The Shape of Code

I spend a lot of time looking at software engineering data, asking, what is the story here?

In a previous post I suggested that the distribution of the number of functions defined to have a given number of parameters, might be a signature of developer beliefs about the relative cost of parameter passing vs accessing globals.

Looking at the data that Iran Rodrigues Gonzaga Junior made available (good man), as part of his thesis Empirical Studies on Fine-Grained Feature Dependencies, I saw it contained information about the number of parameters in a function definition and whether functions accessed a global (Gonzaga’s research question is in another direction; I am always repurposing data).

Are functions that access globals, defined with fewer parameters, compared to those that do not contain any such access? The plot below shows a count of the number of functions defined to have a given number of parameters, for four systems written in C; the solid lines are functions that did not access globals, the dashed lines are functions that accessed globals (code+data).

Over all 50 projects measured, functions that don’t access globals are defined, on average, to have an extra 0.7 parameters (the fitted Poisson regression models are better than a poke in the eye {i.e., the distribution is not really Poisson}, it’s more informative to look at the plotted data).

There is a lot of variation between projects (I picked these four because they were the larger projects and showed variation in behaviors). While the shape of the distributions varies a lot, there is always a noticeable difference in the mean.

Is this difference between projects a difference in developer beliefs, a difference in application requirements, a difference in developer coding habits (and parameter usage is a side effect; are there really that many getters and setters)?

I was hoping for a simple answer, and could not find one. Since I am writing a book and not researching individual issues in detail, it’s time to move on.

Ideas welcome.