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Performance is not a good reason to avoid commenting One recurring attribute of the rolling wave of technology discussed in Section 4.3 is interpreted environments in which commenting imposes a measurable performance penalty. In the 1980s, comments in Basic programs on the original IBM PC slowed programs. In the 1990s, .asp pages did the same thing. In the 2000s, JavaScript code and other code that needs to be sent across network connections presents a similar problem.
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In each of these cases, the ultimate solution has not been to avoid commenting. It has been to create a release version of the code that s different from the development version. This is typically accomplished by running the code through a tool that strips out comments as part of the build process.
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Capers Jones points out that studies at IBM found that a commenting density of one comment roughly every ten statements was the density at which clarity seemed to peak. Fewer comments made the code hard to understand. More comments also reduced code understandability (Jones 2000). This kind of research can be abused, and projects sometimes adopt a standard such as programs must have one comment at least every five lines. This standard addresses the symptom of programmers not writing clear code, but it doesn t address the cause. If you use the Pseudocode Programming Process effectively, you ll probably end up with a comment for every few lines of code. The number of comments,
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32. Self-Documenting Code
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however, will be a side effect of the process itself. Rather than focusing on the number of comments, focus on whether each comment is efficient. If the comments describe why the code was written and meet the other criteria established in this chapter, you ll have enough comments.
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32.5 Commenting Techniques
Commenting is amenable to several different techniques depending on the level to which the comments apply: program, file, routine, paragraph, or individual line.
Commenting Individual Lines
In good code, the need to comment individual lines of code is rare. Here are two possible reasons a line of code would need a comment: The single line is complicated enough to need an explanation. The single line once had an error and you want a record of the error.
Here are some guidelines for commenting a line of code:
Avoid self-indulgent comments Many years ago, I heard the story of a maintenance programmer who was called out of bed to fix a malfunctioning program. The program s author had left the company and couldn t be reached. The maintenance programmer hadn t worked on the program before, and after examining the documentation carefully, he found only one comment. It looked like this:
MOV AX, 723h ; R. I. P. L. V. B.
After working with the program through the night and puzzling over the comment, the programmer made a successful patch and went home to bed. Months later, he met the program s author at a conference and found out that the comment stood for Rest in peace, Ludwig van Beethoven. Beethoven died in 1827 (decimal), which is 723 (hexadecimal). The fact that 723h was needed in that spot had nothing to do with the comment. Aaarrrrghhhhh!
Endline Comments and Their Problems
Endline comments are comments that appear at the ends of lines of code. Here s an example:
Visual Basic Example of Endline Comments
For employeeId = 1 To employeeCount GetBonus( employeeId, employeeType, bonusAmount )
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32. Self-Documenting Code
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If employeeType = EmployeeType_Manager Then PayManagerBonus( employeeId, bonusAmount ) ' pay full amount Else If employeeType = EmployeeType_Programmer Then If bonusAmount >= MANAGER_APPROVAL_LEVEL Then PayProgrammerBonus( employeeId, StdAmt() ) ' pay std. amount Else PayProgrammerBonus( employeeId, bonusAmount ) ' pay full amount End If End If End If Next
Although useful in some circumstances, endline comments pose several problems. The comments have to be aligned to the right of the code so that they don t interfere with the visual structure of the code. If you don t align them neatly, they ll make your listing look like it s been through the washing machine. Endline comments tend to be hard to format. If you use many of them, it takes time to align them. Such time is not spent learning more about the code; it s dedicated solely to the tedious task of pressing the spacebar or the tab key. Endline comments are also hard to maintain. If the code on any line containing an endline comment grows, it bumps the comment farther out, and all the other endline comments will have to be bumped out to match. Styles that are hard to maintain aren t maintained, and the commenting deteriorates under modification rather than improving. Endline comments also tend to be cryptic. The right side of the line usually doesn t offer much room, and the desire to keep the comment on one line means that the comment must be short. Work then goes into making the line as short as possible instead of as clear as possible. The comment usually ends up as cryptic as possible.
Avoid endline comments on single lines In addition to their practical problems, endline comments pose several conceptual problems. Here s an example of a set of endline comments:
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