Sorting algorithms are an essential chapter in undergraduate computer science education. Due to their easy to explain nature and fairly straight-forward analysis, this set of algorithms offers a convenient introduction to the methods and techniques of theoretical computer science and algorithm analysis.

This source archive presents my own demo program for sortings algorithms, called "The Sound of Sorting", which both visualizes the algorithms internals and their operations, and generates sound effects from the values being compared.

The demo is implemented using the cross-platform toolkits wxWidgets and SDL, can be executed on Windows, Linux and Mac, and runs in real time.

There are many resources on the Internet about sorting algorithms, among them are Wikipedia, animated sorting algorithms by David R. Martin and various Java applets by many college or university staff.

The current source package and some binaries can be downloaded from http://panthema.net/2013/sound-of-sorting/

Some YouTube videos are also linked on above webpage.

The program and code is published under the GNU General Public License v3 (GPL), which can also be found in the file COPYING. A few of the sorting algorithms' implementations were written by other authors and may have different licenses.

The Sound of Sorting demo program is very intuitive to use. It contains many sorting algorithms, which are selectable via the list box on the right. For the quick sort variants the pivot rule can be selected separately.

When pressing "Run", the algorithm is started on selected input. "Step" will stop a running algorithm, or start a new one, and halt it after performing one operation. With "Reset" a running algorithm is stopped, and a new random input is created. When "Sound" is activated, the program will generate sound effects on the default audio output. The "Speed" slider changes the algorithms execution speed by adding a delay for each array access.

Due to the 1ms resolution of timers on Windows, the speed scale is smaller. The Linux version has a higher time resoltion < 1ms!

The algorithm's visualization contains mostly white bars representing the value of the array position corresponding to the x-axis. When the algorithm gets or sets an array item, the white bar runs red for one algorithmic step. A swap operation is represented by two bars turning red and their values being exchanged. Some algorithms specially colorize bars which represent indexes, pointers or other information about the internal mechanisms of the algorithm.

Both value comparisons and array accesses are counted and shown in real time. The comparison counter includes ternary comparisons as just one operation. Due to algorithms often using extra memory or local variables, the array access counter highly depends on the actual algorithm implementation.

The generated sound effects depend on the values being compared. Only comparisons yield sound output (except for in radix/bucket sort)! The length of each comparison's sound effect can be modified using the "Sound Sustain" slider. The frequency of the sound is calculated from the compared values. The sound wave itself is triangular and modulated with an ADSR envelope. This yields the "8-bit game tune" feeling. An item value is scaled (with double precision) to the frequency range 120 Hz - 1,212 Hz, which is large but not too high to be annoying.

The demo program uses the wxWidgets toolkit for cross-platform dialogs and painting. For sound output, the audio component of the cross-platform SDL library is used.

All sources resides in src/. The main window's GUI functions are grouped in WMain.h/cpp. The sorting visualization, including the instrumented array class and painting methods are in WSortView.h/cpp.

SortAlgo.h/cpp contains all sorting algorithms. These were mainly modified to operate on a WSortView object, which exposes most of the usual array operators such as operator[], and many special functions to create nicer visualizations. Most notable among these, are a special swap() function and mark() to colorize bars. There is also watch() to do live tracking of indexes stored as local variables (use this with care)!

Comparison counting and sound effects are signaled by the operators of ArrayItem, which is the item class of the instrumented array WSortView. As such, all comparisons of the sorting algorithms will be intercepted by this class.

On each comparison, the values are used to generate sound. All sound generating methods are in SortSound.cpp. The main class here is an Oscillator, which generates an enveloped triangular waveform of a specific frequency. Oscillators are mixed together for the output sound. The output volume is scaled automatically depending on the number of oscillators active.

For (somewhat) rapid development with wxWidgets, the wxGlade dialog generator tool is use. The public version of the Sound of Sorting contains no recording facilities. If you want to contribute a sorting algorithm, please notify me.

Written 2013-05-22 by Timo Bingmann