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<HTML><HEAD><TITLE>Recipe 4.13. Finding All Elements in an Array Matching Certain Criteria (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="1999-07-02T01:32:01Z"><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="ch04_01.htm"TITLE="4. Arrays"><LINKREL="prev"HREF="ch04_13.htm"TITLE="4.12. Finding the First List Element That Passes a Test"><LINKREL="next"HREF="ch04_15.htm"TITLE="4.14. Sorting an Array Numerically"></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="ch04_13.htm"TITLE="4.12. Finding the First List Element That Passes a Test"><IMGSRC="../gifs/txtpreva.gif"ALT="Previous: 4.12. Finding the First List Element That Passes a Test"BORDER="0"></A></TD><TDALIGN="CENTER"VALIGN="TOP"WIDTH="228"><B><FONTFACE="ARIEL,HELVETICA,HELV,SANSERIF"SIZE="-1"><ACLASS="chapter"REL="up"HREF="ch04_01.htm"TITLE="4. Arrays"></A></FONT></B></TD><TDALIGN="RIGHT"VALIGN="TOP"WIDTH="228"><ACLASS="sect1"HREF="ch04_15.htm"TITLE="4.14. Sorting an Array Numerically"><IMGSRC="../gifs/txtnexta.gif"ALT="Next: 4.14. Sorting an Array Numerically"BORDER="0"></A></TD></TR></TABLE></DIV><DIVCLASS="sect1"><H2CLASS="sect1"><ACLASS="title"NAME="ch04-26761">4.13. Finding All Elements in an Array Matching Certain Criteria</A></H2><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch04-pgfId-1131">Problem <ACLASS="indexterm"NAME="ch04-idx-1000006732-0"></A><ACLASS="indexterm"NAME="ch04-idx-1000006732-1"></A><ACLASS="indexterm"NAME="ch04-idx-1000006732-2"></A></A></H3><PCLASS="para">From a list, you want only the elements that match certain criteria.</P><PCLASS="para">This notion of extracting a subset of a larger list is common. It's how you find all engineers in a list of employees, all users in the "staff " group, and all the filenames you're interested in.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch04-pgfId-1139">Solution</A></H3><PCLASS="para">Use <CODECLASS="literal">grep</CODE><ACLASS="indexterm"NAME="ch04-idx-1000006738-0"></A> to apply a condition to all elements in the list and return only those for which the condition was true:</P><PRECLASS="programlisting">@MATCHING = grep { TEST ($_) } @LIST;</PRE></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch04-pgfId-1147">Discussion</A></H3><PCLASS="para">This could also be accomplished with a <CODECLASS="literal">foreach</CODE> loop:</P><PRECLASS="programlisting">@matching = ();foreach (@list) { push(@matching, $_) if TEST ($_);}</PRE><PCLASS="para">The Perl <CODECLASS="literal">grep</CODE> function is shorthand for all that looping and mucking about. It's not really like the Unix <CODECLASS="literal">grep</CODE> command; it doesn't have options to return line numbers or to negate the test, and it isn't limited to regular-expression tests. For example, to filter out just the large numbers from an array or to find out which keys in a hash have very large values:</P><PRECLASS="programlisting">@bigs = grep { $_ > 1_000_000 } @nums;@pigs = grep { $users{$_} > 1e7 } keys %users;</PRE><PCLASS="para">Here's something that sets <CODECLASS="literal">@matching</CODE> to lines from the <EMCLASS="emphasis">who</EM> command that start with <CODECLASS="literal">"gnat</CODE> <CODECLASS="literal">"</CODE>:</P><PRECLASS="programlisting">@matching = grep { /^gnat / } `who`;</PRE><PCLASS="para">Here's another example:</P><PRECLASS="programlisting">@engineers = grep { $_->position() eq 'Engineer' } @employees;</PRE><PCLASS="para">It extracts only those objects from the array <CODECLASS="literal">@employees</CODE> whose <CODECLASS="literal">position()</CODE> method returns the string <CODECLASS="literal">Engineer</CODE>.</P><PCLASS="para">You could have even more complex tests in a <CODECLASS="literal">grep</CODE>:</P><PRECLASS="programlisting">@secondary_assistance = grep { $_->income >= 26_000 && $_->income < 30_000 } @applicants;</PRE><PCLASS="para">But at that point you may decide it would be more legible to write a proper loop instead.</P></DIV><DIVCLASS="sect2"><H3CLASS="sect2"><ACLASS="title"NAME="ch04-pgfId-1187">See Also</A></H3><PCLASS="para">The "For Loops," "Foreach Loops," and "Loop Control" sections of <ICLASS="filename">perlsyn </I>(1) and <ACLASS="olink"HREF="../prog/ch02_01.htm">Chapter 2</A> of <ACLASS="citetitle"HREF="../prog/index.htm"TITLE="Programming Perl"><CITECLASS="citetitle">Programming Perl</CITE></A>; the <CODECLASS="literal">grep</CODE> function in <ICLASS="filename">perlfunc </I>(1) and <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>; your system's <ICLASS="filename">who </I>(1) manpage, if it exists; <ACLASS="xref"HREF="ch04_13.htm"TITLE="Finding the First List Element That Passes a Test">Recipe 4.12</A> <ACLASS="indexterm"NAME="ch04-idx-1000006734-0"></A><ACLASS="indexterm"NAME="ch04-idx-1000006734-1"></A><ACLASS="indexterm"NAME="ch04-idx-1000006734-2"></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="ch04_13.htm"TITLE="4.12. 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