What Is an Algorithm? Representing Algorithms

This lesson introduces the concept of algorithms and how they can be represented, covering the key knowledge required for OCR J277 Section 2.2. Understanding algorithms is fundamental to computational thinking and is central to both Paper 1 and Paper 2 of the OCR GCSE Computer Science exam.

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Defining an Algorithm

An algorithm is a finite sequence of well-defined, step-by-step instructions designed to perform a specific task or solve a particular problem. Algorithms are not computer programs — they are abstract descriptions of a process that can be implemented in any programming language or even followed by a person.

Key properties of an algorithm:

Property	Description
Finite	It must eventually terminate after a finite number of steps
Definite	Each step must be precisely and unambiguously defined
Inputs	It may have zero or more inputs
Outputs	It must produce at least one output
Effective	Each step must be basic enough to be carried out

OCR Exam Tip: If asked to "state what is meant by an algorithm", always mention that it is a set of step-by-step instructions to solve a problem or complete a task. The word "step-by-step" is crucial for full marks.

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Why Algorithms Matter

Before writing any code, programmers plan their solutions using algorithms. This is part of computational thinking, which involves:

• Decomposition — breaking a problem into smaller, manageable sub-problems
• Abstraction — removing unnecessary detail to focus on what matters
• Algorithmic thinking — creating a step-by-step solution

By planning with algorithms first, programmers can identify potential issues early and ensure their solution is logical before committing to code.

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Representing Algorithms: Pseudocode

Pseudocode is a way of writing algorithms using a structured, English-like syntax that resembles a programming language but is not tied to any specific one. OCR uses its own pseudocode reference language in exams.

Here is an example of a simple algorithm in OCR pseudocode that finds the largest of three numbers:

a = input("Enter first number: ")
b = input("Enter second number: ")
c = input("Enter third number: ")

if a >= b AND a >= c then
    print("Largest is " + a)
elseif b >= a AND b >= c then
    print("Largest is " + b)
else
    print("Largest is " + c)
endif

And the equivalent in Python:

a = int(input("Enter first number: "))
b = int(input("Enter second number: "))
c = int(input("Enter third number: "))

if a >= b and a >= c:
    print("Largest is", a)
elif b >= a and b >= c:
    print("Largest is", b)
else:
    print("Largest is", c)

OCR Exam Tip: In the exam, you may be asked to write, complete, or interpret pseudocode. You do not need to write perfect Python — OCR's pseudocode reference language is accepted. However, you must be consistent in whichever style you use.

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Representing Algorithms: Flowcharts

A flowchart is a graphical representation of an algorithm using standard symbols:

Symbol	Shape	Purpose
Terminator	Rounded rectangle (oval)	Start or end of the algorithm
Process	Rectangle	An instruction or action to perform
Decision	Diamond	A yes/no or true/false question
Input/Output	Parallelogram	Data being entered or displayed
Flow line	Arrow	Shows the direction of flow

Example: Is a Number Even or Odd?

A flowchart for this algorithm would flow as follows:

1. Start (terminator)
2. Input number (parallelogram)
3. Is number MOD 2 == 0? (diamond)
4. If Yes → Output "Even" (parallelogram)
5. If No → Output "Odd" (parallelogram)
6. Stop (terminator)

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Pseudocode vs Flowcharts

Feature	Pseudocode	Flowchart
Format	Text-based	Graphical/visual
Best for	Longer, complex algorithms	Shorter algorithms or showing decision logic
Ease of writing	Quick to write	Can be time-consuming to draw neatly
Exam usage	Very common in Paper 2	Common for shorter questions
Modification	Easy to edit lines	May require redrawing

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Worked Example

Problem: Write an algorithm that asks the user for a password. If the password is correct ("secret123"), display "Access granted". Otherwise, display "Access denied".

OCR Pseudocode:

password = input("Enter password: ")
if password == "secret123" then
    print("Access granted")
else
    print("Access denied")
endif

Python:

password = input("Enter password: ")
if password == "secret123":
    print("Access granted")
else:
    print("Access denied")

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Summary

• An algorithm is a step-by-step set of instructions to solve a problem.
• Algorithms can be represented using pseudocode (text-based) or flowcharts (graphical).
• OCR uses its own pseudocode reference language — learn its conventions.
• Flowcharts use standard symbols: terminators, processes, decisions, and input/output.
• Planning algorithms before coding is a key part of computational thinking.

OCR Exam Tip: Always draw flowchart symbols correctly — use diamonds for decisions (not rectangles), and remember to label the Yes/No branches on decision diamonds. Incorrect symbols will lose marks even if the logic is correct.