Cambridge Syllabus Notes

9.2 Algorithms – Cambridge A-Level CS 9618

What is an Algorithm?

An algorithm is a step‑by‑step set of instructions that solves a problem or performs a task. Think of it like a recipe for baking a cake – you follow each step in order to get the final result.

Exam Tip: Remember that an algorithm must be finite, unambiguous and effective. Highlight these properties in your answer.

Structured English Description

Structured English uses plain English with a few key words (IF, THEN, ELSE, WHILE, END) to describe the flow of an algorithm. It’s easier to read than raw code and helps you plan before coding.

START
  INPUT number
  IF number MOD 2 = 0 THEN
    OUTPUT "Even"
  ELSE
    OUTPUT "Odd"
  END IF
END

Flowchart Representation

Flowcharts use shapes to represent steps:

Oval – Start/End
Rectangle – Process / Action
Diamond – Decision (Yes/No)
Arrow – Flow direction

Below is a simple textual flowchart for checking if a number is even or odd.

[Start] → [Input number] → [Decision: number MOD 2 = 0?]
  ├─ Yes → [Output "Even"] → [End]
  └─ No  → [Output "Odd"]  → [End]

Pseudocode Example

Pseudocode is a mix of natural language and programming constructs. It’s less formal than real code but more precise than structured English.

Algorithm CheckEvenOdd
  Input: integer n
  Output: string result
  Begin
    If n mod 2 = 0 Then
      result ← "Even"
    Else
      result ← "Odd"
    End If
    Print result
  End

Algorithm in a Table

Tables can help you organise steps and conditions clearly.

Step	Action	Decision
1	Input number n
2	Check if n mod 2 = 0	Yes → Step 3, No → Step 4
3	Output "Even"
4	Output "Odd"

Exam Tip: When asked to write an algorithm, start with a clear structured English description, then translate it into pseudocode or a flowchart. Use the table format if it helps you organise the logic.

Analogy: The Algorithm as a Road Map

Imagine you’re planning a road trip. The algorithm is your road map:

Start point – Where you begin (input).
Road signs (decisions) – Tell you which direction to take.
Road segments (processes) – The actions you perform.
Destination (output) – The final result.

Just like a map, an algorithm must guide you from start to finish without getting lost.

Key Takeaways for the Exam

Always include start and end points.
Use clear decisions (IF/ELSE) and loops (WHILE).
Show the flow of control – what happens next after each step.
When writing pseudocode, keep it language‑agnostic but precise.
Use tables or flowcharts to visualise complex logic.

Exam Tip: Practice writing algorithms for everyday tasks (e.g., sorting a list of names, calculating the average of grades) to build confidence. The more you practise, the easier it becomes to translate a problem into a clear algorithm. 🚀

Document a simple algorithm using a structured English description, a flowchart or pseudocode