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for $i (0 .. $npts-1) {
   for $j ($i+1 .. $npts-1) {
      $dx = $x[$j] - $x[$i];
...
      $fy[$j] -= $f * $dy / $d;
   }
}

for $i (0 .. $npts-1) {
   $x[$i] += $dt * $fx[$i];
   $y[$i] += $dt * $fy[$i];
}