Edexcel 1CP2 · GCSE Computer Science · ~12 min read · 🔒 Pro
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Designing Algorithms
An algorithm is a precise, finite sequence of steps to solve a problem. Designing a good algorithm requires understanding the problem, planning a solution, and checking it works correctly. In Edexcel 1CP2, you need to be able to design, trace, and evaluate algorithms.
Steps for Designing an Algorithm
Understand the problem — identify inputs, outputs, and any constraints.
Break it down — use decomposition to identify sub-problems.
Plan using pseudocode or flowcharts — write out the logic step by step.
Trace the algorithm — use a trace table to check it produces the correct output.
Evaluate and improve — check efficiency and correctness; refine as needed.
Improving Algorithms
Once an algorithm is working, it can often be improved in two main ways:
Add input validation — reject invalid data before the algorithm processes it.
Efficiency improvements
Reduce unnecessary repetition — avoid doing the same work twice.
Add early exit conditions — e.g., optimised bubble sort exits early when no swaps occur.
Choose a more efficient algorithm — e.g., binary search instead of linear search for sorted lists.
Reduce the number of comparisons or iterations.
Evaluating Algorithms
When evaluating an algorithm, consider:
Criterion
Question to ask
Correctness
Does it produce the right output for all valid inputs?
Efficiency
How many steps does it take? How does it scale with n?
Readability
Can another programmer understand and maintain it?
Robustness
Does it handle invalid or unexpected inputs gracefully?
Example: Improving a Bubble Sort
Standard bubble sort always runs n-1 passes even if the list is already sorted.
Improved bubble sort adds a swapped flag: if no swaps occur in a pass, the list is already sorted → exit early.
This reduces best-case from O(n²) to O(n).
Exam tip: "Evaluate" questions are common. Always state whether the algorithm is correct AND efficient. Give specific improvements, not vague ones. For example: "Add a swapped flag so the algorithm can exit early if no swaps occur in a pass" is better than "make it faster".
⚠️ Common Mistakes
Saying an algorithm is "wrong" without explaining what the error is
Describing efficiency improvements without stating what they improve (e.g., speed/memory)
Forgetting to consider edge cases when evaluating correctness
Confusing correctness (wrong output) with efficiency (slow but correct)
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Key points
Steps for designing a good algorithm from scratch
Correctness vs efficiency improvements explained
How early exit conditions improve bubble sort
Evaluation criteria with worked examples
Handling edge cases in algorithm design
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Worksheet — 1.2e Designing & Improving Algorithms
8 Edexcel-style questions · AI-marked
Q1State two things you should identify before designing an algorithm.[2]
✅ Mark scheme
Any 2: inputs [1] outputs [1] constraints/rules [1] (accept any two of these).
Q2Give one way to check an algorithm is correct after designing it.[1]
✅ Mark scheme
Trace the algorithm using a trace table [1] / test it with sample inputs [1].
Q3A search algorithm always searches every item in the list even when the target is found early. Suggest one improvement to make it more efficient.[2]
✅ Mark scheme
Add an early exit condition [1] so the algorithm stops as soon as the target is found [1].
Q4Describe what is meant by a logic error in an algorithm.[2]
✅ Mark scheme
A logic error is when the algorithm runs without crashing [1] but produces the wrong output [1].
Q5State the four criteria used to evaluate an algorithm.[4]
Q6Explain why adding a 'swapped' flag to bubble sort improves its efficiency.[3]
✅ Mark scheme
The flag checks if any swaps occurred in a pass [1]; if no swaps occur, the list is already sorted [1]; the algorithm exits early rather than making unnecessary passes [1].
Q7A sorting algorithm produces incorrect output for a list with only one item. What type of error is this and how should the algorithm be improved?[3]
✅ Mark scheme
This is a logic error [1] caused by a failure to handle an edge case (single-item list) [1]; the algorithm should check if the list has one or fewer items and return immediately in that case [1].
Q8An algorithm searches a sorted list of 1000 items. The programmer switches from linear search to binary search. Explain the benefit of this change and identify one precondition required.[3]
✅ Mark scheme
Binary search is much more efficient — up to 10 steps vs up to 1000 for linear search [1] as it uses logarithmic (O(log n)) rather than linear (O(n)) complexity [1]; precondition: the list must be sorted [1].
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Mini Test — 1.2e Designing Algorithms
Timed exam-style test. No feedback until submission.
10 questions · 10 marks · 10 minutes
5 MCQ + 5 short answer
⏱10:00
Mini Test · 10 marks
Section A — Multiple Choice [5 marks]
Q1Which is the first step when designing an algorithm?[1]
Q2An algorithm runs without crashing but produces the wrong output. This is:[1]
Q3An algorithm that handles unexpected or invalid inputs gracefully is described as:[1]
Q4Which improvement makes bubble sort exit early if already sorted?[1]
Q5To switch from linear search to binary search, the list must be:[1]
Section B — Short Answer [5 marks]
Q6State two criteria used when evaluating an algorithm.[2]
Q7Describe one way to improve the efficiency of an algorithm.[2]
Mark schemeAdd an early exit condition [1] so the algorithm stops when a result is found, reducing unnecessary iterations [1].
Q8Give one difference between a correctness improvement and an efficiency improvement to an algorithm.[1]
Mark schemeA correctness improvement fixes wrong output / handles edge cases; an efficiency improvement makes the algorithm faster or use less memory (while still producing the same correct output). [1]