ch11_01.htm

By Tom Christiansen and Nathan Torkington ISBN 1-56592-243-3 First Edition, published August 1998

CLASS="replaceable"
><I
>fred</I
></CODE
></B
></CODE
>
print $a[4][23][53];
<CODE
CLASS="userinput"
><B
><CODE
CLASS="replaceable"
><I
>ARRAY(0x81e2494)</I
></CODE
></B
></CODE
>
print $a[4][23];
<CODE
CLASS="userinput"
><B
><CODE
CLASS="replaceable"
><I
>ARRAY(0x81e0748)</I
></CODE
></B
></CODE
>
print $a[4];
<CODE
CLASS="userinput"
><B
><CODE
CLASS="replaceable"
><I
>ARRAY(0x822cd40)</I
></CODE
></B
></CODE
></PRE
><P
CLASS="para"
>The following table shows mechanisms for producing references to both named and anonymous scalars, arrays, hashes, and functions. (Anonymous typeglobs are too scary to show &nbsp;-  and virtually never used. It's best to use <CODE
CLASS="literal"
>Symbol::gensym()</CODE
> or <CODE
CLASS="literal"
>IO::Handle-&gt;new()</CODE
> for them.)</P
><TABLE
CLASS="informaltable"
BORDER="1"
CELLPADDING="3"
><THEAD
CLASS="thead"
><TR
CLASS="row"
VALIGN="TOP"
><TH
CLASS="entry"
ALIGN="LEFT"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>Reference to</P
></TH
><TH
CLASS="entry"
ALIGN="LEFT"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>Named</P
></TH
><TH
CLASS="entry"
ALIGN="LEFT"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>Anonymous</P
></TH
></TR
></THEAD
><TBODY
CLASS="tbody"
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>Scalar</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>\$scalar</CODE
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>\do{my $anon}</CODE
></P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>Array</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>\@array</CODE
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>[ LIST ]</CODE
></P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>Hash</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>\%hash</CODE
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>{ LIST }</CODE
></P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>Code</P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>\&amp;function</CODE
></P
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
><CODE
CLASS="literal"
>sub { CODE } </CODE
></P
></TD
></TR
></TBODY
></TABLE
><P
CLASS="para"
>These diagrams illustrate the differences between named and anonymous values. <A
CLASS="xref"
HREF="ch11_01.htm#ch11-17738"
TITLE="Named values"
>Figure 11.2</A
> shows named values.</P
><H4
CLASS="figure"
><A
CLASS="title"
NAME="ch11-17738"
>Figure 11.2: Named values</A
></H4
><IMG
CLASS="graphic"
SRC="figs/pcb_1102.gif"
ALT="Figure 11.2"><P
CLASS="para"
>In other words, saying <CODE
CLASS="literal"
>$a</CODE
> <CODE
CLASS="literal"
>=</CODE
> <CODE
CLASS="literal"
>\$b</CODE
> makes <CODE
CLASS="literal"
>$$a</CODE
> and <CODE
CLASS="literal"
>$b</CODE
> the <EM
CLASS="emphasis"
>same piece of memory</EM
>. If you say <CODE
CLASS="literal"
>$$a</CODE
> <CODE
CLASS="literal"
>=</CODE
> <CODE
CLASS="literal"
>3</CODE
>, then the value of <CODE
CLASS="literal"
>$b</CODE
> is set to 3.</P
><P
CLASS="para"
><A
CLASS="xref"
HREF="ch11_01.htm#ch11-31355"
TITLE="Anonymous values"
>Figure 11.3</A
> shows anonymous values.</P
><H4
CLASS="figure"
><A
CLASS="title"
NAME="ch11-31355"
>Figure 11.3: Anonymous values</A
></H4
><IMG
CLASS="graphic"
SRC="figs/pcb_1103.gif"
ALT="Figure 11.3"><P
CLASS="para"
>Every reference evaluates as true, by definition, so if you write a subroutine that returns a reference, you can return <CODE
CLASS="literal"
>undef</CODE
> on error and check for it with:</P
><PRE
CLASS="programlisting"
>$op_cit = cite($ibid)       or die &quot;couldn't make a reference&quot;;</PRE
><P
CLASS="para"
>The <CODE
CLASS="literal"
>undef</CODE
><A
CLASS="indexterm"
NAME="ch11-idx-1000004791-0"
></A
> operator can be used on any variable or function in Perl to free its memory. This does not necessarily free memory, call object destructors, etc. It just decrements the reference count by one.<A
CLASS="indexterm"
NAME="ch11-idx-1000004781-0"
></A
> Without an argument, <CODE
CLASS="literal"
>undef</CODE
> produces an undefined value.</P
></DIV
><DIV
CLASS="sect2"
><H3
CLASS="sect2"
><A
CLASS="title"
NAME="ch11-chap11_records_0"
>Records</A
></H3
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="ch11-idx-1000004782-0"
></A
>The classic use of references in Perl is to circumvent the restriction that arrays and hashes may hold scalars only. References are scalars, so to make an array of arrays, make an array of array <EM
CLASS="emphasis"
>references</EM
>. Similarly, hashes of hashes are implemented as hashes of hash references, arrays of hashes as arrays of hash references, hashes of arrays as hashes of array references, and so on.</P
><P
CLASS="para"
>Once you have these complex structures, you can use them to implement records. A record is a single logical unit composed of different attributes. For instance, a name, an address, and a birthday might comprise a record representing a person. C calls such things <EM
CLASS="emphasis"
>structs</EM
>, and Pascal calls them <EM
CLASS="emphasis"
>RECORDs</EM
>. Perl doesn't have a particular name for these because you can implement this notion in different ways.</P
><P
CLASS="para"
>The most common technique in Perl is to treat a hash as a record, where the keys of the hash are the record's field names and the values of the hash are those fields' values.</P
><P
CLASS="para"
>For instance, we might create a "person" record like this:</P
><PRE
CLASS="programlisting"
>$Nat = { &quot;Name&quot;     =&gt; &quot;Leonhard Euler&quot;,
         &quot;Address&quot;  =&gt; &quot;1729 Ramanujan Lane\nMathworld, PI 31416&quot;,
         &quot;Birthday&quot; =&gt; 0x5bb5580,
       };</PRE
><P
CLASS="para"
>Because <CODE
CLASS="literal"
>$Nat</CODE
> is a scalar, it can be stored in an array or hash element, thus creating create groups of people. Now apply the array and hash techniques from Chapters 4 and 5 to sort the sets, merge hashes, pick a random record, and so on.</P
><P
CLASS="para"
>The attributes of a record, including the "person" record, are always scalars. You can certainly use numbers as readily as strings there, but that's no great trick. The real power play happens when you use even more references for values in the record. <CODE
CLASS="literal"
>&quot;Birthday&quot;</CODE
>, for instance, might be stored as an anonymous array with three elements: day, month, and year. You could then say <CODE
CLASS="literal"
>$person-&gt;{&quot;Birthday&quot;}-&gt;[0]</CODE
> to access just the day field. Or a date might be represented as a hash record, which would then lend itself to access such as <CODE
CLASS="literal"
>$person-&gt;{&quot;Birthday&quot;}-&gt;{&quot;day&quot;}</CODE
>. Adding references to your collection of skills makes possible many more complex and useful programming strategies.</P
><P
CLASS="para"
>At this point, we've conceptually moved beyond simple records. We're now creating elaborate data structures that represent complicated relationships between the data they hold. Although we <EM
CLASS="emphasis"
>can</EM
> use these to implement traditional data structures like linked lists, the recipes in the second half of this chapter don't deal specifically with any particular structure. Instead, they give generic techniques for loading, printing, copying, and saving generic data structures. The final program example demonstrates how to manipulate binary trees.<A
CLASS="indexterm"
NAME="ch11-idx-1000004783-0"
></A
></P
></DIV
><DIV
CLASS="sect2"
><H3
CLASS="sect2"
><A
CLASS="title"
NAME="ch11-pgfId-180"
>See Also</A
></H3
><P
CLASS="para"
><A
CLASS="olink"
HREF="../prog/ch04_01.htm"
>Chapter 4</A
> of <A
CLASS="citetitle"
HREF="../prog/index.htm"
TITLE="Programming Perl"
><CITE
CLASS="citetitle"
>Programming Perl</CITE
></A
>; <I
CLASS="filename"
>perlref </I
>(1), <I
CLASS="filename"
>perllol</I
> (1), and <I
CLASS="filename"
>perldsc</I
> (1)</P
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