What is cyclomatic complexity and why is it important?

What is cyclomatic complexity and why is it important?

Cyclomatic complexity is a computer science metric (measurement) developed by Thomas McCabe used to generally measure the complexity of a program. It directly measures the number of linearly independent paths through a programs source code.

The concept, although not the method, is somewhat similar to that of general text complexity
measured by the Flesch-Kincaid Readability Test.

Cyclomatic complexity is computed using a graph that describes the control flow of the program. The nodes of the graph correspond to the commands of a program. A directed edge connects two nodes, if the second command might be executed immediately after the first command. By definition,

CC = E - N + P
where
CC = cyclomatic complexity
E = the number of edges of the graph
N = the number of nodes of the graph
P = the number of connected components