sub mark_attacks {
   my ($r,$c,$array) = @_;
   my ($u,$d) = ($r,$r);

   # to be removed - just for visualization during debugging
   $array->[$r]->[$c] = 'Q';

   for ($c+1..$size-1) {   
      $array->[$r]->[$_] = 0;
      $array->[--$u]->[$_] = 0 if $u > 0;
      $array->[++$d]->[$_] = 0 if $d < $size-1;
   }

}