Algorithm Basics (Cambridge (CIE) A Level Computer Science) : Revision Note

Definition of algorithm

What is an algorithm?

• An algorithm is a solution to a problem expressed as a sequence of defined steps

• Methods of writing an algorithm before attempting to program a solution include:

  • Structured English

  • Pseudocode

  • Flowcharts

Structured English

• Structured English is a human-readable method for describing algorithms using a combination of natural English language and programming logic

  • It uses clear English phrases to describe each step

  • Logic structures like IF...THEN, REPEAT, and WHILE may appear, but without strict syntax rules

  • Often used in the early planning stages before converting to pseudocode

Ask the user to enter their age
If the age is 18 or over
→ Display a welcome message
Otherwise
→ Display an access denied message

Pseudocode

• Pseudocode is a precise, structured, and language-independent way of describing an algorithm that resembles a programming language

• It follows specific exam-board-defined syntax

  • It includes formal elements like IF, THEN, ELSE, WHILE, REPEAT, DECLARE, and ← for assignment

  • Students must follow CIE’s pseudocode format in exams

INPUT Age
IF Age >= 18 THEN
    OUTPUT "Welcome to the site"
ELSE
    OUTPUT "Sorry, this site is for users 18 and over"
ENDIF

Flowcharts

• Flowcharts are a visual tool that uses shapes to represent different functions to describe an algorithm

• Used to visualise the flow of control in a system

• Standard symbols include:

  • Oval for Start/End

  • Parallelogram for Input/Output

  • Rectangle for Processes

  • Diamond for Decisions

• Arrows show the sequence of operations

Identifier tables

What is an identifier table?

• An identifier table is used when writing pseudocode to keep track of all the identifier names used in an algorithm

• An identifier is the name given to a variable, constant, array, procedure, or any other named element in the pseudocode

Why use an identifier table?

• It helps you stay organised when designing an algorithm

• Ensures consistent naming

• Makes it easier to understand what each identifier stores or does

• Useful in exam questions when you are asked to declare variables clearly

Identifier naming rules

• Must start with a letter (A–Z or a–z)

• Can include letters, digits (0–9), and underscores (_)

• Accented characters and symbols are not allowed

• Identifiers are case insensitive (e.g. Total and total are treated the same)