SE 2040 Debugging Notes

Ideas for how to debug program crashes and logic errors:

• Try running your code from the command prompt (outside the IDE). Eclipse, CLion, and other IDEs sometimes write zeros to memory and store other information that can make it less likely your code will fail.
• Once you find an error, keep re-using that set of values until it's fixed. Randomly using other inputs can mean debugging many issues at once.
• Ensure you are reading your input from a file, not the keyboard. It's too easy to mistype data; reading your input from a file allows you to focus on what's wrong with the code instead of typing.
• Try using smaller inputs and see if you get the same error. You can simply cut the input in half, check for the error. If you get the error, cut the remaining input in half. If not, add back half of the remaining input. You should be able to identify a minimal input that fails fairly quickly.
• Remember that standard output (such as cout << x;) is buffered: if the program crashes, the last part of the output may not get written. Flush the output by calling cout.flush() in key places, or write debug messages to standard error; for instance,

          cerr << x;
  

  Standard error is not buffered.
• Add checks (assertions) for function/method preconditions; for example:

          #include <cassert>
          double average(const double nums[], int size) {
             assert(size > 0);  // OR: if ( size <= 0 ) throw "bad size";
             ...
          }
  

  Once you find the condition that fails, start working backwards through the execution sequence to find the first incorrect computed result. Then figure out what to fix.

  Note the order: find the first place your code goes wrong and then find the error. You are more likely to be struck by lightening than to fix code without knowing the cause.

• Set breakpoints and check your calculations mid-stream. If you are not sure how to do the calculation by hand, then find out.
• To see the backtrace before an assertion fails, add an if statement:

          if ( size <= 0 )
             assert(size > 0);
  

  Place a breakpoint on the assertion so you can see what lead to it.
• Draw pictures of your data to help visualize the algorithm.
• Rewrite the code you are trying to debug.
• Explain your code to your rubber ducky.