Tuesday, August 11, 2015

Learning How Computers Represent Information

The third and fourth CS Unplugged lessons were about representing information.  We gave students abbreviated ASCII tables and they collaboratively decoded a brief text message – the TechKobwa motto “Girls love tech”.  Of course, several of them also took it upon themselves to decode my t-shirt. 

The rest of the third lesson was on representing numbers in binary, following the CS Unplugged curriculum.   I created two ‘flip boards’ as visual aids—one with binary dot-cards affixed to the board and construction paper cards that could be flipped to reveal or hide the card underneath, and the other with 0-cards affixed to the board and 1-cards that could be flipped to make any 5-digit binary number. 

Starting with the former, we gave the students binary “dot cards” and started them off counting up from 0 (all cards face down).  Most caught on very quickly.  A few required some extra help from their family leaders.  All counted from 0 to 31.   

 Then we substituted the dot board for the 0-1 board.  We also found it useful to add a strip showing the powers of 2 beneath the board positions to remind them of the position values.  Students were quickly converting from binary to decimal and back again.  Several volunteered to demonstrate their algorithms for doing so on the board.

The fourth CS Unplugged lesson started with a review and then dove into representation of images.  We passed around copies of the TechKobwa girl at different magnification levels to introduce the concepts of pixels and resolution.  These concepts were all the more real when used to explain why our Skype session from the afternoon before had been blurry. 

I simplified the “Kid Fax” from the CS Unplugged curriculum a bit so we could easily transmit images to Louise.  We first transmitted the resolution and then the run lengths for each row, inclusive of the length of the last run of white pixels, if any.  After walking through our run-length encoding of a simple 5x6 letter “a”, the students decoded the first “Kid Fax” to see the image it created.  Then they transmitted it to Louise, who was sitting with a blank graph paper outside the classroom where she couldn’t see the image, by calling out the series of numbers.  Each time they called a number, they waited for an “ack”.  When done, they were happy to see that she had exactly reconstructed the image from their transmission.

We ran out of time to decode the remaining “Kid Fax” images.  But I noticed many students working them out in their free time, curious to see what image they would find.  We sent the teachers back with materials for expanding this lesson to include representation of color images and creating their own images.

The lesson on image representation was a new one for TechKobwa this year.  Given the enthusiasm, it’s one we’ll want to expand on in future camps.  

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