For me this is also a point about writing code that can easily be mentally translated to human language. It might seem simple enough, but this bug shows that

For me this is also a point about writing code that can easily be mentally translated to human language. It might seem simple enough, but this bug shows that if (Current < Withdrawal): Current -= Withdrawal requires a few mental steps in working memory that can easily get missed Whereas isValidTransaction = Current - Withdrawal >= 0 if (isValidTransaction): NewBalance = Current - Withdrawal reads like English, and is hard to get wrong in my view. In the first case, the reader is expected to do mental math (albeit easy) in order to verify correctness. Whereas in the second, they're only expected to read a logical statement and confirm that the logic checks out.