C1.1 · Model modules, classes and abstract data types

Ontario Grade 12 Computer Science

Designing Modular Programs

Ontario ICS4U study lesson for expectation C1.1

What you will learn

• Explain how modules, classes and abstract data types model different aspects of a program.
• Represent class responsibilities using attributes and methods.
• Describe an abstract data type through its permitted operations and expected behaviour.
• Model how related modules and classes divide responsibilities.

1. Choose the model that answers the question

• Module: groups related program elements.
• Class: describes objects, their information and their actions.
• ADT: specifies permitted operations and expected behaviour, independently of storage details.

2. Model a class through information and actions

• Attributes describe an object’s information.
• Methods describe actions or services associated with the object.
• Trace attribute values before and after an action to check that the model is clear.

3. Describe ADTs through their operation rules

• Specify operation inputs, results and expected behaviour.
• A queue removes the earliest item that is still waiting.
• State the expected result for an operation such as dequeue on an empty queue.

4. Combine modules and classes in one model

• A module can group related classes; a program part can use their services.
• Assign a method to the class whose responsibility it supports.
• Make assumptions and expected outcomes explicit when tracing a model.

Worked example

Example 1: Model a book as a class

class Book:
    def __init__(self, title):
        self.title = title
        self.checked_out = False

    def borrow(self):
        if self.checked_out:
            return "Unavailable"
        self.checked_out = True
        return "Borrowed: " + self.title

book = Book("River Road")
message = book.borrow()

1. Set the precondition and input
   Assume the book is available before the request. The constructor input is the title River Road; the borrow action uses the resulting Book object’s current status.
2. Identify the responsibilities
   The title and checked_out attributes describe the book. The borrow method changes that book’s status, so the method belongs with the Book responsibility in this model.
3. Trace the action
   A new object starts with checked_out set to False. The method therefore changes it to True and returns the success message.
4. State the expected output
   The expected returned message is Borrowed: River Road, and the object’s checked_out value becomes True. This is a code trace, not a report of execution.

Worked example

Example 2: Specify a queue ADT

ADT Queue
    enqueue(item): add item at the back
    dequeue():
        if empty, use the specified empty-queue result
        otherwise, remove and return the front item

1. State the operation rules
   Enqueue takes one item and adds it at the back. Dequeue takes no item input and removes and returns the front item when the queue is not empty. The ADT description must also specify what happens if dequeue is requested while empty.
2. Apply the operations in order
   Start with an empty queue. After enqueueing A and then B, A is at the front and B follows it.
3. Trace the removal
   Dequeue removes the front item, so it returns A. B remains in the queue.
4. Keep storage separate
   The result follows from the queue’s first-in, first-out rule. The ADT model does not need to name how the items are stored.

Worked example

Example 3: Organize a small library program

class Book:
    def __init__(self, title, checked_out):
        self.title = title
        self.checked_out = checked_out

class Library:
    def __init__(self, books):
        self.books = books

    def find_book(self, title):
        for book in self.books:
            if book.title == title:
                return book
        return None

def availability(library, title):
    book = library.find_book(title)
    if book is None:
        return "Not found"
    if book.checked_out:
        return "Checked out"
    return "Available"

1. Set assumptions and input
   Assume the Library already holds Book objects, each with a title and checked-out status. The input is the title to look up.
2. Assign responsibilities
   Book represents one book and its status. Library manages the collection and looks up a title. The program module handles the user request and presents the response. In this Python example, a module is represented by a file grouping related definitions and code.
3. Trace the request
   For the input River Road, Library checks its Book objects in order. If it finds a match whose checked_out value is False, the expected response is Available. If no title matches, the expected response is Not found.
4. Check that the model agrees
   The diagram shows the program using the library module and the library module grouping Book and Library. The trace follows the responsibilities shown; it is not a claim that the code was executed.

Common mistakes and how to avoid them

Lesson summary

• Model modules as groups of related program elements.
• Model classes through objects’ attributes and methods, with clear responsibilities.
• Model ADTs through permitted operations and their expected behaviour, not a particular storage choice.
• Check that assumptions, responsibilities, diagrams and traces agree.

Check your understanding

Key terms

Module

A named unit for grouping related program elements; the exact convention depends on the programming language.

Class

A description used to create objects with associated information and actions.

Attribute

Information associated with an object.

Method

An action or service associated with a class or its objects.

Abstract data type (ADT)

A model defined by its permitted operations and expected behaviour, rather than by a particular storage implementation.

Precondition

A condition assumed to be true before an operation is performed.

Continue through ICS4U

View the complete ICS4U Ontario Grade 12 Computer Science curriculum and lessons

• B2.3 · Reflect on individual and team progress
• C1.2 · Use encapsulation in program design
• C1.3 · Decompose a task into subprograms
• C1.4 · Design reusable modules and methods
• C2.1 · State algorithm preconditions and postconditions
• C2.2 · Compare linear and binary search costs

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About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Computer Science (ICS4U), expectation C1.1. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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