Impact of function size on number of reported faults

Are longer functions more likely to contain more coding mistakes than shorter functions?

Well, yes. Longer functions contain more code, and the more code developers write the more mistakes they are likely to make.

But wait, the evidence shows that most reported faults occur in short functions.

This is true, at least in Java. It is also true that most of a Java program’s code appears in short methods (in C 50% of the code is contained in functions containing 114 or fewer lines, while in Java 50% of code is contained in methods containing 4 or fewer lines). It is to be expected that most reported faults appear in short functions. The plot below shows, left: the percentage of code contained in functions/methods containing a given number of lines, and right: the cumulative percentage of lines contained in functions/methods containing less than a given number of lines (code+data):

left: the percentage of code contained in functions/methods containing a given number of lines, and right: the cumulative percentage of lines contained in functions/methods containing less than a given number of lines.

Does percentage of program source really explain all those reported faults in short methods/functions? Or are shorter functions more likely to contain more coding mistakes per line of code, than longer functions?

Reported faults per line of code is often referred to as: defect density.

If defect density was independent of function length, the plot of reported faults against function length (in lines of code) would be horizontal; red line below. If every function contained the same number of reported faults, the plotted line would have the form of the blue line below.

Number of reported faults in C++ classes (not methods) containing a given number of lines.

Two things need to occur for a fault to be experienced. A mistake has to appear in the code, and the code has to be executed with the ‘right’ input values.

Code that is never executed will never result in any fault reports.

In a function containing 100 lines of executable source code, say, 30 lines are rarely executed, they will not contribute as much to the final total number of reported faults as the other 70 lines.

How does the average percentage of executed LOC, in a function, vary with its length? I have been rummaging around looking for data to help answer this question, but so far without any luck (the llvm code coverage report is over all tests, rather than per test case). Pointers to such data very welcome.

Statement execution is controlled by if-statements, and around 17% of C source statements are if-statements. For functions containing between 1 and 10 executable statements, the percentage that don’t contain an if-statement is expected to be, respectively: 83, 69, 57, 47, 39, 33, 27, 23, 19, 16. Statements contained in shorter functions are more likely to be executed, providing more opportunities for any mistakes they contain to be triggered, generating a fault experience.

Longer functions contain more dependencies between the statements within the body, than shorter functions (I don’t have any data showing how much more). Dependencies create opportunities for making mistakes (there is data showing dependencies between files and classes is a source of mistakes).

The previous analysis makes a large assumption, that the mistake generating a fault experience is contained in one function. This is true for 70% of reported faults (in AspectJ).

What is the distribution of reported faults against function/method size? I don’t have this data (pointers to such data very welcome).

The plot below shows number of reported faults in C++ classes (not methods) containing a given number of lines (from a paper by Koru, Eman and Mathew; code+data):

Number of reported faults in C++ classes (not methods) containing a given number of lines.

It’s tempting to think that those three curved lines are each classes containing the same number of methods.

What is the conclusion? There is one good reason why shorter functions should have more reported faults, and another good’ish reason why longer functions should have more reported faults. Perhaps length is not important. We need more data before an answer is possible.