Follow Sets Construction (Python)

2015-12-22T10:49:21-08:00

Python recipe 579140 by Narayana Chikkam (cfg, first, follow, nullable). Revision 2.

Language Processors: FOLLOW(A) = {t | (t is a terminal and S ⇒+ α A t β) or (t is EOF and S ⇒∗ αA)}

Simple graph algorithms with a modular design (Python)

2011-04-21T13:40:32-07:00

Python recipe 577668 by jimmy2times (algorithms, breadth, depth, directed, first, graph, object, oriented, python, search, theory, undirected, visit). Revision 7.

The purpose of this recipe is to look at algorithmic graph theory from an object-oriented perspective.

A graph is built on top of a dictionary indexed by its vertices, each item being the set of neighbours of the key vertex. This ensures that iterating through the neighbours of a vertex is still efficient in sparse graphs (as with adjacency lists) while at the same time checking for adjacency is expected constant-time (as with the adjacency matrix).

Any valid class of graph must implement the interface defined by AbstractGraph.

A generic search algorithm takes as input a graph, source and target vertices and a queue. A queue must implement the methods Q.get(), Q.put() and Q.empty() in such a way to get the desired order in visiting the vertices.

Given this pattern, breadth-first and depth-first search are essentially defined by the corresponding expansion policies: the first one uses an actual FIFO queue, the second one a LIFO queue (or stack).

Popular recipes tagged "first" but not "a"

Follow Sets Construction (Python)

Simple graph algorithms with a modular design (Python)