language agnostic - 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 the minimum amount of CC", but I'm not sure I really understand what it is.

On the NDepend Website, I saw an explanation that basically says "The number of decisions in a method. Each if, for, && etc. adds +1 to the CC "score"). Is that really it? If yes, why is this bad? I can see that one might want to keep the number of if-statements fairly low to keep the code easy to understand, but is this really everything to it?

Or is there some deeper concept to it?

Answer 1

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.