Adjacent Looping

neversaint has asked for the wisdom of the Perl Monks concerning the following question:

Dear Masters,
I have a simple array which I want to print the adjacent element together. But my script below doesn't seem to do the job:

# Number of elements in the array are always EVEN
my @arr = qw (J K L M N O);
my %done;

OUT:
foreach my $elem(@arr) {

   IN:
   foreach my $elem2(@arr) {
   
      next IN if ($elem eq $elem2);
      next IN if ($done{$elem2." - ". $elem});

      print "$elem - $elem2\n";
      
      $done{$elem." - ".$elem2} = 1;

    }
 
}
[download]

It somehow doesn't print the desired result like this:

J - K
L - M
N - O
[download]

What's wrong with my script above?

---
neversaint and everlastingly indebted.......

Comment on Adjacent Looping Select or Download Code

Replies are listed 'Best First'.
Re: Adjacent Looping by moritz (Cardinal) on Aug 22, 2007 at 07:16 UTC
For adjacent entries you don't have to iterate over the array twice: `use warnings; use strict; my @a = qw(J K L M N O); my %done; for (my $i = 0; $i < @a; $i += 2){ my $str = $a[$i] . ' ' . $a[$i+1]; print $str, "\n"; $done{$str} = 1; }` [download] In this case `%done` isn't really used, if you want each pair to be printed only once you can check if `$done{$str}` is already set. Perl 6 in German -- Difficult Sudoku	[reply] [d/l] [select]
Re: Adjacent Looping by GrandFather (Saint) on Aug 22, 2007 at 07:36 UTC
If you don't mind clobbering the array you could just: `use warnings; use strict; my @arr = qw (J K L M N O); print join (' - ', splice @arr, 0, 2), "\n" while @arr;` [download] Prints: `J - K L - M N - O` [download] or if you need the unique check: `use warnings; use strict; my @arr = qw (J K L M N O J K); my %done; while (@arr) { my $str = join ' - ', splice @arr, 0, 2; next if exists $done{$str}; $done{$str}++; print "$str\n"; }` [download] DWIM is Perl's answer to Gödel	[reply] [d/l] [select]
Re: Adjacent Looping by hangon (Deacon) on Aug 22, 2007 at 07:33 UTC
Too many loops. Try this: `my @arr = qw (J K L M N O); my $i = 0; while($i < scalar @arr){ print "$arr[$i] - $arr[$i+1]\n"; $i += 2; }` [download]	[reply] [d/l]
Re: Adjacent Looping by ikegami (Patriarch) on Aug 22, 2007 at 14:04 UTC
A more funcional approach: `use List::MoreUtils qw( pairwise ); my @arr = qw( J K L M N O ); my %done; my @pairs = grep !$done{$_}++, # Remove duplicates pairwise { "$a - $b" } @arr; print map "$_\n", @pairs;` [download] The above is basically the same as: `my @arr = qw( J K L M N O ); my %done; my @pairs; while (@arr) { my $pair = join(' - ', splice(@arr, 0, 2)); next if $done{$pair}++; push @pairs, $pair; } print "$_\n" for @pairs;` [download]	[reply] [d/l] [select]
Re: Adjacent Looping by wfsp (Abbot) on Aug 22, 2007 at 13:27 UTC
In your example the input is sorted. A hash could help: `my %hash = qw (J K L M N O); print qq{$_ => $hash{$_}\n} for sort keys %hash;` [download]	[reply] [d/l]