The Structure of a ProgramMichael Heron

Introduction

• Computer programs in C++ are broken up into several parts.

• For various reasons, which we will discuss as we go through the module.

• In this lecture, we’re going to look at the big picture of what parts make up a program.

• We’ll get to the detail in later lectures.

• Some of this will be quite abstract at the moment.

A C++ Program

• A properly designed C++ program consists of:

• One or more objects.

• These objects are defined by classes.

• Each object consists of:

• Attributes

• Behaviours

• Each behaviour consists of:

• Variables

• Selection structures

• Repetition structures

• Data structures

Objects and Classes

• Conceptually the most difficult thing to visualise.

• So don’t worry about it too much for now.

• C++ is an object oriented programming language.

• Except, not really.

• We will be looking at writing object oriented programs.

• This is a framework common to many programming languages.

• Java and C# being two very popular examples.

The Structure of an Object

• We’ll begin closer to home.

• With the idea of an object.

• An object is an instance of a class.

• A class is like a blueprint telling the object what structure it has.

• A class defines:

• Attributes

• Behaviours

• The object defines

• State

Eh?

• There exists, somewhere, a blueprint for a chair.

• Blueprints for the chairs on which you are sitting.

• The blueprint defines what the chair looks like.

• It defines the structure of the chair

• It defines the relationship of the legs to the seat

• This blueprint would be the class.

• The specific chairs on which you are sitting would be objects.

Uh…

• The blueprint tells us how the chair is supposed to behave and what information about that chair may be mutable.

• The colour of the chair

• The material of the chair

• The size of the chair

• The class says:

• A chair has a colour, material, and size

• The object says:

• I am blue, made of leather, and is medium sized.

Okay!

• We’ll come back to this subject later.

• Because regardless of what people may tell you, object orientation does not come naturally to most people.

• Suffice to say that an object is one of the building blocks of an object oriented program.

• Java and C# require you to use objects.

• C++ lets you use them if you like.

Attributes

• Attributes are things that an object will have.

• Usually things that are mutable (they can change).

• Consider a human face.

• It has eyes

• It has a nose

• It has a mouth

• These are modeled in a computer program as variables.

• A class is thus a collection of variables.

Behaviours

• As well as these variables, a class contains behaviours for acting upon those variables.

• If the class is a human face:

• Attributes: Eyes, Mouth, Nose

• Behaviours: blink, smile, sniff

• These are modeled in a computer program as functions.

• Also called methods.

• Two names for the same thing, we’ll use these interchangeably.

Behaviours

• Behaviours can be broken down into further parts.

• Behaviours may have variables of their own.

• Temporary variables that only exist as long as the method is executing.

• Behaviours will usually incorporate programming structures.

• Some structures allow the programmer to choose between courses of actions.

• Some structures allow the programmer to repeat blocks of code.

My First Program

• The simplest program you can write in C++ is the following:

#include <iostream>

int main() {cout << "Hello Dundee University!" << endl;return 1;

}

The Program

• The first part is known as a preprocessor directive.• More on this later.

• Main is the starting point of the program.• It’s a function.

• C++ will look for this when you tell it to compile and execute a program.

• cout is a special keyword in C++• It refers to the standard output device.

The Program

• The << symbols are known as operators.• These are a big part of C++, so we’ll come back to these.

• In this case, they mean ‘send the follow text to the standard output device’

• endl is a special keyword.• It means ‘a line end symbol’

• Return means very little in this context.• We’ll come back to it in future lectures.

Experimentation

• The key to successful programming is experimentation.

• You have to be willing to play around with code to see what happens.

• One of the questions I am asked most often is ‘what happens when I do this?’

• You don’t need to ask me.• Make a backup of your code, and try it yourself.

• Your code is just plain text

• You can save it in notepad

A Second C++ Program

#include <iostream>

int main() {int age;

cout << "How old are you?" << endl;cin >> age;cout << "You are " << age << " years old" << endl;

}

New Features

• A variable!

• int age

• User input

• cin is keyboard input

• Notice that the >> goes the other way

• Display of the contents of a variable to the user

• Sending age to the standard output

Variables

• Variables are little boxes we use to hold data when we don’t know in advance what’s going to go in them.• Common when dealing with user input.

• Variables have two parts to them.• A name

• A type

• The type we have used here is int• Short for integer, which is a whole number.

Variables

• A variable has three parts to its lifecycle.• It is created.

• Or declared.

• It is manipulated• It has values assigned to it.

• We can set the variable to have certain values.• age = 100;

• It has the value queried.

• It is destroyed.• This is done automatically for this variable.

• Not so automatically for others.

Variables

• C++ offers support for different kinds of variable.

• float

• Floating point numbers

• char

• Single alphanumeric characters

• bool

• True or false

Variables

• C++ also offers support for strings.

• But not natively.

• String support can be enabled by adding the following to your program after the #include:

• using namespace std;

• From that point on, the string data type is available to you.

A Third Program

#include <iostream>

using namespace std;

int main() {string name;

cout << "What is your name?" << endl;cin >> name;cout << "Your name is " << name << endl;

}

Arithmetic Operators

• For the numeric data types, we can perform simple arithmetic on them using the arithmetic operators.• +

• Addition

• -• Subtraction

• *• Multiplication

• / • Division

• %• Modulo division

A Fourth Program

#include <iostream>

using namespace std;

int main() {int num1, num2;int answer;

cout << "What is your first number?" << endl;cin >> num1;cout << "What is your second number?" << endl;cin >> num2;

answer = num1 + num2;

cout << "Your answer is " << answer << endl;}

Summary

• C++ Programs are made up of many parts• Objects, classes

• Variables and Structures

• We’ve looked at one of these today in any depth.• The variable.

• Variables are the building blocks of a program.• They let us deal with ambiguity, the unknown, and complexity.