Structure charts (Cambridge International AS Computer Science)

Exam board Cambridge International · ASUpdated 2025-05-09

Structure charts

What is a structure chart?

A structure chart is a modelling tool used in the design stage of program development
It helps to decompose a problem into smaller, manageable sub-tasks, representing each as a module
Structure charts focus on what the program does, not how it does it

Example

A program that asks a user to enter their username and password
Checks if the password matches the stored password and uses a counter to track how many times it is checked
If the password matches, a successful message is displayed, else after three failed attempts a 'denied' message is displayed
A structure chart might look like:

Flowchart of login process: input username, enter password, check password with conditions, possible outputs are "Login successful" or "Access denied".

Key features

Feature	Explanation
Top-down design	The chart starts with the main program and breaks it into sub-modules
Decomposition	Each module represents a specific task or function
Module boxes	Each module is shown as a box
Vertical lines	Show control flow – which module calls which
Parameter arrows	Arrows pointing into modules show data being passed in or returned
Stepwise refinement	Each level adds more detail to the task above
Diamond shape	Shows a condition that could be true or false
Semi-circular arrow	Indicates repetition

Pseudocode from structure chart

The first step is to create an identifier table

Identifier	Data type	Description
`Username`	STRING	Stores the username entered by the user
`Password`	STRING	Stores the password entered by the user
`StoredPass`	STRING	The correct password stored in the system for comparison
`NoAttempts`	INTEGER	Counts the number of failed login attempts
`LoginSuccess`	BOOLEAN	Indicates whether the login was successful

Next, identify is any functions or procedures could be used

Module	Type	Purpose
`GetCredentials`	PROCEDURE	Asks the user to input username and password
`CheckPassword`	FUNCTION	Compares input password with stored password and returns TRUE/FALSE
`ShowAccessMessage`	PROCEDURE	Displays success or failure message

Any finally, write the pseudocode

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Feature

Explanation

Top-down design

The chart starts with the main program and breaks it into sub-modules

Decomposition

Each module represents a specific task or function

Module boxes

Each module is shown as a box

Vertical lines

Show control flow – which module calls which

Parameter arrows

Arrows pointing into modules show data being passed in or returned

Stepwise refinement

Each level adds more detail to the task above

Diamond shape

Shows a condition that could be true or false

Semi-circular arrow

Indicates repetition

Identifier

Data type

Description

Username

STRING

Stores the username entered by the user

Password

STRING

Stores the password entered by the user

StoredPass

STRING

The correct password stored in the system for comparison

NoAttempts

INTEGER

Counts the number of failed login attempts

LoginSuccess

BOOLEAN

Indicates whether the login was successful

Module

Type

Purpose

GetCredentials

PROCEDURE

Asks the user to input username and password

CheckPassword

FUNCTION

Compares input password with stored password and returns TRUE/FALSE

ShowAccessMessage

PROCEDURE

Displays success or failure message