What is Cyclomatic Complexity?

Question

A term that I see every now and then is \"Cyclomatic Complexity\". Here on SO I saw some Questions about \"how to calculate the CC of Language X\" or \"How do I do Y with th

Answer 1

That's it, the idea is that a method which has a low CC has less forks, looping etc which all make a method more complex. Imagine reviewing 500,000 lines of code, with an analyzer and seeing a couple methods which have oder of magnitude higher CC. This lets you then focus on refactoring those methods for better understanding (It's also common that a high CC has a high bug rate)

Answer 2

Each decision point in a routine (loop, switch, if, etc...) essentially boils down to an if statement equivalent. For each if you have 2 codepaths that can be taken. So with the 1st branch there's 2 code paths, with the second there are 4 possible paths, with the 3rd there are 8 and so on. There are at least 2**N code paths where N is the number of branches.

This makes it difficult to understand the behavior of code and to test it when N grows beyond some small number.

Answer 3

Consider the control flow graph of your function, with an additional edge running from the exit to the entrance. The cyclomatic complexity is the maximum number of cuts we can make without separating the graph into two pieces.

For example:

function F:
    if condition1:
       ...
    else:
       ...
    if condition2:
       ...
    else:
       ...

Control Flow Graph

You can probably intuitively see why the linked graph has a cyclomatic complexity of 3.

Answer 4

Yep, that's really it. The more execution paths your code can take, the more things that must be tested, and the higher probability of error.

Answer 5

I'm not aware of a deeper concept. I believe it's generally considered in the context of a maintainability index. The more branches there are within a particular method, the more difficult it is to maintain a mental model of that method's operation (generally).

Methods with higher cyclomatic complexity are also more difficult to obtain full code coverage on in unit tests. (Thanks Mark W!)

That brings all the other aspects of maintainability in, of course. Likelihood of errors/regressions/so forth. The core concept is pretty straight-forward, though.

Answer 6

Wikipedia may be your friend on this one: Definition of cyclomatic complexity

Basically, you have to imagine your program as a control flow graph and then

The complexity is (...) defined as:

M = E − N + 2P

where

• M = cyclomatic complexity,
• E = the number of edges of the graph
• N = the number of nodes of the graph
• P = the number of connected components

CC is a concept that attempts to capture how complex your program is and how hard it is to test it in a single integer number.