Oh bugger me, I had to see this at 1:00AM when I was about to go in for the night...

I'm not sure that I see a quicksort coming out of this. With the limitation on accessing the stack and all, plus no allowances for making arrays or new stacks, handling the recursion would be a nightmare. I once had to write a non-recursive implementation of quicksort as a class exercise, and it damn near drove me into the arms of the music department as a result.

Here's a bubblesort. I'm not sure how you count instructions. If I count every assignment, count each conditional as one (each of the two while's and an if), then a cond-clause as well (the else), plus one count for calls like pop, push and rotate_up, then I get somewhere around 24. That's probably not quite right, though, or your challenge would have been for a lower number.

Bubble-sort is still an O(N2) algorithm, though it is better in most cases than a selection sort. There is an early-termination form of the algorithm, but I'm already up past my bedtime. If I get a chance to look at this again before deadline, I'll see if I can adapt that. Big raspberries to the people who say that studying computer science in universities is a waste of time (and I know a lot of them at my day-job).

If I take complete leave of my senses, I'll see if I can remember that non-recursive q-sort...

--rjray

# Assume that rotate_up as defined in the original problem
# statement has been defined.

sub sordid
{
    local $len = pop(@stack);
    local $bum = $len;

    local ($x, $y, $limit);

    while ($bum > 1)
    {
        $limit = $bum;
        while (--$limit)
        {
            $x = pop(@stack);
            $y = pop(@stack);

            if ($x gt $y)
            {
                push(@stack, $x);
                push(@stack, $y);
            }
            else
            {
                push(@stack, $y);
                push(@stack, $x);
            }

            rotate_up($bum);
        }
        # At end of the $limit loop, top element is the max, and
        # top+1 to end is semi-sorted. One more rotate_up()
        # is needed before moving the floor up one notch.
        rotate_up($bum);
        $bum--;
    }

    push(@stack, $len);
}

@stack = qw(d b f a e c 6);  # <-- bottom .. top -->

print "(@stack)\n";
# Prints: (d b f a e c 6)
sordid();
print "(@stack)\n";
# Prints: (f e d c b a 6)

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In reply to Re: Algorithm Pop Quiz: Sorting by rjray
in thread Algorithm Pop Quiz: Sorting by clintp

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