that is probably the best solution, so let's see some real (untested) code! 
use POSIX qw(floor);

my @lat = ...;
my @long = ...;

my $radius = 0.01;
my $radius2 = $radius ** 2;

my @flat = map { floor($_/$radius) } @lat;
my @flong = map { floor($_/$radius) } @long;

my %cell;
for my $i (0..$#lat) {
  push @{$cell{$flat[$i], $flong[$i]}}, $i
}

for my $i (0..$#lat) {
  my @near;
  for my $dflat(-1, 0, 1) {
    my $flat = $flat[$i] + $dflat;
    for my $dflong(-1, 0, 1) {
      my $flong = $flong[$i] + $dflong;
      if (my $cell = $cell{$flat, $flong}) {
        for my $j (@$cell) {
          if ((($lat[$j] - $lat[$i]) ** 2 + ($long[$j] - $long[$i]) **
+ 2) < $radius) {
            push @near, $j
          }
        }
      }
    }
  }
  @near = sort @near;
  print "points near $i: @near\n";
}

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In reply to Re^2: fastest way to compare numerical strings? by salva
in thread fastest way to compare numerical strings? by Anonymous Monk

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