systematic-debugging

# 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 | Error type | Where to look | |-----------|---------------| | Compile error | The FIRST error (later ones are often cascading) | | Segfault | Last function in the stack trace, check array bounds and null pointers | | Python traceback | The innermost frame (bottom), but also check the middle for context | | Wrong output | Diff expected vs actual: `diff <(expected) <(actual)` | | Timeout/hang | Is it an infinite loop? Deadlock? I/O bound? Add a timer or counter | ### 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 fail...