Hofmator,

One more small thing. Hope you won't mind to look at it. I should add that when the very first element doesn't have its neighbour then it forms another cluster. In other words we ignore the first element only when it has neighbour within tolerance. So for example:

my @nlist     = ( 2,4,5,6,7 );
my $tolerance = 1;

We would like to have:

A => [2]
B => [4 5]
C => [4 5 6]
D => [5 6 7]
E => [6 7]
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How can I modify your code to accomodate this?

Update: I think I got it.

#my @nlist = (0,0,1,2,3,3,4,5,6,8,8,10);
#my @nlist = (0,1,2,3,4,5,6,8,10);
my @nlist = (2,4,5,6,7);

my @key_list = ('A'..'Z');
my $tolerance = 1;
my %hoa;

my %uniq;

# Check if first element has a neighbour
if ( felem_has_nbr( $nlist[0], $nlist[1],$tolerance ) == 1 ) {
    @uniq{ @nlist[ 1 .. $#nlist ] } = ();

}
else {
    @uniq{ @nlist[ 0 .. $#nlist ] } = ();
}


for my $centroid ( sort { $a <=> $b } keys %uniq ) {
    my $key = shift @key_list;
    $hoa{$key} = [ grep in_range( $centroid, $_ ), @nlist ];
}

print "$_ => [@{$hoa{$_}}]\n" for sort keys %hoa;

sub in_range {
    my ( $centroid, $testnum ) = @_;
    return abs( $centroid - $testnum ) <= $tolerance;
}


sub felem_has_nbr {

    my ( $felem, $sec_in_arr, $tl ) = @_;
    abs( $felem - $sec_in_arr ) <= $tl ? return 1 : return 0;

}
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