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C'est la Z

When Your Program Crashes

We all have to deal with bugs - students, teachers, hobbyists, professionals. It's rare that we'll take a concept to working program without some bumps along the way. Well, once or twice I did but that's once or twice over the course of a lifetime, those cases are the exceptions.

The errors and the bugs are inevitable so what can we do to help students deal with the invariable frustration?

Why am I thinking about this now?

For two reasons.

One is that I was recently observed - it was the usual peer observation. At Hunter, everyone except tenured full (research) professors get peer reviews and those faculty members along with other full time instructors do the reviews. I can't take it too seriously since in my experience, of all those teaching at the college level, research professors are the least qualified to productively comment on teaching.

Anyway, I was observed in a remote lesson that included a video and in the video I was doing some live coding and got hung up on a bug. I worked my way through it, got stymied, paused the video and resumed it but when I resumed, I made sure to show what I found, how I found it and how I fixed it.

The observer commented that this was very bad - I should have edited the video so that I coded the program perfectly since making an error shows that I don't know what I'm doing. Nope, my observer was 100% wrong. I made the error and left it in and that was the correct choice. Model mistakes for the students. Show them that we don't get it right the first or even the second time and that we can struggle finding errors losing huge chunks of time over a a missing semicolon.

The other reason was that a student recently submitted a program that didn't work. First, I was upset that the student hadn't asked for help. I know it's hard for many for many of them but it's always been frustrating for me when they don't use their human resources - me, classmates, and mentors. Second, when I did start working with the student to find and fix the error I saw they were looking in the wrong place.

The assignment was a recursive search - knights tour or something similar. The crash was a segmentation fault and the student was looking at the recursive logic struggling to see where they went wrong and was making no headway.

I suggested we use the debugger. Not to trace through the code but to get a stack trace. We're using C++ and if you run a program under the gdb debugger you can use the bt command to get a backtrace which is a stacktrace - where the program crashed and the sequence of function calls made to get there. You can also easily see what all the variables were all the way down.

The students had been exposed to gdb last semester but too few of them use it when appropriate.

Looking at the stack trace we quickly saw that there wasn't anything wrong with any program logic and in fact the problem was that the data file had an extra blank line at the end. We deleted that line and everything worked.

So, even getting by the asking for help how do we get students to use the tools they have available to help them?

This even goes for the compiler itself - the error messages. True many error messages are cryptic, particularly for beginners and it's hard when you get that wall of text but how do we get them to learn to read those messages.

I try two things. Read the first message since that's usually the root cause and then, even if the message doesn't make sense, when you do figure out the error you'll start to develop a mapping from weir error messages to actual problems

Of course when debugging, I, like many others fall back to, if I may quote Brian Kernighan "..careful thought, couple widh judiciously placed print statements" but even educating students on doing this effectively takes work.

I think the biggest challenge is getting them to see that it's not them - we all make errors and all struggle fixing them.

I've just mentioned a couple of things here and of course there are many other debugging and development strategies that we use as teachers and as professionals. Rubber Duck debugging, code reading, incremental development etc.. What else? I'd love to hear any strategies others have used to help students along when dealing with errors.

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