Algorithms is a core part of GCSE Computer Science. Revise the key concepts and common mistakes below, then lock them in with the free games.
Key concepts
AlgorithmA precise step-by-step method for solving a problem.
Pseudocode & flowchartsWays to express an algorithm before coding.
SearchingLinear search (any list) and binary search (sorted list).
SortingOrdering data, e.g. bubble, merge and insertion sort.
DecompositionBreaking a problem into smaller parts.
AbstractionHiding unnecessary detail to focus on what matters.
O(n)Linear time — linear search, bubble sort traversal.
O(log n)Logarithmic — binary search.
O(n^2)Quadratic — bubble and insertion sort.
O(n log n)Efficient sort time — merge sort.
Best caseFastest possible scenario for the algorithm.
Worst caseSlowest possible scenario for the algorithm.
ComparisonChecking which of two items is bigger.
SwapExchange two items' positions.
Common mistakes to avoid
Questions where students often pick the tempting wrong answer — make sure you know the right one:
What does a binary search need to work?✗ It works on any unsorted list. ✓ A sorted list.
Must an algorithm be written in code?✗ Yes, an algorithm has to be program code. ✓ No — it's a step-by-step method (e.g. pseudocode or a flowchart).
Is an algorithm the same thing as a computer program?✗ Yes - algorithm is just the formal word for program code. ✓ No - an algorithm is the abstract sequence of steps; a program is one implementation of it in a particular language. The same algorithm can be coded many ways.
Which best defines an algorithm?✗ Any piece of computer hardware ✓ A precise sequence of steps that solves a problem
Are abstraction and decomposition the same thing?✗ Yes - both just mean simplifying a problem, so they are identical. ✓ No - decomposition splits a problem into smaller parts, while abstraction removes irrelevant detail from the problem; they are separate, complementary techniques.
Practise Algorithms — free games
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