How To Debug
Author: Benjamin Qi
General tips for identifying errors within your program.
This module is based on both of these resources. I've included the content that is most relevant to USACO.
- Don't repeat yourself!
This section is not complete.
- Your code should be consistent and readable (to yourself at the very least).
- See style tips.
- Give variables meaningful names.
- Use plenty of print statements.
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 ) / special cases?
Any undefined behavior?
can result in different outputs locally vs online
not returning anything from non-
- USACO problems usually contain a note of the following form if the output format requires 64-bit rather than 32-bit integers, but it's easy to miss:
Note that the large size of integers involved in this problem may require the use of 64-bit integer data types (e.g., a
long longin C/C++)."
C++ - Compiling with additional options can help catch these.
- Confusing and , and , etc.?
- Shadowed or unused variables?
- C++ - Compile with warning options (
- Are you clearing all data structures between test cases?
- Any NaNs (ex. taking the square root of a negative number)?
- 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).
- 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 ...
- Rewrite your solution from the start.
- Make a new file rather than overwriting your previous program! It's always possible that you might introduce more bugs.
Of course, many of these are also relevant for RE / TLE.
- Have you tested all corner cases locally?
- Any undefined behavior? (see above)
- 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?
- Did you remove debug output before submitting (ex. are you printing a lot of information to
- 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.
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