Systematic Debugging

A structured approach to finding and fixing bugs.

The Debugging Loop

1. REPRODUCE → 2. ISOLATE → 3. DIAGNOSE → 4. FIX → 5. VERIFY

Never skip steps. Never guess-and-check repeatedly.

Step 1: Reproduce

• Run the exact command that fails
• Capture FULL output (stdout AND stderr)
• Note: exit code, error message, stack trace
• If intermittent, identify what changes between runs

Step 2: Isolate

• What's the MINIMAL input that triggers the error?
• Which specific line/function fails? (read the traceback bottom-up)
• Is it a compile error, runtime error, or wrong output?
• Does it fail on all inputs or specific ones?

Step 3: Diagnose

Read the error message carefully

diff <(expected) <(actual)

Add minimal instrumentation

• C:

  fprintf(stderr, "reached checkpoint %d\n", __LINE__);

• Python:

  print(f"DEBUG: {var=}", file=sys.stderr)

• Check intermediate values, not just final output
• Remove debug prints after fixing

Step 4: Fix

• Change ONE thing at a time
• If the same approach fails 3 times → completely different strategy
• Don't add workarounds — fix the root cause
• Common root causes:
  • Off-by-one errors (loop bounds, array indexing)
  • Type mismatches (int vs float, signed vs unsigned)
  • Encoding issues (UTF-8 vs bytes)
  • Path errors (relative vs absolute, missing trailing slash)
  • Race conditions (file not written yet, process not started)

Step 5: Verify

• Run the same command that failed before
• Test with multiple inputs, not just the one that was failing
• Check edge cases: empty input, single element, very large input
• Run any existing test suite

Common Failure Patterns

"It compiles but gives wrong output"

1. Print all intermediate values
2. Compare with a known-correct reference implementation
3. Check: integer overflow, floating point precision, endianness

"It works on small input but times out on large"

1. Check algorithm complexity — O(n²) on 1M items = timeout
2. Profile: which loop/function takes the most time?
3. Restructure: hash maps, sorting, streaming

"It works locally but fails in the test"

1. Check: absolute vs relative paths
2. Check: different working directory
3. Check: different input format than expected
4. Read the test script to understand what it actually checks