How To Debug

Author: Benjamin Qi

General tips for identifying errors within your program.

KACTLthings to try in an ICPC contest

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


This section is not complete.

Any help would be appreciated! Just submit a Pull Request on Github.

todo - find particularly bad example of repetition

  • Don't repeat yourself!
  • Your code should be readable (to yourself at the very least). Variables should be given meaningful names.
  • 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=1N=1) / special cases?
  • Any undefined behavior?
    • Includes (but not limited to) uninitialized variables, array out of bounds, shifting int by 32\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 NN and MM, ii and jj, etc.?
  • Shadowed or unused variables?
  • 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.

Module Progress:

Join the USACO Forum!

Stuck on a problem, or don't understand a module? Join the USACO Forum and get help from other competitive programmers!

Give Us Feedback on How To Debug!

Leave suggestions for us by making a post on the USACO Forum! Ex: Unclear sections, mislabeled problems, etc.