A-LevelComputer ScienceSystem software9618/33 · May/June 2025 · Q96 Marks

Several syntax diagrams are shown. [Figure depicting 'upper', 'digit', and 'passcode' syntax diagrams]

(a)State why JJ90 is not a valid passcode for the given syntax diagrams.[1]
(b)Complete the Backus-Naur Form (BNF) for <upper> and <passcode>. <upper> ::= <passcode> ::=[3]
(c)A character can be an upper, a lower or a digit. The rules for passcode have been changed so that the third character may also be selected from upper and the final character may be repeated one or more times. Complete the syntax diagram for passcode to show these changes.[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2025). The mark or mark type is shown in brackets.

[1] The second character must come from either lower or digit. It can't be upper.
[3] Max 3
[1] One mark
MP1 <upper> ::= J|K|L|V|X|Z
Either: fully written out answer
MP2 Any two correct options for <passcode>
MP3 Remaining two options correct for <passcode>
Example answer <passcode> ::= <upper><lower><lower><digit>| <upper><lower><digit><digit>| <upper><digit><digit><digit>| <upper><digit><lower><digit>
Or: answer with interim expression
MP4 <passcode> ::= <upper><middle><middle><digit>
MP5 <middle> ::= <lower>|<digit>
[2] One mark per mark point
MP1 Box for upper in correct place with correct connections
MP2 Correct repetition arrow for final digit

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