Which line should I fix?

When you encounter a bug, the first step is to pinpoint the problematic line. Start by reproducing the error and examining the stack trace to find the exact file and line number. If the stack trace is unclear, add temporary logging statements before and after suspect lines to narrow down the culprit. Once you isolate the line, analyze the logic: check for null references, incorrect indices, or logic errors. Consider edge cases that might trigger the failure. After identifying the root cause, modify the line to correct the logic—this could involve adding a guard clause, adjusting a calculation, or correcting a data type. Finally, run the relevant unit tests or re‑execute the scenario to ensure the fix resolves the issue without introducing new bugs.