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Edexcel 4CP0 · Topic 1 · 1.1a
What are
Algorithms?
Definitions · Properties · Representation · Efficiency
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Edexcel iGCSE Computer Science 4CP0
Learning Objectives
By the end of this lesson you will be able to...
Define the term algorithm and state its key properties
Describe the three ways algorithms can be represented: pseudocode, flowcharts, and natural language
Explain why algorithm efficiency matters and how it is measured
Confidently answer Edexcel 4CP0 exam questions on this topic
Core Definition
What is an Algorithm?
"A finite, precise, and unambiguous set of step-by-step instructions for solving a problem or completing a task."
REAL-WORLD EXAMPLE
A recipe — each step is clear, in order, and eventually ends
A sat-nav route — precise directions to a destination
COMPUTING EXAMPLE
Searching a list for a name
Sorting numbers into order
Algorithm Properties
Three Essential Properties
1
Precise — every step must be clear and exact. No ambiguity in what to do. "Sort of add them" is not precise. "Add x to y" is precise.
2
Unambiguous — each instruction can only be interpreted one way. Different people following the same algorithm must get the same result.
3
Finite — the algorithm must eventually stop. An infinite loop that never terminates is not a valid algorithm.
⚠ Exam note:
The Edexcel spec expects you to know that algorithms must be precise, unambiguous, and finite
Representation
Three Ways to Write an Algorithm
📝
Pseudocode
Structured code-like text. Not a real programming language but follows logic clearly. Used throughout the Edexcel spec.
🔷
Flowcharts
Visual diagrams using standard shapes. Easy to follow for simple algorithms. Shows flow of control clearly.
📖
Natural Language
Plain English steps. Easy to understand but can be ambiguous and imprecise — not ideal for complex algorithms.
💡 Edexcel exams primarily test pseudocode — you must know the Edexcel pseudocode syntax
Efficiency
Why Algorithm Efficiency Matters
"A correct algorithm that takes too long is useless in practice. Efficiency measures how the time or memory needed grows as the input size grows."
TIME EFFICIENCY
How many steps does the algorithm need? Fewer steps = faster. A linear search checks each item one by one. A binary search eliminates half the list each time.
COMPARING ALGORITHMS
Two correct algorithms can produce the same answer but one may be far more efficient. Choosing the right algorithm for the situation is a key skill.
Designing Algorithms
How to Develop an Algorithm
1
Understand the problem — what are the inputs? What output is needed? What constraints exist?
2
Plan the steps — break the problem into smaller sub-tasks (decomposition). What must happen first, second, last?
3
Write the algorithm — express the steps in pseudocode or a flowchart. Each step must be precise and unambiguous.
4
Trace and test — follow the algorithm manually with example inputs. Does it produce the correct output for all cases?
Exam Practice
Question 1
Define the term "algorithm" in the context of computer science.
2 marks
✓ MARK SCHEME
1 mark
A set of step-by-step instructions / rules for solving a problem or completing a task
1 mark
That is precise / unambiguous / finite (any one of these qualifiers)
Exam Practice
Question 2
State two properties that a correctly designed algorithm must have.
2 marks
✓ MARK SCHEME — any two of:
1 mark
Precise — every instruction is clear and exact
1 mark
Unambiguous — each step can only be interpreted one way
1 mark
Finite — the algorithm must eventually terminate / stop
Exam Practice
Question 3
Give one advantage and one disadvantage of representing an algorithm as a flowchart compared to pseudocode.
2 marks
✓ MARK SCHEME
Advantage
Visual / easier to follow / shows flow of control clearly / no specialist knowledge of pseudocode needed
Disadvantage
Harder to scale for complex algorithms / takes longer to draw / less compact than pseudocode
Common Mistakes
Don't lose easy marks
1
Saying an algorithm is just "a set of instructions" without adding that they must be precise, unambiguous, and finite — the definition answer needs at least one qualifier for the second mark.
2
Confusing efficiency with correctness. An algorithm can be correct but inefficient. Efficiency is about how many steps are needed, not whether it gets the right answer.
3
Saying pseudocode is "a programming language" — it is NOT. Pseudocode cannot be executed by a computer. It is a planning/design tool.
Topic Summary — 1.1a
What You Need to Know
DEFINITION
Algorithm = finite, precise, unambiguous set of step-by-step instructions for solving a problem
PROPERTIES
Precise — exact steps
Unambiguous — only one interpretation
Finite — must eventually stop
REPRESENTATIONS
Pseudocode — structured, code-like
Flowcharts — visual diagrams
Natural language — plain English (can be ambiguous)
EFFICIENCY
Two algorithms can both be correct but one may be faster. Fewer steps = more efficient. Choose the right algorithm for the situation.
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1.1b
Flowcharts
Symbols · Drawing · Reading flowcharts
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