A-LevelComputer ScienceSystem software9618/31 · May/June 2023 · Q68 Marks

Several syntax diagrams are shown.

(a)State whether each of the following passwords is valid or invalid and give a reason for your choice. DPAD99$ Reason DAD#95 Reason ADY123? Reason[3]
(b)Complete the Backus-Naur Form (BNF) for the syntax diagrams shown. <symbol> ::= <letter> ::=[1]
(c)An identifier begins with one or more letters, followed by zero digits or one digit or more digits. Valid letters and digits are shown in the syntax diagrams on page 6. Draw a syntax diagram for an identifier.[4]

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 2023). The mark or mark type is shown in brackets.

[3] (a) One mark per correct valid/invalid and reason combination (Max 3) DPAD99$ - Valid Reason - 4/multiple letters followed by 2/multiple digits followed by a symbol. DAD#95 - Invalid Reason - The symbol comes before the digits – it should be after. ADY123? - Invalid Reason – The ? is not a valid symbol.
[1] (b) <symbol> ::= $ | % | & | @ | # <letter> ::= A | D | P | R | Y
[4] (c) One mark per mark point (Max 4) • begins with a letter • letter can repeat and digit present • digit can repeat or can be bypassed • correct structure – name, boxes and arrows (in and out). Example answers: identifier --> letter --> (loop back to letter) --> digit --> (loop back to digit or bypass digit) --> (end)

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