As another example of unhelpful behavior (similar to No Pause on Elsif in Debugger), here's some code I was modifying today:

###############################
# try to sort fields as numbers first, then as strings
# keep going until there's a difference

sub smart_sort
{
  my @a = split /[\s_]+/, lc($a);
  my @b = split /[\s_]+/, lc($b);
  
  for my $i ( 0..(@a < @b ? $#a : $#b) )
  {
    if ( $a[$i] =~ $NUMBER_ONLY_REGEX  # line 214
     and $b[$i] =~ $NUMBER_ONLY_REGEX
     and $a[$i] <=> $b[$i] )
    {
      return $a[$i] <=> $b[$i];
    }
    elsif ( ( my @ai = /^(\d+)(.+)/ )  # error/bug is here
        and ( my @bi = /^(\d+)(.+)/ ) )
    {
      if ( $ai[0] <=> $bi[0] )
      {
        return $ai[0] <=> $bi[0];
      }
      elsif ( $ai[1] cmp $bi[1] )
      {
        return $ai[1] cmp $bi[1];
      }
    }  
    elsif( $a[$i] cmp $b[$i] )
    {
      return $a[$i] cmp $b[$i];
    }
  }
  return $a cmp $b;
}
[download]

gives the error:

Use of uninitialized value in pattern match (m//) at /user/sgriffit/perl/mergen.pl line 214.

Line 214 is only the start of the if/elsif/... chain. Not too helpful. Glad there's only 1 place where a match is happening.

Now I realize there are several things there that could be changed to improve the error reporting. For instance, since most of the blocks return, the elsifs could be ifs instead.

I don't really want to spend time thinking "If this code has a bug, what's the best way of writing it so it's easy to spot?" Perhaps I'm flogging a dead horse...

Here's the cleaned up version, with more comments:

sub smart_sort
{
  my @a = lc($a) =~ /(\d+|[a-z]+)/ig; # better split on fields
  my @b = lc($b) =~ /(\d+|[a-z]+)/ig;

  for my $i ( 0..(@a < @b ? $#a : $#b) )
  {
    # try numbers first
    if ( $a[$i] =~ $NUMBER_ONLY_REGEX 
     and $b[$i] =~ $NUMBER_ONLY_REGEX
     and $a[$i] <=> $b[$i] )
    {
      return $a[$i] <=> $b[$i];
    }
    
    # fallback to string compare
    if( $a[$i] cmp $b[$i] )        # no "elsif"
    {
      return $a[$i] cmp $b[$i];
    }
  }
  
  # can't decide by fields, decide by whole string
  return $a cmp $b;
}
[download]

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