Object Mentor Blog: How Big Should a Function Be?

How Big Should a Function Be?

Uncle Bob — Mon, 29 Oct 2007 22:25:37 -0500

In the ‘80s we had a rule that a function should be no longer than a screenful. Few, if any, of us followed that rule. Our functions were typically much longer than the 20 lines that would fit on a VT100.

Nowadays, on a 17” laptop screen with the font cranked way down low, you can get 60, 80, maybe even 100 lines of code on the screen. Does the same rule still apply? Should functions be no longer than a screenful?

Of course the idea is ridiculous. Functions should be small. Very small. A well written function might be 4, 5, perhaps 8 lines long; but no longer.

There are exceptions of course. Some functions will have long switch/case statements (We’ll talk about those later), or long array initializations, or some other mindless structure that simply takes space to fill. But the fast majority of functions are not of that kind, and should be very short. Specifically, functions that process information as part of an algorithm should be very small.

A function should have zero or one level of indentation. Nested ifs and while’s should be avoided in most cases. Moreover, the body of an if or while should be one or two lines long. Typically, the body of an if or while statement should be no more than a single function call.

The body of a try block should also be a function call; and the word try should be the first word in the function.

Each function should have a descriptive name, two or three words in length, that makes it clear what the function does. Moreover the function should have between zero and three arguments.

A source file should read like a newspaper article. Newspaper articles begin with a headline, followed by a single synoptic paragraph, followed by paragraphs full of ever increasing detail. The ideal article can be read until it gets boring. A source file should read the same way. High level functions should be followed by lower level functions that should be followed by still lower level functions. The calls should all point downwards in the source file if possible.

Of course these rules cannot be followed 100% of the time, but they are good rules nonetheless. Our goal, as programmers, is to write programs that humans can understand and maintain.

It is not good enough that a program work. A program must also be written well. As a programmer you should take pride in your work and never leave a mess under the hood. Remember, a product that works, but that has a bad internal structure is a bad product.

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In theory, this idea sounds neat. However, I object to the idea. While I certainly don’t advocate writing goliath (hercules) functions, I do believe there should be a balance; functions should neither be TOO large nor TOO small. The problem with scads of tiny functions is that you have to hunt through multiple layers of code to debug the top level function. It also makes the class way too “busy” with lots of function noise all over the place.

Granted, if code is going to be re-used, it should definitely be in its own function. However, if it’s not going to be re-used, it makes agile sense (needless complexity) to keep related code as close as possible to the rest (in a single function).

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