Wednesday, November 18, 2009

CS Unplugged: Image Representation and Searching Algorithms

When I did some computer science outreach in the summer for the Girls @ Virtual Ventures camp, I promised them that if they came back for the weekend clubs in the fall, I'd make sure I came back to teach them something new. This was definitely a promise that would be easy to break, since the first semester of a PhD is pretty busy and it's not likely any of them would have remembered, but I'm glad I kept it anyway. I got a chance to try out a couple of activities from CS Unplugged for the first time.

The girls had been spending a lot of time in Photoshop and making videos and such, so I wanted to pick something that would relate back to that. The first natural choice was Image Representation. In this activity, students essentially 'fax' encoded images to each other and rebuild them in a pre-made grid. It was a lot of fun telling them all about fax machines and dial-up Internet, since none really knew about fax machines and only one had even heard of dial-up. (This made me feel a lot older than 25!) The only thing was that the activity took far longer than I expected, so be sure to leave close to an hour if you want to give them time to make the images that come with the activity in addition to creating their own.

Here's a little video about this activity by the creators of CS Unplugged (it just came out):

The second activity I decided on was Searching Algorithms. I introduced it by showing how an image management program can find all images with a certain tag even though there are thousands of pictures in its database. The activity has a set of battleships, each with a letter and number. The students give each other the number and try to find the corresponding letter. The first search has a random permutation of ships, the second search has all the numbers in sorted order, and the third one makes use of a hash code. I made a small mistake in the first type of searching (linear search) by not telling them to give each other the numbers of their battleships before searching for the letter it was at. Luckily, it didn't matter, because their search strategy didn't change as a result. Phew!

In the end, with an introduction to what computer science is (including the Pathways in Computer Science video from the University of Washington's Why Choose CSE? series), and the two activities, I spent 2.5 hours with the girls. It was a little longer than I had intended, and I could have left after the first activity, but they had the time so I stayed. :)


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