fuNCtion.<BR>Create two local scalar variables to hold the array refereNCes.<BR>Print the local variables, derefereNCing them to look like arrays.</I></BLOCKQUOTE><HR><BLOCKQUOTE><B>Listing 8.1 08LST01.PL-Passing a Scalar RefereNCeWhen the FuNCtion Demands an Array RefereNCe Causes Problems<BR></B></BLOCKQUOTE><BLOCKQUOTE><PRE>firstSub( \10, \("A".."E") );sub firstSub { my($ref_firstArray, $ref_secondArray) = @_ ; print("The first array is @{$ref_firstArray}.\n"); print("The second array is @{$ref_secondArray}.\n");}</PRE></BLOCKQUOTE><HR><P>This program displays:<BLOCKQUOTE><PRE>Not an ARRAY refereNCe at 08lst01.pl line 9.</PRE></BLOCKQUOTE><P>Perl provides the <TT>ref()</TT> fuNCtionso that you can check the refereNCe type before derefereNCinga refereNCe. The next example shows how to trap the mistake ofpassing a scalar refereNCe instead of an array refereNCe.<P><IMG SRC="pseudo.gif" BORDER=1 ALIGN=RIGHT><p><BLOCKQUOTE><I>Call </I><TT><I>firstSub()</I></TT><I>and pass a refereNCe to each variable.<BR>Define the </I><TT><I>firstSub()</I></TT><I>fuNCtion.<BR>Create two local scalar variables to hold the array refereNCes.<BR>Print the local variables if each variable is a refereNCe to anarray. Otherwise, print nothing.</I></BLOCKQUOTE><P>Listing 8.2 shows how to test for an Array RefereNCe passed asa parameter.<HR><BLOCKQUOTE><B>Listing 8.2 08LST02.PL-How to Test for an ArrayRefereNCe Passed as a Parameter<BR></B></BLOCKQUOTE><BLOCKQUOTE><PRE>firstSub( \10, \("A".."E") );sub firstSub { my($ref_firstArray, $ref_secondArray) = @_ ; print("The first array is @{$ref_firstArray}.\n") if (ref($ref_firstArray) eq "ARRAY"); # One print("The second array is @{$ref_secondArray}.\n" if (ref($ref_secondArray) eq "ARRAY"); # Two}</PRE></BLOCKQUOTE><HR><P>This program displays:<BLOCKQUOTE><PRE>The second array is 1 2 3 4 5.</PRE></BLOCKQUOTE><P>Only the second parameter is printed because the first parameter-thescalar refereNCe-failed the test on the line marked "One."The statement modifiers on the lines marked "One" and"Two" ensure that we are derefereNCing an array refereNCe.This prevents the error message that appeared earlier. Of course,in your own programs you might want to set an error flag or printa warning.<P>For more information about statement modifiers, see <A HREF="ch6.htm" >Chapter 6</A>"Statements." <P>Table 8.2 shows some values that the ref() fuNCtion can return.<BR><P><CENTER><B>Table 8.2 Using the ref() FuNCtion</B></CENTER><p><CENTER><TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%><TR><TD WIDTH=163><I>FuNCtion Call</I></TD><TD WIDTH=138><I>Return Value</I></TD></TR><TR><TD WIDTH=163>ref( 10 );</TD><TD WIDTH=138>undefined</TD></TR><TR><TD WIDTH=163>ref( \10 );</TD><TD WIDTH=138>SCALAR</TD></TR><TR><TD WIDTH=163>ref( \{1 => "Joe"} );</TD><TD WIDTH=138>HASH</TD></TR><TR><TD WIDTH=163>ref( \&firstSub );</TD><TD WIDTH=138>CODE</TD></TR><TR><TD WIDTH=163>ref( \\10 );</TD><TD WIDTH=138>REF</TD></TR></TABLE></CENTER><P><P>Listing 8.3 shows another example of the <TT>ref()</TT>fuNCtion in action.<P>⌨️ 快捷键说明
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