Advent Calendar - December 8, 2020

Wednesday, Dec 16, 2020| Tags: Perl, Raku

Advent Calendar 2020


The gift is presented by Roger Bell_West. Today he is talking about his solution to the task Sort Email Adresses of “The Weekly Challenge - 062”. This is re-produced for Advent Calendar 2020 from the original post by Roger Bell_West.


Write a script that takes a list of email addresses (one per line) and sorts them first by the domain part of the email address, and then by the part to the left of the @ (known as the mailbox).

Note that the domain is case-insensitive, while the mailbox part is case sensitive. (Some email providers choose to ignore case, but that’s another matter entirely.)

If your script is invoked with arguments, it should treat them as file names and read them in order, otherwise your script should read email addresses from standard input.

BONUS: Add a -u option which only includes unique email addresses in the output, just like sort -u.


The trick here is to construct an easily-sortable representation. This code will fail on some of the more exotic constructs possible in a user-part.

my %o;
getopts('u',\%o);
my %l;
while (<>) {
  chomp;
  my @e=split /@/,$_;
  my $k=lc($e[1]).'@'.$e[0];

That’s the sortable representation. billG@microSOFT.com becomes microsoft.com@billG – which I can feed to a standard sort to get the desired results.

If we’re being unique, delete any previous addresses with this key.

  if ($o{u}) {
    delete $l{$k};
  }

Under this key, store a list of email addresses that match it.

  push @{$l{$k}},$_;
}

Then for output just look at each key and print the addresses stored under it.

foreach my $k (sort keys %l) {
  print map {"$_\n"} @{$l{$k}};
}

In Raku it was more work to sort out the option parsing than to rewrite the code. The built-in libraries will let me have command-line options, certainly – but not, at least not trivially, combined with raw filenames. So in this first part I iterate through the command-line arguments, putting any that are the names of files into the files list, and parsing “-u” separately.

(If there exists a file called “-u”… frankly, you deserve all you get.)

my $u=0;
my @fn;
for @*ARGS -> $p {
  if ($p.IO.e) {
    push @fn,$p;
  } elsif ($p eq '-u') {
    $u=1;
  }
}
unless (@fn) {
  push @fn,'-';
}

The rest is basically the same algorithm, with syntactic tweaks for Raku.

my %l;
for @fn -> $fn {
  my $fh=open :r,$fn;
  for $fh.lines {
    .chomp;
    my @e=comb(/<-[@]>+/,$_);
    my $k=lc(@e[1]) ~ '@' ~ @e[0];
    if ($u) {
      %l{$k}:delete;
    }
    push %l{$k},$_;
  }
  close $fh;
}

for (sort keys %l) -> $k {
  my @q=%l{$k}.flat;
  for @q -> $e {
    say $e;
  }
}

If you have any suggestion then please do share with us perlweeklychallenge@yahoo.com.

Advent Calendar 2020

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