Saturday, September 25, 2010

Bored in Vegas

As usual, my advice on Vegas is, don't bother. This trip, I've gotten sick (something flu-like), been fed far too much, sucked in a lot of signature smoke, been too hot outside and too cold inside, and now my flight home is indefinitely delayed due to mechanical problems (I'm now in the airport).

Unfortunately, I'm coming back in 2 weeks for the TopCoder Open. Oh well.

Sunday, September 05, 2010

Visualising sorting algorithms

This is another one of my rare technical posts, as opposed to news of which countries I've been visiting.

If you're in computer science, you've probably seen an animation of sorting algorithms, maybe heard a rendition, or seen a visual representation. I have, somewhat by accident, discovered a different way to visualise a sorting algorithm: plot points for memory accesses, with address on the X axis and time (counted by accesses) on the Y axis, and different colours for reads and writes. It produces some rather pretty pictures. Note that these are not to scale relative to each other - the Y axis has been compressed to fit the entire sort into a fixed height.

Ye olde bubblesort. Some of the patterns are an optical illusion due to aliasing, but the green spikes are a feature of the algorithm.

Insertion sort - the version optimized for mostly sorted content. Although the data is random, you can see that in many cases it reduces the search distance.Shellsort, clearly showing the phases.

Selection sort:Heapsort: the solid lines at the top are the heap-building phase, while the rest shows the extraction. Note the very slight slope to the bottom-right line: as the heap gets smaller, the heap extraction gets faster, but only as O(log N).
Divide-and-conquer algorithms have a pretty fractal nature. This is quicksort - the perturbations in the fractal indicate the random selection of pivots (it just picks the middle, rather than median-of-3). Mergesort: this diagram is twice as wide as the others because it uses temporary storage on the right.