Algorithm basics (Cambridge International AS Computer Science)

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

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

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 sensitive (e.g. Total and total are NOT treated the same)