Editing Cyclomatic number (section)

===Generalizations to directed graphs===
The [[cycle rank]] is an invariant of [[directed graph]]s that measures the level of nesting of cycles in the graph. It has a more complicated definition than cyclomatic number (closely related to the definition of [[tree-depth]] for undirected graphs) and is more difficult to compute. Another problem for directed graphs related to the cyclomatic number is the minimum [[feedback arc set]], the smallest set of edges whose removal breaks all directed cycles. Both cycle rank and the minimum feedback arc set are [[NP-hard]] to compute.

It is also possible to compute a simpler invariant of directed graphs by ignoring the directions of the edges and computing the circuit rank of the underlying undirected graph. This principle forms the basis of the definition of [[cyclomatic complexity]], a software metric for estimating how complicated a piece of computer code is.