Teaching Students to Comment Part 1

Computer Science Teacher
Computer Science Teacher - Thoughts and Information from Alfred Thompson

Teaching Students to Comment Part 1

  • Comments 2

Getting students (actually just about any programmer) to write comments is an uphill battle. I tried to convince my students of the value of comments by something from my own experience.

I had written a graphics program back in college many years ago. A few years later (OK about 20) I decided to write the same program again. This time I was programming in Visual Basic. FORTRAN seems not to be so easily available any more. I remembered most of the program and knew I could figure the rest out as I went alone. The program was supposed to draw geometric shapes but when I ran it lines went everywhere but where they were supposed to go. I stared at the image on the screen and realized that I’d had a similar problem when I had written the original program. What I could not remember was how I’d fixed it.

Believe it or not I still had a copy of the original code. It was on punch cards (yeah, I go back that far) and I was able to find it in a box that my wife thought I had thrown out. Fortunately I had written comments for the code. I quickly found the problem. I was passing angle information in degrees rather than in radians. A comment explaining the parameter for a function call made this clear. A quick change and the program was working just great.

That’s a good story for justifying commenting code that is non-obvious or to explain something that was not easy to understand. It’s also a good example of why interfaces to methods, modules, classes and the like should be commented. But we all know that those kinds of comments are still too rare.

I recently read a blog entry by Justin Creasy, CTO of Immerge Technologies on the subject of commenting. I recommend Justin’s version but the short version is that his team writes to comments before the code. In short they build the outline of the program using comments during the architecture and design phase of development. This has several advantages. One is that it aids the planning process. Another is that the comments are already there when the code is written. The trick of course is keeping to comments updated as the program changes.

  • Before commenting, I would recommend making sure students use spaces and making sure students write readible code. Self-documenting code does not get out of date as quickly as comments.

    I am not saying don't comment but make sure they do the other things first. When they start to comment the rule is that comments should be at a different level of abstraction from the code they describe.

    A good way to introduce effective comments is to have students start by writing pseudocode and then write the real code while maintaining the pseudocode as comments. This works very well in my experience for programmers who are strong on lingual skills but don't have the traditional skillset or are weak in that area.
    It also stresses that comments are at a different level of abstraction.

    Hopefully, some of the tools that tie design documents more firmly to code (traceability) will also help make sure code is well documented though this is not the same as commenting. Unfortunately, the cases which demand commenting are unlikely to be found by programming students. These include large programs (which really need design work too) or low level code like Assembly or CIL.
  • Over at Teaching Students to Comment Part 1, AlfredTh blogs about the importance of comments and the...
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