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C'est la Z

Quick Sort over Zoom

I haven't been blogging much this year. Not sure why - probably pandemic fatigue. It's also affected my blog reading - more scanning, less deep reading. I've also been trying to spend mode time off screen learning how to paint (with,let's say mixed results :-) ) leaving less time and energy to blog. Hopefully I'll pick up on both ends as more people get vaccinated and we can get to a more normal life.

In any event, I tried a new motivation for Quicksort today so I thought I'd share.

I've approached teaching the Quicksort in a variety of ways (here, here, here, here) but never had a great lead in activity. Merge sort was easy in person since you can "Tom Sawyer" shuffling a deck of cards in class. We adapted this activity pretty well to work over Zoom this past summer in our teacher certification program. Quicksort however, haven't found a good one.

Thinking about how to leverage remote, I had an idea to try - use polls. I use Zulip for class discussions. It's like Slack but is open source, can be self hosted, you can post from email, and has much better threading. I wrote a small bot to quickly create emoji polls.

First I asked for the students to share their birtdays - just month and day in the form MMDD so if you were born on March 12th you would write 0312 in the chat.

Then, I picked one birthday at random, let's say 0515.

I then created a poll:

🐶 My birthday is earlier in the year than 0515

🐱 My birthday is later in the year than 0515

The bot tags the post with the emoji and the class can click on their choice.

We then talk about what we now know -

  1. We now know the exact location of 0515 with respect to all the dates
  2. The data set is now partially ordered (everything less than 0515 to the left, greater to the right).

I also drew out some diagrams showing what was happening

This is also one iteration of the Quicksort.

We do this again on one side and continue to discuss.

From here we finish developing the algorithm and we're off to the races.

As usual, I won't know for a while how well this worked or if it did at all but the students seemed to be engaged and so far I think it helped get a good feel for how the algorithm worked.

Using Zulip or other chat polls in this case made the motivating activity easy and seamless. I'm not sure that this particular activity would be as easy to implement in person unless you used some technology like clickers.

We finished off the class discussing how we should test our implementations. They're going to test the sorts on a variety of data sets including sets where all values are the same or are already sorted. Given our naive pivot selection this should lead to some pretty bad runtimes – fodder to motivate the next class.

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