Structure Diagrams and Problem Representation

This lesson covers structure diagrams as a method for representing decomposed problems, as required by OCR J277 Section 2.1. Structure diagrams provide a clear, visual way to show how a complex problem is broken down into smaller sub-problems.

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What Is a Structure Diagram?

A structure diagram (also called a hierarchy chart or top-down design diagram) is a visual representation of how a problem has been decomposed into sub-problems. It uses a tree-like structure where:

• The top level shows the overall problem
• Each lower level shows the sub-problems at increasing levels of detail
• Lines connect each problem to its sub-problems

Structure diagrams are a direct visual representation of the decomposition process.

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How to Read a Structure Diagram

A structure diagram is read from top to bottom and (generally) from left to right:

graph TD
  A["Quiz Game"] --> B["Setup"]
  A --> C["Game Play"]
  A --> D["End Game"]
  B --> B1["Load Questions"]
  B --> B2["Show Menu"]
  C --> C1["Show Question"]
  C --> C2["Get Input"]
  C --> C3["Check Answer"]
  D --> D1["Show Score"]
  D --> D2["Ask Play Again"]

Level	Content
Level 0	The overall problem: Quiz Game
Level 1	Main sub-problems: Setup, Game Play, End Game
Level 2	Detailed sub-problems: Load Questions, Show Menu, etc.

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Creating a Structure Diagram

Step-by-Step Process

1. Identify the main problem — Write this at the top of the diagram.
2. Decompose into main sub-problems — These become the Level 1 boxes.
3. Decompose each sub-problem further — These become Level 2 boxes.
4. Continue until sub-problems are simple enough to be solved directly.
5. Draw connecting lines from each problem to its sub-problems.

Rules for Structure Diagrams

Rule	Explanation
One box at the top	The overall problem is a single box at Level 0
Each box connects to its parent	Lines show which sub-problems belong to which parent
Read top to bottom	Each level represents increasing detail
Each box is a sub-problem	Boxes represent tasks, not data
Sufficient detail	Decompose until each box represents a single, manageable task

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Worked Example 1: Library Management System

graph TD
  A["Library Management System"] --> B["Manage Books"]
  A --> C["Manage Loans"]
  A --> D["Manage Members"]
  A --> E["Reports"]
  B --> B1["Add Book"]
  B --> B2["Edit Book"]
  B --> B3["Search Book"]
  C --> C1["Borrow Book"]
  C --> C2["Return Book"]
  D --> D1["Register Member"]
  D --> D2["Edit Member"]
  E --> E1["Overdue Books"]
  E --> E2["Popular Books"]
  E --> E3["Member History"]

Explanation:

Level	Sub-Problems
Level 0	Library Management System
Level 1	Manage Books, Manage Loans, Manage Members, Reports
Level 2	Add Book, Edit Book, Search Book, Borrow Book, Return Book, etc.

Each Level 2 sub-problem is simple enough to be solved directly with an algorithm.

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Worked Example 2: Login System

graph TD
  A["Login System"] --> B["Get Credentials"]
  A --> C["Validate"]
  A --> D["Respond"]
  B --> B1["Input Username"]
  B --> B2["Input Password"]
  C --> C1["Hash Password"]
  C --> C2["Compare With Stored"]
  C --> C3["Check Attempts & Lock"]
  D --> D1["Grant Access"]
  D --> D2["Deny Access"]

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Structure Diagrams vs Other Representations

Representation	Shows	Best For
Structure diagram	How a problem is decomposed into sub-problems	Planning the overall structure of a solution
Flowchart	The step-by-step flow of an algorithm	Showing the logic and decision points in an algorithm
Pseudocode	The detailed logic of an algorithm in code-like text	Writing out the specific instructions for a solution

Structure diagrams are used at the planning stage — before you write flowcharts or pseudocode. They help you understand the overall shape of the problem.

OCR Exam Tip: In the exam, you may be given a partially completed structure diagram and asked to fill in the missing boxes. Read the existing boxes carefully to understand the pattern and level of detail, then add boxes at the same level of detail. You may also be asked to create a structure diagram from a given scenario — start by identifying the main sub-problems (Level 1) before adding detail (Level 2).

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Benefits of Using Structure Diagrams

Benefit	Explanation
Visual clarity	Easy to see the overall structure at a glance
Shows decomposition	Clearly demonstrates how a problem is broken into parts
Identifies sub-programs	Each low-level box can become a function or procedure
Supports teamwork	Different team members can be assigned different branches
Planning tool	Helps plan before coding — reduces errors and rework

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Common Exam Questions

Question Type 1: Complete a Structure Diagram

"The structure diagram below shows part of a school timetable system. Complete the missing boxes."

Approach: Look at the existing boxes, identify what is missing logically, and add boxes at the appropriate level with clear, specific labels.

Question Type 2: Create a Structure Diagram

"Draw a structure diagram for a system that manages a sports day event."

Approach:

1. Write "Sports Day Management" at the top.
2. Identify Level 1 sub-problems: e.g. Manage Events, Manage Participants, Record Results, Generate Reports.
3. Decompose each into Level 2: e.g. under Manage Events: Add Event, Edit Event, Delete Event, Schedule Event.

Question Type 3: Explain the Purpose

"Explain how a structure diagram is used in the development of a computer system."

Approach: Explain that it visually represents decomposition, shows sub-problems at different levels of detail, helps plan before coding, and allows tasks to be assigned to team members.

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Summary

Structure diagrams visually represent how a problem has been decomposed into sub-problems using a hierarchical tree structure. They are created by identifying the main problem, decomposing it into sub-problems, and continuing until each sub-problem is simple enough to solve. For the OCR J277 exam, practise creating, completing, and interpreting structure diagrams for different scenarios.

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Worked Example: School Canteen Ordering App

A local secondary school wants to build an app so students can pre-order their lunch in the morning. The kitchen uses the orders to know how much to prepare, parents can top up their child's balance online, and teachers can see who has not paid.

Step 1 — Write down the overall problem. "Canteen Ordering App" goes at Level 0.

Step 2 — Identify the Level 1 sub-problems. Ask: what broad areas must the app cover?

• Account management (login, top-up, balance)
• Menu (display today's options)
• Ordering (choose items, confirm)
• Kitchen reports (aggregated totals)
• Administration (add staff, edit menu, view unpaid accounts)

Step 3 — Decompose each Level 1 sub-problem.