OCR A-Level Computer Science Computer systems (01), June 2025: Question 6

✅ Acceptable Points (Any 4, 1 mark each)

• Reduces development time or debugging time (or saves developer effort).
• Reduced programming knowledge needed (complex subroutines are pre-written by experts).
• Libraries will already be tested and error-free.
• Libraries are often optimised for performance / speed.
• Libraries are designed to work across multiple platforms / cross-platform.
• Provides consistency between applications (e.g. standard file dialogues, UI consistency).
• Reduces file sizes if dynamic libraries (DLLs) are shared.

❌ Common Errors & Mark Scheme Traps

• Vague time references: Writing just "saves time" is Do Not Accept (DNA). You must specify that it saves development or debugging time, not runtime!
• Conflating runtime speed: Merely saying "it runs faster" without context is insufficient; mention that library routines are pre-optimised.

💡 The 4 Stages in Order

1. A: Lexical Analysis: Strips whitespace & comments, turns lexemes into tokens, creates symbol table.
2. B: Syntax Analysis: Checks grammar rules, reports syntax errors, produces Abstract Syntax Tree (AST).
3. C: Code Generation: Translates intermediate code/AST into machine code or intermediate byte code.
4. D: Code Optimisation: Removes redundant code and replaces inefficient instructions to boost runtime performance and reduce footprint.

🧠 Exam Technique

Notice the chronological sequence of compilation: Lexical → Syntax → Generation → Optimisation. In matching questions, anchor keywords first: "tokens" = Lexical; "tree / grammar" = Syntax; "efficiency" = Optimisation; "low-level" = Generation.

✅ (b)(ii) Two Benefits of an Interpreter (Max 2)

• Easier / faster to debug and maintain (stops immediately when an error is encountered).
• Can test code line by line without needing full compilation each time.
• Code will still run up to the first error encountered.
• Source code is portable across different machine architectures (as long as an interpreter exists).

✅ (b)(iii) Two Benefits of a Compiler (Max 2)

• Protects intellectual property / source code: No need to distribute the human-readable source code.
• No interpreter / runtime environment required on the end user's machine.
• Faster execution / run-time once compiled (already machine code; no line-by-line translation).
• Produces a standalone executable file (.exe).
• Code is optimised during compilation.

❌ Common Errors

• Claiming compilers "run faster": Compilers do not run faster—the compiled code executes faster at runtime. Be precise!
• Forgetting that interpreted code must ship with source code, exposing proprietary logic to clients.

🧠 Top Mark Differentiator

Always state when the advantage occurs: Interpreters hold advantages during development and debugging; Compilers hold advantages at deployment and runtime execution.

📐 Step-by-Step Mark Breakdown (3 Marks)

1. Mark 1: Correct syntax: INSERT INTO TblSoftware (...) . Note: Field names are optional if all fields are supplied in order, but strongly recommended.
2. Mark 2: Keyword VALUES correctly placed.
3. Mark 3: Correct data items supplied: ('DCC07', 'DCCCast', 'Sound', 89.99) .

❌ Common Errors & Mark Scheme Conditions

• Case-sensitivity: Table names and field names must preserve the correct casing as given in the specification: TblSoftware , ProductID .
• Quotation marks: Text/string values ( 'DCC07' , 'DCCCast' , 'Sound' ) must have quotes. Price can be numeric or quoted as per the mark scheme allowance ( 89.99 or '89.99' ).
• Order constraint: If the order of clauses ( INSERT INTO , VALUES , contents) is incorrect, mark is capped at a maximum of 2.

📐 Step-by-Step Mark Breakdown (5 Marks)

1. Mark 1: Outer query projection: SELECT Name FROM TblSoftware
2. Mark 2: Comparison clause: WHERE Price < ( ... )
3. Mark 3: Subquery projection: SELECT Price
4. Mark 4: Subquery source: FROM TblSoftware
5. Mark 5: Subquery criteria: WHERE Name = 'DCCDraw' (or == )

❌ Common Traps to Avoid

• Hardcoding the price: Writing WHERE Price < 119.99 scores 0 for the nested logic. The question explicitly mandates a nested SELECT statement.
• Missing parentheses: Subqueries in SQL must always be wrapped inside parentheses: (SELECT ...) . Missing brackets may be penalised or rely on Follow-Through (FT).
• Selecting the wrong column in subquery: Writing SELECT * or SELECT Name inside the subquery makes a numeric comparison with Price < invalid! The subquery must return the scalar Price .