Do you need to find at what indexes the minima/maxima occur, or just the values? Assuming values for the moment: 
  my @minima;
  my @maxima;
  my $prev_cmp = 0;

  for (0 .. $#array - 1) {
    my $cmp = $array[$_] <=> $array[$_+1];
    if ($cmp != $prev_cmp) {
      push @minima, $array[$_] if $cmp < 0;
      push @maxima, $array[$_] if $cmp > 0;
      # when this and next elements are ==, defer checking for
      # minima/maxima till next loop iteration
      $prev_cmp = $cmp if $cmp;
    }
  }
  if (@array) {
    push @minima, $array[-1] if $prev_cmp >= 0;
    push @maxima, $array[-1] if $prev_cmp <= 0;
  }

[download]
(untested, probably at least one bug). Then select your top k from @minima and @maxima (or replace the pushes with an insertion sort and do it on the fly.)

Update: wow, seems to actually work. Note that I assume no NaNs in the array (see perlop for the effect of NaNs on <=>) and that the endpoints are considered minima/maxima/both (slight changes would be needed to do otherwise).

Update2: fixed bug when array is empty :), moved $cmp check (with no effect on results) & added a comment

In reply to Re: finding local minima/maxima for a discrete array by ysth
in thread finding local minima/maxima for a discrete array by Anonymous Monk

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