Debugging code in a competitive setting, Part 1

Writing code in a competitive setting is an especially tiring and frustrating task, for a multitude of reasons. The rush of wanting to code fast prevents one from applying good coding standards and leads to trivial errors. Also, there is no time to set up a testing framework etc. How do you prevent (or at least find and correct) any errors in the code you write?

What I personally do is to use a combination of gdb and a custom debug message function.

The first part of this series of posts will explain the function

First, I always put the following code at the start of my programs:

#ifdef DEBUGIT
  #define DEBUG(X) cerr << ">>> DEBUG(" << __LINE__ << ") " << #X << " = " << X << endl
#else
  #define DEBUG(X) (void)0
#endif

Now, whenever there is a need to check the value of a variable (or expression), all that is left to be done is to call the DEBUG() function like so:

DEBUG(x); // debugging variable x
DEBUG((a<<3)+2); // debugging an expression

The way that the "function" (or macro, to be precise) is defined, whenever DEBUGIT is defined at compilation, debug statements are shown, and otherwise, they are just skipped.

Hence, whenever you want to look at the debug statements, use g++ -DDEBUGIT to compile instead of just g++.

Here's an example program and its output:

#include <iostream>

using namespace std;

#ifdef DEBUGIT
  #define DEBUG(X) cerr << ">>> DEBUG(" << __LINE__ << ") " << #X << " = " << X << endl
#else
  #define DEBUG(X) (void)0
#endif

int main() {
  int x = 5, y = 10;
  DEBUG(x);
  DEBUG(y);
  ++x += y++;
  DEBUG(y+1);
  cout << x << endl;
}

Output:

>>> DEBUG(13) x = 5
>>> DEBUG(14) y = 10
>>> DEBUG(16) y+1 = 12
16

The advantage with the DEBUGIT technique is that you can directly submit code (without any modifications) and the grader will not run the DEBUG statements at all.

I have added the following line to my .bashrc file:

alias d++='g++ -DDEBUGIT'

Hence, I can directly run d++ for compiling with the debug statements.

In my next post, I will explain how to use gdb to further make life easier.