More thoughts on debugging

Yesterday's post onerrors led to some interesting discussion over on Facebook. Specifically on teaching students to use a debugger.

My contention is that while it's easy to demonstrate a debugger it's hard to get student buy in. In my CS0, we start with Thonny which has a great integrated debugger. In my CS1 I also show and use gdb. Regardless of my approach the subset of students who actively end up using the debugger has remained both constant and small.

A few students will use the debugger in the very early stages to help understand basic constructs but fewer will go back to the tool later when a real error occurs. One of the difficulties probably relates to the fact that it's hard to present a natural, organic example of using a debugger. You either have to force an appropriate error situation or you have to wait and hope for the right situation to present itself at a good time.

Truth be told, I don't use debuggers all that much. Mostly, I'll use them in C or C++ to get in the ballpark of where my error is - usually via postmortem debugging.

In any event, while the students should know of the existence and power of debugging tools the real skill is how to think about your craft.

Whether one uses a debugger or print statements the questions are really:

• what problem is the student trying to solve - do they understand it?
• How are they solving it?
• What specifically are they telling their program to do?

That last one is important. I always tell my students that "Computers are really bad at doing what you want them to do but they're really good at what you tell them to do!"

Here are some errors that I've seen stump kids time and time again:

A typical intro to recursion problem is the Fibonacci numbers. The recursive definition gives:

\[ f(n) = \left\{ \begin{array}{ll} 1 & if x \leq 2 \\ f(n-1) + f(n-2) & otherwise \end{array} \right. \]

This translates directly to code

int fib(int n){
      if (n<2)
          return 1;
      else
          return (n-1) + (n-2);
  }

See the problem?

I see this at least a couple of times a year.

The student writes (n-1) + (n-2) instead of f(n-1) + f(n-2) . In fact, when they read off their code they're more likely than not to say it correctly - the brain autocorrects. It's just when you read an essay with a glaring mistake over and over. A debugger might bring attention to this but then so can careful code tracing. This is a time that rubber duck debugging can also be effective. Another good strategy would be to just take a break and come back with fresh eyes.

Another one is when students are starting to work with loops and arrays. I've seen all sorts of variations on the error so will only share one. Suppose the students are looking for the largest in a data set. I might see code like this:

smallestIndex = 0;
  for (i=1;i<=listSize;i++){
      if (a[i] < a[i-1]){
          smallestIndex = i;
      }
  }

Once again, the student will usually describe the algorithm correctly but their code is comparing adjacent elements not each element against a "source of truth." The computer is doing what they told it to do not what they want it to do.

A debugger would probably be more helpful here but again, it all comes down to careful analysis of the code.

In the previous two examples, a debugger could help but the real requirement is careful code tracing be it via tool, print statements or by hand. Here's an example where a debugger can really shed some light for a beginner. This time in C++:

#include <iostream>

  int main(){}
  {
    std::cout << "Why does my program crash before it even gets to this??????";

    int a;
    int b;
    // more code here

    // something here that results in a segfault and a crashed program
  
    return 0;
  }

What causes the error isn't important. What happens is that students start to add print statements and ultimately get to the point where a print is the very first line of the program and it still outputs nothing.

The culprit here is output buffering. cout is the same as System.out.print - no newline. The output buffers and won't actually appear until you send a \n . If your program crashes before then you won't see anything. This is why we have cerr in C++, stderr for C and System.err in Java. The error streams are unbuffered and will output immediately. Not as good but still effective would be adding a newline to the original print statement.

In any event, this is a case where stepping through with a debugger can be truly enlightening.

Now with all of this, the real key is to carefully consider what you're trying to do and how you end up communicating it in program. I'd love to find a way to convince more students of the utility of a good toolset that includes debuggers but I'd also just love to be more effective in getting them to carefully read and analyze their code.