How To Debug

This module is based on both of these resources. I've included the content that is most relevant to USACO.

Pre-Submit

• Don't repeat yourself!

public static void main(String[] args) {
	//swap first item with third
	int first = arr[0];
	int last = arr[2];
	arr[0] = last;
	arr[2] = first;

	//swap second item with fourth
	int second = arr[1];
	int secondLast = arr[3];
	arr[1] = secondLast;
	arr[3] = second;
}

private void swap(int[] arr, int i, int j) {
	int first = arr[i];
	arr[i] = arr[j];
	arr[j] = first;
}

public static void main(String[] args) {
	swap(arr, 0, 2);
	swap(arr, 1, 3);
}

• Your code should be readable (to yourself at the very least). Variables should be given meaningful names.
  • See style tips.
• Use plenty of print statements.

Wrong Answer

• Read the full problem statement again.
• Read your code again.
• Have you understood the problem correctly?
• Is your output format correct?
  • Did you remove debug output before submitting?
• Add some assertions, resubmit.
• Are you sure your algorithm works?
  • Go through the algorithm for a simple case / write some testcases to run your algorithm on.
• Do you handle all corner cases (such as $N=1$) / special cases?
• Any undefined behavior?
  • Includes (but not limited to) uninitialized variables, array out of bounds, shifting int by $\ge 32$ bits, not returning anything from non-void functions.
  • Can result in different outputs locally / on the USACO server.
  • Compiling with additional options can help catch these.
  • C++ - can use array::at or vector::at instead of [] to throw exceptions when the index is out of bounds.
• Any overflows or NaNs?
• Confusing $N$ and $M$, $i$ and $j$, etc.?
• Shadowed or unused variables?
  • Compile with warning options.
• Write a test case generator and a (simpler) slow solution and compare the outputs
  • See stress testing.
  • You can compare against a model solution (if available).
• Rewrite your solution from the start.
  • Don't delete your previous program. Make a new file! It's always possible that you might introduce more bugs.
• Use a debugger (if you're using an IDE).
  • Set breakpoints to stop your program at certain lines.
  • Step through lines one by one and watch how the values of your variables change.
  • Probably slower than well-placed print statements ...

Of course, many of these are also relevant for RE / TLE.

Runtime Error

• Have you tested all corner cases locally?
• Any undefined behavior?
• Are you reading or writing outside the range of any vector?
• Any assertions that might fail?
• Any possible division by 0? (mod 0 for example)
• Any possible infinite recursion?
• Invalidated pointers or iterators?
• Are you using too much memory?

Time Limit Exceeded

• Do you have any possible infinite loops?
• What is the complexity of your algorithm?
• Avoid vector, map. (use arrays / unordered_map)
• Did you remove debug output before submitting (ex. are you printing a lot of information to stderr)?

Before Posting on the Forum

• Try everything above first.
  • If you've found a small counter-test, you should be able to figure out why your code isn't working on your own via print statements.
• Provide a link to the problem (and the relevant module, if applicable).
• Include what you've attempted so far.
• If you have code, post it (formatted as described here).
  • Add comments.
  • If you know which part doesn't work, mention it.