This student contacted me off-forum, so I won't make a full reply here, but briefly:

• Anything from the lectures is non-normative - it's the project spec you should be working off
• What sort of key the Vigenere functions are required to accept depends entirely on the values of low_range and high_range. If a caller ever supplies a key containing a character outside those ranges - they've violated the function's preconditions, and the function can do anything it likes.
• The purpose of a test is to invoke some function or component, and to ask: when invoked, is the function's behaviour in accordance with its specification? It never makes sense to write a test that violates the preconditions, so we (the markers) promise never to do so.
• That said: we will try to call your functions in the most unexpected and unpleasant ways we can, while remaining within the letter of the preconditions.

cheers,

Arran

ANONYMOUS wrote:

Based on the project spec and the most recent lecture, the vigenere function can accept a key that contains both numbers and letters. What is the expected logic to process such a key, and could you please explain how this would allowed with the pre condition: \pre key must not be an empty string, and all characters in key must be within the range from range_low to range_high (inclusive).

To me this seems contradictory, if the range passed in is A-Z. My thinking for possible logic is that if a character or number is encountered in the string that is not within the range, then do not apply a shift using that character/number, instead iterate through the key string to find the next occurance of a suitable character.