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<h1 class="chapter">Chapter 4. Subroutines</h1>
<div class="htmltoc"><h4 class="tochead">Contents:</h4>
<p> <a href="#lperl3-CHP-4-SECT-1">System and User Functions</a><br />
<a href="ch04_02.htm">Defining a Subroutine</a><br />
<a href="ch04_03.htm">Invoking a Subroutine</a><br />
<a href="ch04_04.htm">Return Values</a><br />
<a href="ch04_05.htm">Arguments</a><br />
<a href="ch04_06.htm">Private Variables in Subroutines</a><br />
<a href="ch04_07.htm">The local Operator</a><br />
<a href="ch04_08.htm">Variable-length Parameter Lists</a><br />
<a href="ch04_09.htm">Notes on Lexical (my) Variables</a><br />
<a href="ch04_10.htm">The use strict Pragma</a><br />
<a href="ch04_11.htm">The return Operator</a><br />
<a href="ch04_12.htm">Exercises</a><br /></p></div>
<div class="sect1"><a name="lperl3-CHP-4-SECT-1" /></a>
<h2 class="sect1">4.1. System and User Functions</h2>
<p><a name="INDEX-313" /></a>We've
already seen and used some of the builtin <a name="INDEX-314" /></a>system functions, such as
<tt class="literal">chomp</tt>, <tt class="literal">reverse</tt>,
<tt class="literal">print</tt>, and so on. But, as other languages do, Perl
has the ability to make <em class="firstterm">subroutines</em>, which are
<a name="INDEX-315" /></a>user-defined
functions.<a href="#FOOTNOTE-97">[97]</a> These
let us recycle one chunk of code many times in one program.<a href="#FOOTNOTE-98">[98]</a>
</p><blockquote class="footnote"> <a name="FOOTNOTE-97" /></a><p>[97]In Perl, we don't generally make the
distinction that Pascal programmers are used to, between
<em class="emphasis">functions</em>, which return a value, and
<em class="emphasis">procedures</em>, which don't. But a
<em class="emphasis">subroutine</em> is always user-defined, while a
<em class="emphasis">function</em> may or may not be. That is, the word
<em class="emphasis">function</em> may be used as a synonym for
<em class="emphasis">subroutine</em>, or it may mean one of Perl's
builtin functions. That's why this chapter is titled
<em class="emphasis">Subroutines</em>, because it's about the ones
you can define, not the builtins. Mostly.</p> </blockquote><blockquote class="footnote">
<a name="FOOTNOTE-98" /></a><p>[98]The code examples used in this book are recycled from at least
40% post-consumer programming, and are at least 75% recyclable into
your programs when properly decomposed. </p> </blockquote>
<p>The name of a subroutine is another Perl identifier (letters, digits,
and underscores, but can't start with a digit) with a
sometimes-optional ampersand (<tt class="literal">&</tt>) in front.
There's a rule about when you can omit the ampersand and when
you cannot; we'll see that rule by the end of the chapter. For
now, we'll just use it every time that it's not
forbidden, which is always a safe rule. And we'll tell you
every place where it's forbidden, of course.
</p>
<p>That subroutine name comes from a separate namespace, so Perl
won't be confused if you have a subroutine called
<tt class="literal">&fred</tt> and a scalar called
<tt class="literal">$fred</tt> in the same program -- although
there's no reason to do that under normal circumstances.
</p>
</div>
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