RogerBW's Blog

Perl Weekly Challenge 40: mucking about with lists 29 December 2019

I’ve been doing the Perl Weekly Challenges. The latest was about list manipulation.

To print entries in multiple lists in parallel:

my @a=(
    [qw(I L O V E Y O U)],
    [qw(2 4 0 3 2 0 1 9)],
    [qw(! ? £ $ % ^ & *)],
      );

my $ix=0;
my $r=1;
while ($r) {
  $r=0;
  my @out;
  foreach my $iy (0..$#a) {

This is pretty much the only clever bit; if a list entry is undefined, substitute a space to keep the columns lined up, and if none of them is defined then end the loop.

    if (defined $a[$iy][$ix]) {
      push @out,$a[$iy][$ix];
      $r=1;
    } else {
      push @out,' ';
    }
  }
  if ($r) {
    print join(' ',@out),"\n";
  }
  $ix++;
}

Perl6 is pretty similar except that you can use @a.end instead of $#a.

For part 2 the challenge is to extract elements of a list, sort them, and return them to their places in the list.

my @list=(10, 4, 1, 8, 12, 3);
my @indices=(0,2,5);

my @s=sort {$a <=> $b} map {$list[$_]} @indices;
map {$list[$indices[$_]]=$s[$_]} (0..$#indices);

What can I say, I like map. Again, Perl6 has different syntax but basically works the same way.


  1. Posted by Peter at 11:36pm on 29 December 2019

    Perl's obscure—but occasionally inordinately useful—hash slice makes the latter code neater and faster. Something like (untested):

    my @list=(10, 4, 1, 8, 12, 3);
    my @indices=(0,2,5);
    @list{@indices} = sort @list{@indices};
    
  2. Posted by RogerBW at 11:56pm on 29 December 2019

    True. I use hash slices so rarely that they aren't really part of my idiom.

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