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Free A-Level Computer Science guide
Work from a precise scenario to an algorithm rather than copying a familiar loop. Use a trace and a practical IDE activity to locate a fault, explain the correction and test the rule's boundaries.
Content owner: Michael Print · Written for A-Level learners · Checked against official specifications
Algorithm design begins with what a result must achieve. State the input representation and rules, then select operations that maintain those rules. Trace the exact assignments and compare the calculated result with the scenario's success condition.
An IDE helps expose evidence: stop execution at a breakpoint, inspect variables, and step through the first place the result diverges from the requirement. Fix the cause and rerun relevant tests. Successful execution alone does not establish a correct answer.
OCR H446 · 2.2.1(a,d–e), 2.3.1(a); reinforces 2.3.1(b)
Useful foundations
By the end, you should be able to
Records are (code, stock, minimum, pack_size). When stock falls below minimum, order the smallest positive whole number of packs that restores minimum stock. Otherwise order nothing.
Output (code, units_to_order) only for records needing orders, preserving their input order. The input can be empty.
Stock and minimum are non-negative integers; pack_size is positive. Assume four correctly structured fields already parsed into integers. Production handling would also validate structure and conversion.
Stock 3, minimum 10 and packs of 4 need two packs: 8 units bring stock to 11. One pack leaves only 7; ordering the shortage violates whole packs and three packs violate minimality.
Iterate over independent records and select those needing orders. Keep calculation in a function returning data, with presentation in its caller. That separates the rule from screen output and shared state.
For a positive shortage, round up packs with (shortage + pack_size - 1) // pack_size, then multiply by pack_size for units. Use this only inside stock<minimum.
Every returned order must be a pack multiple and bring stock to at least minimum. Removing one pack must leave stock below minimum; this verifies the smallest valid order.
Each record is inspected once, with at most n outputs: O(n) time and O(n) output storage under constant-sized numeric operations.
In a temporary copy, deliberately replace the calculation with packs = shortage // pack_size. This runs, but orders only 4 units for stock 3, minimum 10, pack size 4. Logical errors need not raise exceptions.
A breakpoint pauses at a selected location without permanent print statements. Step over runs a call without pausing inside it; step into follows its internals. A watch tracks variables or conditions.
IDE buttons vary: record values and location rather than one interface label. Preserve the independently expected result instead of replacing it with whatever the program prints.
Pause before units is calculated
Run the failing record. Inspect shortage=7, pack_size=4 and packs=1.
Step over the assignment
units becomes 4; stock+units is 7, below minimum 10.
Identify the first wrong decision
The pack count rounded down. Appending and printing merely expose that earlier error.
Correct and retest
Restore ceiling division. Rerun the failing input plus boundaries.
Keep evidence
Record input, expected/actual output, inspected values, correction and retest outcome.
Worked example
| Code | stock < minimum | Shortage | Packs | orders after record |
|---|---|---|---|---|
| P | True | 7 | 2 | [('P', 8)] |
| Q | False | Not calculated | Not calculated | [('P', 8)] |
| R | True | 1 | 1 | [('P', 8), ('R', 8)] |
def reorder(records):
orders = []
for code, stock, minimum, pack_size in records:
if stock < 0 or minimum < 0 or pack_size <= 0:
raise ValueError("invalid stock or pack size")
if stock < minimum:
shortage = minimum - stock
packs = (shortage + pack_size - 1) // pack_size
units = packs * pack_size
orders.append((code, units))
return orders
records = [("P", 3, 10, 4),
("Q", 10, 10, 6),
("R", 0, 1, 8)]
print(reorder(records))
print(reorder([]))Worked example
5 original questions total 25 marks. Attempt each before opening the independently written indicative marking guidance.
A record has stock 6, minimum 17 and pack size 5. Calculate the shortage, packs and units ordered, and verify minimality.
Indicative answer
For stock 9, minimum 10, pack size 4, a faulty calculation uses shortage // pack_size. Give its result, explain the fault and identify a useful breakpoint and inspected value.
Indicative answer
Give a test input and independently calculated expected outcome for each: exact pack multiple, stock equal to minimum, and invalid pack size. Explain what each tests.
Indicative answer
Design an algorithm for sequential requests for seats on a twelve-seat minibus. Each request has an ID and a positive passenger count. Accept it only if enough seats remain; otherwise add its ID to a waiting list. Preserve request order and return both lists.
Indicative answer
Distinguish step into from step over when stopped at a call to reorder. Explain why either tool still requires a known expected outcome.
Indicative answer
This guide addresses OCR H446 2.2.1(a,d–e), 2.3.1(a); reinforces 2.3.1(b). Check your examination year and the complete specification for the assessment scope.
These are independently written explanations and practice questions. CompSciTutoring.co.uk is not affiliated with or endorsed by an examination board. The marking guidance is indicative; always check the syllabus for your examination year.
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