Recipe 10.18 Program: Sorting Your Mail

The program in Example 10-1 sorts a mailbox by subject by reading input a paragraph at a time, looking for one with a "From" at the start of a line. When it finds one, it searches for the subject, strips it of any "Re: " marks, and stores its lowercased version in the @sub array. Meanwhile, the messages themselves are stored in a corresponding @msgs array. The $msgno variable keeps track of the message number.

Example 10-1. bysub1

  #!/usr/bin/perl    # bysub1 - simple sort by subject   my(@msgs, @sub);   my $msgno = -1;   $/ = '';                    # paragraph reads   while (<>) {       if (/^From/m) {           /^Subject:\s*(?:Re:\s*)*(.*)/mi;           $sub[++$msgno] = lc($1) || '';       }       $msgs[$msgno] .= $_;   }    for my $i (sort { $sub[$a] cmp $sub[$b] || $a <=> $b } (0 .. $#msgs)) {       print $msgs[$i];   }

That sort is only sorting array indices. If the subjects are the same, cmp returns 0, so the second part of the || is taken, which compares the message numbers in the order they originally appeared.

If sort were fed a list like (0,1,2,3), that list would get sorted into a different permutation, perhaps (2,1,3,0). We iterate across them with a for loop to print out each message.

Example 10-2 shows how an awk programmer might code this program, using the -00 switch to read paragraphs instead of lines.

Example 10-2. bysub2

  #!/usr/bin/perl -n00   # bysub2 - awkish sort-by-subject   INIT { $msgno = -1 }   $sub[++$msgno] = (/^Subject:\s*(?:Re:\s*)*(.*)/mi)[0] if /^From/m;   $msg[$msgno] .= $_;   END { print @msg[ sort { $sub[$a] cmp $sub[$b] || $a <=> $b } (0 .. $#msg) ] }

Perl programmers have used parallel arrays like this since Perl 1. Keeping each message in a hash is a more elegant solution, though. We'll sort on each field in the hash, by making an anonymous hash as described in Chapter 11.

Example 10-3 is a program similar in spirit to Example 10-1 and Example 10-2.

Example 10-3. bysub3

  #!/usr/bin/perl -00   # bysub3 - sort by subject using hash records   use strict;   my @msgs = ( );   while (<>) {       push @msgs, {           SUBJECT => /^Subject:\s*(?:Re:\s*)*(.*)/mi,           NUMBER  => scalar @msgs,   # which msgno this is           TEXT    => '',       } if /^From/m;       $msgs[-1]{TEXT} .= $_;   }       for my $msg (sort {                                $a->{SUBJECT} cmp $b->{SUBJECT}                                           ||                            $a->{NUMBER}  <=> $b->{NUMBER}                      } @msgs            )   {       print $msg->{TEXT};   }

Once you have real hashes, adding further sorting criteria is simple. A common way to sort a folder is subject major, date minor order. The hard part is figuring out how to parse and compare dates. Date::Manip does this, returning a string you can compare; however, the datesort program in Example 10-4, which uses Date::Manip, runs more than 10 times slower than the previous one. Parsing dates in unpredictable formats is extremely slow.

Example 10-4. datesort

  #!/usr/bin/perl -00   # datesort - sort mbox by subject then date   use strict;   use Date::Manip;   my @msgs = ( );   while (<>) {       next unless /^From/m;       my $date = '';       if (/^Date:\s*(.*)/m) {           ($date = $1) =~ s/\s+\(.*//;  # library hates (MST)           $date = ParseDate($date);       }        push @msgs, {           SUBJECT => /^Subject:\s*(?:Re:\s*)*(.*)/mi,           DATE    => $date,           NUMBER  => scalar @msgs,           TEXT    => '',       };    } continue {       $msgs[-1]{TEXT} .= $_;   }      for my $msg (sort {                                $a->{SUBJECT} cmp $b->{SUBJECT}                                           ||                            $a->{DATE}    cmp $b->{DATE}                                           ||                            $a->{NUMBER}  <=> $b->{NUMBER}                         } @msgs            )   {       print $msg->{TEXT};   }

Example 10-4 is written to draw attention to the continue block. When a loop's end is reached, either because it fell through to that point or got there from a next, the whole continue block is executed. It corresponds to the third portion of a three-part for loop, except that the continue block isn't restricted to an expression. It's a full block, with separate statements.

10.18.1 See Also

The sort function in Chapter 29 of Programming Perl and in perlfunc(1); the discussion of the $/ ($RS, $INPUT_RECORD_SEPARATOR) variable in Chapter 28 of Programming Perl, in perlvar(1), and in the Introduction to Chapter 8; Recipe 3.7; Recipe 4.16; Recipe 5.10; Recipe 11.9