File handling, exceptions and the ethics of computing

Serial, sequential and random access files

A serial file stores records in the order they arrive — fine for a transaction log, useless for finding one record quickly. A sequential file keeps records in key order, so it can be searched more efficiently and merged. A random access (direct) file computes a record's position from its key using a hashing algorithm, giving near-instant access — the method used when a single record must be retrieved on demand.

Hashing and collisions

A hashing algorithm converts a key into an address. Two keys can hash to the same address — a collision — resolved by placing the record in the next free slot (open addressing) or by chaining records at that address. A good hash spreads records evenly; a poor one produces long chains and destroys the speed advantage.

File operations

Open for read, write or append; read or write a record; close. Forgetting to close a file risks losing data still sitting in the buffer, and opening for write rather than append silently destroys the existing contents.

Exception handling

A run-time error such as a missing file, a division by zero or invalid input should be caught, not allowed to crash the program: TRY … EXCEPT … FINALLY. The FINALLY block runs whether or not an error occurred, which is where files get closed. Catching every exception and doing nothing is worse than not catching it — the program then fails silently with corrupt data.

Ethics and ownership

Copyright protects the expression of a program; licences (proprietary, free, open source) define what users may do. Professional codes of conduct require competence, honesty about limitations, protection of client data, and refusing work beyond one's ability. The exam also asks about the ethics of automated decisions — who is accountable when software trained on biased data refuses someone a loan.