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<HTML><HEAD><TITLE>Recipe 10.6. Detecting Return Context (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="1999-07-02T01:39:47Z"><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="ch10_01.htm"TITLE="10. Subroutines"><LINKREL="prev"HREF="ch10_06.htm"TITLE="10.5. Passing Arrays and Hashes by Reference"><LINKREL="next"HREF="ch10_08.htm"TITLE="10.7. Passing by Named Parameter"></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="ch10_06.htm"TITLE="10.5. Passing Arrays and Hashes by Reference"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 10.5. Passing Arrays and Hashes by Reference"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1"><ACLASS="chapter"REL="up"HREF="ch10_01.htm"TITLE="10. Subroutines"></A></FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch10_08.htm"TITLE="10.7. Passing by Named Parameter"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 10.7. Passing by Named Parameter"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch10-27272">10.6. Detecting Return Context</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch10-pgfId-494">Problem</A></H3><PCLASS="para"><ACLASS="indexterm"NAME="ch10-idx-1000004677-0"></A><ACLASS="indexterm"NAME="ch10-idx-1000004677-1"></A><ACLASS="indexterm"NAME="ch10-idx-1000004677-2"></A><ACLASS="indexterm"NAME="ch10-idx-1000004677-3"></A><ACLASS="indexterm"NAME="ch10-idx-1000004677-4"></A>You want to know whether your function was called in scalar context or list context. This lets you have one function that does different things, like most of Perl's built-in functions.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch10-pgfId-500">Solution</A></H3><PCLASS="para">Use the <CODECLASS="literal">wantarray()</CODE> function, which has three possible return values depending on how the current function was called:</P><PRECLASS="programlisting">if (wantarray()) { # list context} elsif (defined wantarray()) { # scalar context} else { # void context} </PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch10-pgfId-524">Discussion</A></H3><PCLASS="para">Many built-in functions act differently when called in scalar context than in list context. A user-defined function can learn the context it was called in by examining the return value from the <CODECLASS="literal">wantarray</CODE> built-in. List context is indicated by a true return value. If it returns a value that is false but defined, then the function's return value will be used in scalar context. If it returns <CODECLASS="literal">undef</CODE>, it isn't being asked to provide a value at all.</P><PRECLASS="programlisting">if (wantarray()) { print "In list context\n"; return @many_things;} elsif (defined wantarray()) { print "In scalar context\n"; return $one_thing;} else { print "In void context\n"; return; # nothing}mysub(); # void context$a = mysub(); # scalar contextif (mysub()) { } # scalar context@a = mysub(); # list contextprint mysub(); # list context<ACLASS="indexterm"NAME="ch10-idx-1000004679-0"></A><ACLASS="indexterm"NAME="ch10-idx-1000004679-1"></A><ACLASS="indexterm"NAME="ch10-idx-1000004679-2"></A><ACLASS="indexterm"NAME="ch10-idx-1000004679-3"></A><ACLASS="indexterm"NAME="ch10-idx-1000004679-4"></A></PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch10-pgfId-566">See Also</A></H3><PCLASS="para">The <ACLASS="olink"HREF="../prog/ch03_125.htm"><CODECLASS="literal">return</CODE></A> and <ACLASS="olink"HREF="../prog/ch03_191.htm"><CODECLASS="literal">wantarray</CODE></A> functions in <ACLASS="olink"HREF="../prog/ch03_01.htm">Chapter 3</A> of <ACLASS="citetitle"HREF="../prog/index.htm"TITLE="Programming Perl"><CITECLASS="citetitle">Programming Perl</CITE></A> and <ICLASS="filename">perlfunc </I>(1)</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="ch10_06.htm"TITLE="10.5. Passing Arrays and Hashes by Reference"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 10.5. Passing Arrays and Hashes by Reference"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><ACLASS="book"HREF="index.htm"TITLE="Perl Cookbook"><IMGSRC="../gifs/txthome.gif"ALT="Perl Cookbook"BORDER="0"></A></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch10_08.htm"TITLE="10.7. Passing by Named Parameter"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 10.7. Passing by Named Parameter"BORDER="0"></A></TD></TR><TR><TDALIGN="LEFT"VALIGN="TOP"WIDTH="228">10.5. Passing Arrays and Hashes by Reference</TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><ACLASS="index"HREF="index/index.htm"TITLE="Book Index"><IMGSRC="../gifs/index.gif"ALT="Book Index"BORDER="0"></A></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228">10.7. 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