ch04_16.htm

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

CLASS="programlisting"
>@sorted = map  { $_->[1] }
          sort { $a->[0] <=> $b->[0] }
          map  { [ length $_, $_ ] }
          @strings;</PRE
><P
CLASS="para"
>The operations now appear in reverse order. When you meet a <CODE
CLASS="literal"
>map</CODE
>-<CODE
CLASS="literal"
>sort</CODE
>-<CODE
CLASS="literal"
>map</CODE
>, you should read it from the bottom up to determine the function:</P
><DL
CLASS="variablelist"
><DT
CLASS="term"
><CODE
CLASS="literal"
>@strings</CODE
> </DT
><DD
CLASS="listitem"
><P
CLASS="para"
>The last part is the data to be sorted. Here it's just an array, but later we'll see that this can be a subroutine or even backticks. Anything that returns a list to be sorted is fair game.</P
></DD
><DT
CLASS="term"
><CODE
CLASS="literal"
>map</CODE
> </DT
><DD
CLASS="listitem"
><P
CLASS="para"
>The <CODE
CLASS="literal"
>map</CODE
> closest to the bottom builds the temporary list of anonymous arrays. This list contains the precomputed fields (<CODE
CLASS="literal"
>length</CODE
> <CODE
CLASS="literal"
>$_ </CODE
>) and also records the original element (<CODE
CLASS="literal"
>$_ </CODE
>) by storing them both in an anonymous array. Look at this <CODE
CLASS="literal"
>map</CODE
> line to find out how the fields are computed.</P
></DD
><DT
CLASS="term"
><CODE
CLASS="literal"
>sort</CODE
> </DT
><DD
CLASS="listitem"
><P
CLASS="para"
>The <CODE
CLASS="literal"
>sort</CODE
> line sorts the list of anonymous arrays by comparing the precomputed fields. It won't tell you much, other than whether the list is sorted in ascending or descending order.</P
></DD
><DT
CLASS="term"
><CODE
CLASS="literal"
>map</CODE
> </DT
><DD
CLASS="listitem"
><P
CLASS="para"
>The <CODE
CLASS="literal"
>map</CODE
> at the top of the statement turns the sorted list of anonymous arrays back into a list of the sorted original elements. It will generally be the same for every <CODE
CLASS="literal"
>map</CODE
>-<CODE
CLASS="literal"
>sort</CODE
>-<CODE
CLASS="literal"
>map</CODE
>.</P
></DD
></DL
><P
CLASS="para"
>Here's a more complicated example, which sorts by the first number that appears on each line in <CODE
CLASS="literal"
>@fields</CODE
>:</P
><PRE
CLASS="programlisting"
>@temp = map { [ /(\d+)/, $_ ] } @fields;
@sorted_temp = sort { $a-&gt;[0] &lt;=&gt; $b-&gt;[0] } @temp;
@sorted_fields = map { $_-&gt;[1] } @sorted_temp;</PRE
><P
CLASS="para"
>The regular expression mumbo-jumbo in the first line extracts the first number from the line being processed by <CODE
CLASS="literal"
>map</CODE
>. We use the regular expression <CODE
CLASS="literal"
>/(\d+)/</CODE
> in a list context to extract the number.</P
><P
CLASS="para"
>We can remove the temporary arrays in that code, giving us:</P
><PRE
CLASS="programlisting"
>@sorted_fields = map  { $_-&gt;[1] }
                 sort { $a-&gt;[0] &lt;=&gt; $b-&gt;[0] }
                 map  { [ /(\d+)/, $_ ] }
                 @fields;</PRE
><P
CLASS="para"
>This final example compactly sorts colon-separated data, as from Unix's <EM
CLASS="emphasis"
>passwd</EM
> file. It sorts the file numerically by fourth field (group id), then numerically by the third field (user id), and then alphabetically by the first field (user name).</P
><PRE
CLASS="programlisting"
>print map  { $_-&gt;[0] }             # whole line
      sort {
              $a-&gt;[1] &lt;=&gt; $b-&gt;[1]  # gid
                      ||
              $a-&gt;[2] &lt;=&gt; $b-&gt;[2]  # uid
                      ||
              $a-&gt;[3] cmp $b-&gt;[3]  # login
      }
      map  { [ $_, (split /:/)[3,2,0] ] }
      `cat /etc/passwd`;</PRE
><P
CLASS="para"
>This compact, <CODE
CLASS="literal"
>map</CODE
>-<CODE
CLASS="literal"
>sort</CODE
>-<CODE
CLASS="literal"
>map</CODE
> technique is more reminiscent of the functional world of Lisp and Scheme programming than Perl's normal C and <EM
CLASS="emphasis"
>awk</EM
> heritage. Because it was first pointed out by Randal <A
CLASS="indexterm"
NAME="ch04-idx-1000006804-0"
></A
>Schwartz, this black art is often referred to as the <I
CLASS="firstterm"
>Schwartzian Transform</I
>.</P
></DIV
><DIV
CLASS="sect2"
><H3
CLASS="sect2"
><A
CLASS="title"
NAME="ch04-pgfId-1569"
>See Also</A
></H3
><P
CLASS="para"
>The <CODE
CLASS="literal"
>sort</CODE
> function in <I
CLASS="filename"
>perlfunc </I
>(1) and <A
CLASS="olink"
HREF="../prog/ch03_01.htm"
>Chapter 3</A
> of <A
CLASS="citetitle"
HREF="../prog/index.htm"
TITLE="Programming Perl"
><CITE
CLASS="citetitle"
>Programming Perl</CITE
></A
>; the <CODE
CLASS="literal"
>cmp</CODE
> and <CODE
CLASS="literal"
>&lt;=&gt;</CODE
> operators in <I
CLASS="filename"
>perlop </I
>(1) and <A
CLASS="olink"
HREF="../prog/ch02_01.htm"
>Chapter 2</A
> of <A
CLASS="citetitle"
HREF="../prog/index.htm"
TITLE="Programming Perl"
><CITE
CLASS="citetitle"
>Programming Perl</CITE
></A
>; <A
CLASS="xref"
HREF="ch04_15.htm"
TITLE="Sorting an Array Numerically"
>Recipe 4.14</A
><A
CLASS="indexterm"
NAME="ch04-idx-1000006760-0"
></A
><A
CLASS="indexterm"
NAME="ch04-idx-1000006760-1"
></A
><A
CLASS="indexterm"
NAME="ch04-idx-1000006760-2"
></A
><A
CLASS="indexterm"
NAME="ch04-idx-1000006760-3"
></A
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