Thursday, November 24, 2011

The Khan academy on insertion sort

The Khan academy puts 8-minute youtube video lectures (and a few exercises) on the internet to teach all sorts of topics in an online fashion. I just realized that there are some lectures on computer science, and I have watched a few with great interest. I was mesmerized by the lectures on insertion sort - I taught that in class just a few weeks ago. My thinking was: could I borrow some ideas from that renowned teaching style?

The instructor is careful, thorough, and incredibly patient. He follows a well-trodden path to communicate, but does not announce signposts explicitly as a general paradigm for problem-solving.

Signpost 1. Define the problem: he does that in a very concrete way, using an example. For sorting, there is only one possible ambiguity in the problem definition: what about duplicates? He deals with that without explicitly drawing the student's attention to it, just by checking his program with an example that has duplicates.

Although I would also use an example, I am more explicit and state the problem definition in terms of a pair (input, output). Why? Because I don't want students to only learn about sorting: I also want to inculcate the notion of computational problem, that they will see later in many more advanced Theory courses.

Signpost 2. Explain the algorithm by executing it slowly on an example.

I would also do that if it was the beginning of the semester, but in addition to what he wrote on his electronic blackboard, I would write down comparisons as I perform them: instead of just saying "we compare 3 to 7", I would write down "3<7" on the side. Why? Because I will at some later point want to analyze the runtime, and for that, I might want to focus on the number of comparisons: highlighting the comparisons is a good preliminary perspective that will come in handy later.

One minor thing I really did not like was that he kept calling elements "the zeroth element" for the first element, "the first element" for the second element, and so on. I am resolutely opposed to molding the English language after conventions used in programming languages.

One major thing is that the instructor explains how the algorithm works, but not how one would come up with that algorithm in the first place. In his place, I might try to ask a leading question to get my unknown audience to come up with the algorithm. Why? Because I try to teach, not so much specific algorithms, but how to design algorithms. For example: "One way to go about it could be called "sort as you go": scan the entire list, and, at every step of the scan, try to make sure that everything you have scanned so far is sorted." Then I can show intermediate steps in the execution - the desired state of the list after processing the first item, the first two items, the first three items, etc. And only then would I discuss how to fill in by processing each additional item scanned. That's a modular approach, and hopefully after I have stated the "sort as you go" idea, many students can figure out the rest of the algorithm. It would, maybe, stimulate the students' creativity a little bit more.

Signpost 3. Write code for insertion sort in Python.

Before that, in his place I would first have written some high-level pseudo-code. Why? As a preparation, I suppose, for more difficult problems, settings when it is imperative to lay out the algorithm before starting to code.

The way in which he explained his code was slow and very intuitive. However, I noticed that he wrote and explained the code in a very linear fashion, line by line. That's not the way we do it: we always go for a modular presentation, starting with incomplete code that contains some "..." in places, and filling in the details during a second pass. For insertion sort, the difference is not huge, but that bothered me slightly.

He then ran it on an example in the end, as a way to implicitly suggest the need for testing. I would have been more explicit about that need.

Signpost 4.Execute the code step-by-step on a small example.

I skipped much of that presentation, but he did exactly what I do in class after I have presented an algorithm, when I am not 100 percent sure that the students understood it. Fortunately it's automated in our scheme (Racket) environment: one of the options when running a procedure in our programming environment is to do it not all at once but step by step, with highlights showing what parts of the code are being executed and what the current values of interest are.

Signpost 5. Do some small code optimization.

I really liked that short explanation. He replaced "While A: if B: ... else break" by "While (A and not B) ...". That was quite natural, and his last few words indicated that it was a general pattern. I had never thought it through quite so explicitly, so I was happy to learn that.

No video about runtime, but maybe that'll come some day.

My main reservation was that he taught one particular algorithm, but did not take that opportunity to teach how to design algorithms. One could present insertion sort as an example of problem-solving by reducing to an easier subproblem (namely, inserting a single element in an already sorted list). Making that explicit introduces the idea of reductions.

Overall, if it is a way to teach insertion sort for the sake of teaching insertion sort, it is great. If it is a way to introduce students to algorithmic design through the example of insertion sort as a stepping stone, it is less impressive. Obviously, he is targeting a lower level than my students, but I would still have liked him to embed some deeper level of understanding in his presentation (even if it's done in an implicit manner), like sowing seeds of knowledge, just to prepare students, so that, when they meet more advanced material, that material will seem natural to them. They would be ready without even knowing that they have been prepared for it!

In short: very pedagogical, very engaging, superbly clear, but there is a little bit of a lack of vision.


  1. I found the dancing part on quicksort pretty "refreshing" as well:

    PS: nice blog Professor Mathieu!

  2. Nice dance! No time to show it in class, but I can at least point students to it.

  3. Love this blog, keep up the great work wish you all the best.

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  4. Wow!... students at brown are really lucky to have a professor who is as thoughtful about teaching as you are.

  5. Thanks Anonymous...but it's a lot easier to criticize other people's teaching than to do it right myself... thanks anyway!


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