AlgorithmAnimations: Displaying how algorithms compute

Graph algorithms are easy to visualize and indeed there already exists a variety of packages and programs to animate the dynamics when solving problems from graph theory. Still, and somewhat surprisingly, it can be difficult to understand the ideas behind the algorithm from the dynamic display alone.

CATBox consists of a software system for animating graph algorithms and a course book which we developed simultaneously. The software system presents both the algorithm and the graph and puts the user always in control of the actual code that is executed. He or she can set breakpoints, proceed in single steps and trace into subroutines. The graph, and additional auxiliary graphs like residual networks, are displayed and provide visual feedback. The course book, intended for readers at advanced undergraduate or graduate level, introduces the ideas and discusses the mathematical background necessary for understanding and verifying the correctness of the algorithms and their complexity. Computer exercises and examples replace the usual static pictures of algorithm dynamics.

For this volume we have chosen solely algorithms for classical problems from combinatorial optimization, such as minimum spanning trees, shortest paths, maximum flows, minimum cost flows, weighted and unweighted matchings both for bipartite and non-bipartite graphs.

We consider non-bipartite weighted matching, in particular in the geometrical case, a highlight of combinatorial optimization. In order to enable the reader to fully enjoy the beauty of the primal-dual solution algorithm for weighted matching, we present all mathematical material not only from the point of view of graph theory, but also with an emphasis on linear programming and its duality. This yields insightful and aesthetically pleasing pictures for matchings, but also for minimum spanning trees.

For further information see the main website at or contact Alexander Schliep ( This project is connected to the following projects: Gato.


Members: Alexander Schliep, Carolyn Keenan, Matthew R. Mitsui, Scott Merkling, Alice Paul. Collaborators: Winfried Hochstättler (Fernuniversität Hagen, Institut für Mathematik).


Hochstättler et al.. CATBox – An Interactive Course in Combinatorial Optimization. Springer, 2010.

Schliep et al.. Developing Gato and CATBox with Python: Teaching graph algorithms through visualization and experimentation. In Multimedia Tools for Communicating Mathematics, Springer-Verlag, 291–310, 2002.