OCR GCSE Computer Science Computational thinking, algorithms and programming (02), June 2025: Question 2

📐 Step-by-Step Flowchart Trace

Trace for input value 3:

1. Outputs "OK" (Count = 1) → x = 3 - 2 = 1 → Is 1 < 0? FALSE (loops back).
2. Outputs "OK" (Count = 2) → x = 1 - 2 = -1 → Is -1 < 0? TRUE → Ends.

Trace for input value 10:

1. Outputs "OK" (1) → x = 8 → 8 < 0 is FALSE
2. Outputs "OK" (2) → x = 6 → 6 < 0 is FALSE
3. Outputs "OK" (3) → x = 4 → 4 < 0 is FALSE
4. Outputs "OK" (4) → x = 2 → 2 < 0 is FALSE
5. Outputs "OK" (5) → x = 0 → 0 < 0 is FALSE (0 is not less than 0!)
6. Outputs "OK" (6) → x = -2 → -2 < 0 is TRUE → Ends.

✅ Completed Table

❌ Common Errors to Avoid

• Assuming 0 terminates the loop: The condition is x < 0 , not x ≤ 0 . When x = 0 , the decision is FALSE, leading to one final iteration where "OK" is printed a 6th time.
• Off-by-one errors: Notice that the output is displayed before x is decremented and checked. Always trace strictly in order.

🧠 Exam Technique: Quick Scratch Trace

Always draw a mini trace table in the blank space on the exam paper:

✅ Model Answers (Choose Any Two)

• Option 1: FOR loop
  Description: A count-controlled loop that repeats a specified / predetermined number of times.
• Option 2: WHILE loop
  Description: A condition-controlled loop that repeats while a condition is true (or until it becomes false), testing the condition before the code runs.
• Option 3: DO UNTIL / REPEAT UNTIL loop
  Description: A condition-controlled loop that repeats until a condition becomes true, testing the condition after the code runs (always executes at least once).

❌ Common Errors & Examiner Warnings

• Writing "repeats infinitely": This is incorrect; loops only run forever if there is a logic bug where the exit condition is never met.
• Mismatched conditions: Stating that a WHILE loop "repeats until a condition is true" loses the description mark (a WHILE loop runs while true, stopping when false).
• Duplicate descriptions: If you write both WHILE and DO UNTIL , simply writing "condition-controlled" for both will only earn the description mark once unless you explain their difference (e.g. pre-check vs post-check).

✅ Model Answer

1. Sequence
2. Selection (or branching)

💡 The Three Golden Building Blocks

All structured computer programs are built from just three basic constructs:

• Sequence: Executing instructions one after another in order.
• Selection: Making decisions based on conditions ( IF / ELSE , SWITCH ).
• Iteration: Repeating blocks of code ( FOR , WHILE ).

✅ Model Answers (Any Two Pairs)

• Tool 1: Code / Text Editor
  Description: Allows the programmer to enter, write, and modify the high-level program code directly.
• Tool 2: Translator / Compiler / Interpreter
  Description: Converts the high-level source code into low-level machine code / binary so the processor can execute it.
• Tool 3: Debugger / Error Diagnostics
  Description: Identifies and highlights errors (syntax/runtime), displays their line numbers, and suggests fixes.
• Tool 4: Run-time Environment
  Description: Allows the program to be run and tested directly within the IDE, displaying output to the user.

🧠 Exam Technique: Clear Descriptions

Your description must provide more detail than just repeating the tool's name:

• Editor: ✗ "Edits code" → ✓ "Allows code to be typed, formatted, and changed."
• Translator: ✗ "Translates code" → ✓ "Converts high-level code to machine code."
• Debugger: ✗ "Debugs errors" → ✓ "Locates bugs and provides line numbers and error details."