Top-Down Design with Functions and Classes

Chapter 3

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3.1 Building Programs from Existing Information

– Reuse of existing programs

– Develop program in stages

– Compile along the way

– Keep it small

– Comments to describe actions

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Washers.cpp

// File: washers.cpp

// Computes the weight of a batch of flat

// washers.

#include <iostream>

using namespace std;

int main()

{

const float PI = 3.14159;

float holeDiameter; // input -

4

Washers.cpp

float edgeDiameter; // input - diameter

float thickness; // input - thickness

float density; // input - density

float quantity; // input - number

float weight; // output - weight

float holeRadius; // radius of hole

float edgeRadius; // radius of outer edge

float rimArea; // area of rim

float unitWeight; // weight of 1 washer

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Washers.cpp

cout << "Inner diameter in centimeters: ";

cin >> holeDiameter;

cout << "Outer diameter in centimeters: ";

cin >> edgeDiameter;

cout << "Thickness in centimeters: ";

cin >> thickness;

cout << "Material density in grams per

cubic centimeter: ";

cin >> density;

cout << "Quantity in batch: ";

cin >> quantity;

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Washers.cpp

// Compute the rim area.

holeRadius = holeDiameter / 2.0;

edgeRadius = edgeDiameter / 2.0;

rimArea = PI * edgeRadius * edgeRadius -

PI * holeRadius * holeRadius;

// Compute the weight of a flat washer.

unitWeight = rimArea * thickness * density;

// Compute the weigh

weight = unitWeight * quantity;

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Washers.cpp

// Display the weight

cout << "The expected weight of the batch

is " << weight;

cout << " grams." << endl;

return 0;

}

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Washers.cpp

Program OutputInner Diameter in centimeters: 1.2

Outer Diameter in centimeters: 2.4

Thickness in centimeters: 0.1

Material density in grams per cubic centimeter: 7.87

Quantity in batch: 1000

The expected weight of the batch is 2670.23 grams

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3.2 Library Functions

– Goal of Structured Programming• Error free code

– Reusability• Don’t reinvent the wheel

– C ++ provides collection of functions

– Organized in Libraries• Library examples Table 3.1

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C++ Math Library

– Functions in the Math library• sqrt cos sin pow

• Examples Table 3.1

– Function use in Assignmentsy = sqrt (x);

sqrt is function name

x is function argument

– Activated by a “function call”– Result of execution is assigned to variable y

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C++ Math Library (cont)

Function sqrt as a “black box”

Square rootcomputation

X is 16.0 Result is 4.0

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C++ Library Functions

– We can effectively utilize existing functions by learning to read function prototypes with the preconditions and postconditions.

– Example prototype (or signature) double sqrt(double x) // PRE: x >= 0.0 // POST: Returns the square root of x.

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Preconditions and Postconditions

– Comments that represents a contract between the implementor of a function and the user (client) of that function.

– We'll look at two such comments:• Precondition: What the function requires.• Postcondition: What the function will do if

the precondition is met.

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Preconditions and Postconditions

– The preconditions are the circumstances that must be true before the function can successfully fulfill the promised postconditions.

– Example (Precondition abbreviates to PRE:double sqrt(double x);

// PRE: x >= 0

// POST: Returns square root of argument X

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SquareRoot.cpp

// File: squareRoot.cpp// Performs three square root computations

#include <cmath> // sqrt function#include <iostream> // i/o functionsusing namespace std;

int main(){ float first; float second; float answer;

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SquareRoot.cpp

// Get first number and display its square root. cout << "Enter the first number: "; cin >> first; answer = sqrt(first); cout << "The square root of the first

number is " << answer << endl;

// Get second number and display its square root. cout << "Enter the second number: "; cin >> second; answer = sqrt(second); cout << "The square root of the second

number is " << answer << endl;

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SquareRoot.cpp

// Display the square root of the sum of first// and second.

answer = sqrt(first + second); cout << "The square root of the sum of

both numbers is " << answer << endl;

return 0;}

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SquareRoot.cpp

Program Output

Enter the first number: 9The square root of the first number is 3Enter the second number: 16The square root of the second number is 4The square root of the sum of both numbers is 5

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3.3 Top-Down Design and Structure Charts

D raw aC irc le

D rawin te rsec tin g

lin es

D raw ab ase

D raw aTrian g le

D rawin te rsec tin g

lin es

D raw afig u re

Original Problem

Detailed

subproblems

Level 0

Level 1

Level 2

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3.4 Functions without Arguments

– Functions used in Top-Down Design– main() is just a function

• called by OS

– C++ program is a collection of Functions• top level function is called the main()• lower level functions

– User Defined or Libraries– Example StkFigMn.cpp

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StickFigure.cpp

// File: stickFigure.cpp// Draws a stick figure

#include <iostream>

using namespace std;

// Functions used ...

void drawCircle(); // Draws a circlevoid drawTriangle(); // Draws a trianglevoid drawIntersect(); // Draws intersecting linesvoid drawBase(); // Draws a horizontal line

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StickFigure.cpp

int main(){// Draw a circle. drawCircle();

// Draw a triangle. drawTriangle();

// Draw intersecting lines. drawIntersect();

return 0;}

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Function Calls

– We can call a function and get results without knowing the implementation of that function.• pow(x, y) returns x to the yth power.

– For now, we need not know exactly how a function is implemented.

– However, we do need to know how to use the function.

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Function Calls

– This general form of a function call:

function-name ( optional argument-list );– Example function call:

drawCircle ();– The function name is drawCircle – No arguments to the function

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Function Prototype

– This general form of a function prototype:

type function-name ( optional argument-list );– Example function prototype:

void skipThree ();– Type

• int - float - char– Name– ( );– Descriptive comment

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Function Definition

– General form of a function definition:

type function-name ( optional argument-list )

{

local-declarations - function body

executable-statements

}– Example function definition:

void drawTriangle ()

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Function Definition

void drawTriangle ()

{

// Draw a triangle.

drawIntersect ();

drawBase ();

}

function header

function body

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StickFigure.cpp

// Draws a circlevoid drawCircle(){ cout << " * " << endl; cout << " * *" << endl; cout << " * * " << endl;} // end drawCircle

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StickFigure.cpp

void drawCircleChar(char symbol){ cout << " " << symbol << " " << endl; cout << " " << symbol << " " << symbol <<

endl; cout << " " << symbol << " " << symbol <<

endl;}

// Draws a trianglevoid drawTriangle(){ drawIntersect(); drawBase();}

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StickFigure.cpp

// Draws intersecting lines

void drawIntersect(){ cout << " / \\ " << endl; cout << " / \\ " << endl; cout << " / \\" << endl;}

// draws a horizontal linevoid drawBase(){ cout << " -------" << endl;}

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Order of Execution

int main()

{

drawCircle();

drawTriangle();

drawIntersect();

return 0;

}

void drawCircle()

{

cout << “ * “ << endl;

cout << “ * * “ << endl;

cout << “ * * “ << endl;

}

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Function Advantages

– Program team on large project

– Simplify tasks

– Each Function is a separate unit

– Top-down approach

– Procedural abstraction

– Information hiding

– Reuse (drawTriangle)

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Abstraction

– Abstraction:• Refers to the act of ignoring details to

concentrate on essentials.• Allows us to use complicated things with

little effort (CD players, automobiles, computers).

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Displaying User Instructions

We still have not covered passing in and out of a function

Following example shows displaying info– instruct(); function call in main

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Instruct.cpp

// DISPLAYS INSTRUCTIONS TO THE USER // OF AREA/CIRCUMFERENCE PROGRAM

int instruct (){

cout << "This program computes the area and " <<

endl;cout << "circumference of a circle. " <<

endl << endl;

cout << "To use this program, enter the radius of the " <<

endl; cout << "circle after the prompt" << endl;

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Instruct.cpp

cout << "Enter the circle radius: " << endl << endl;

cout << "The circumference will be computed in the same ” << endl;

cout << "units of measurement as the radius. The area "

<< endl;cout << "will be computed in the same units

squared." << endl << endl;}

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Program Output

This program computes the area and circumference of a circle.

To use this program, enter the radius of the circle after the prompt

Enter the circle radius:

The circumference will be computed in the same units of measurement as the radius. The area will be computed in the same units squared.

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3.5 Functions with Input Arguments

– Functions used like building blocks

– Build systems one functions at a time• Stereo Components

– Use function return values and arguments to communicate between functions

– Discuss AreaMain.cpp• Flow of arguments and returns

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Function Call

Form: fname (actual arg list);

Example: scale (3.0, z);

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Function Return

Functions must return a value unless declared as void

Form: return expression;

Example: return x * y;

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Function Definition

Form: type fname (formal arg list){

function body

}Example: float scale(float x, int n)

{

float scaleFactor;

scaleFactor = pow(10, n);return (x * scaleFactor);

}

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Function Prototype

Form: type fname (formal arg type list);

Example: float scale (float x, int n);

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TestScale.cpp

// File testScale.cpp// Tests function scale. #include <iostream>#include <cmath>

using namespace std;

// Function prototype float scale(float, int);

int main(){

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TestScale.cpp

float num1; int num2;

// Get values for num1 and num2 cout << "Enter a real number: "; cin >> num1; cout << "Enter an integer: "; cin >> num2;

// Call scale and display result. cout << "Result of call to function scale

is " << scale(num1, num2) << endl;

return 0;}

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TestScale.cpp

float scale(float x, int n){ float scaleFactor; scaleFactor = pow(10, n); return (x * scaleFactor);}

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Argument / Parameter List Correspondence

– Functions can have more than 1 arg– Correspondence between Actual & Formal

arguments

Function call scale (3.0, z);

Actual Argument Formal Argument

3.0 x

z n

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Argument / Parameter List Correspondence

float scale(float x, int n)

{float scaleFactor;

scaleFactor = pow(10, n);return (x * scaleFactor);

}

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Argument / Parameter List Correspondence

Function call scale (x + 2.0, y);

Actual Argument Formal Argument

x + 2.0 x

y y

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Argument / Parameter List Correspondence

Function call scale (y, x);

Actual Argument Formal Argument

y x

x y

Watch for type matches in formal and actual arguments

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Key Points

The substitution of the value of an actual argument in a function call for its corresponding formal argument is strictly positional. That is, the value of the first actual argument is substituted for the first formal argument; the second and so on

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Key Points

The names of these corresponding pairs of arguments are no consequence in the substitution process. The names may be different, or they may be the same.

The substituted value is used in place of the formal argument at each point where that argument appears in the called function.

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3.6 Scope of Names

Variable declared in a function has a local scope within the function

Same for variables declared in the main Variable declared before main is global

scope– Call anywhere in program

Functions declared globally

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Scope of Names

Positional correspondence Type consistent is key because of positional

correspondence– Argument types– Return types

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Scope of Names

Type of a value returned by a called function must be consistent with the type expected by the caller as identified in the prototype

Type of an actual argument in a function call must be consistent with the type of its corresponding formal argument

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3.7 Extending C++ through Classes

– Discuss two classes• String class part of compiler• Money class is a user defined class

– #include <string>

– #include “money.h”

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String Class

Declaring string objects Reading & Displaying strings Assignment & Concatenation Operator Overloading Dot Notation Member Functions Object Assignments

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Declaring string Objects

A number of ways to declare

string firstName, lastName;

string wholeName;

string greeting = “Hello “;

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Reading & Displaying string Objects

Use extraction operator >> & the stream cin for input– cin >> firstName;

Use insertion operator << & the stream cout for output– cout << greeting << wholeName <<

endl;

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Reading & Displaying string Objects

getline (cin, lastName, ‘\n’);

reads all characters typed in from the keyboard up to the new line into the string object

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StringOperations.cpp

// FILE: StringOperations.cpp// ILLUSTRATES STRING OPERATIONS#include <iostream>#include <string>using namespace std;

int main (){

string firstName, lastName; string wholeName; string greeting = "Hello ";

cout << "Enter your first name: "; cin >> firstName;

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StringOperations.cpp

cout << "Enter your last name: "; cin >> lastName;

// Join names in whole name wholeName = firstName + " " + lastName;

// Display results cout << greeting << wholeName << '!' << endl; cout << "You have " <<

(wholeName.length () - 1) << " letters in your name." <<

endl;

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StringOperations.cpp

// Display initials cout << "Your initials are " <<

(firstName.at(0)) << (lastName.at(0))

<< endl;return 0;

}

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StringOperations.cpp

Program outputEnter your first name: Caryn

Enter your last name: Jackson

Hello Caryn Jackson!

You have 12 letters in your name.

Your initials are CJ

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Assignment

Stores the first and last name– wholeName = firstName + “ “ +

lastName; Concatenation

– + joins the two objects together “ “ for string values not ‘ ‘

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Operator Overloading

+ normally means addition but with strings it means concatenation

The + can take on many meanings– Operator Overloading– C++ can have multi meanings to 1 operator– >> & << are overloaded operators– * / - == = all can be overloaded

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Dot Notation

Dot notation used to call an objects member functions

wholeName.length();– Applies member function length to string object

wholeName– Function returns the objects length– Table 3.3 lists some additional functions

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Money Class

Process money objects Two attributes

– dollars– cents

Why do this ?? Why not float ??

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Money Class

Allocate a money object– money creditLimit = 5000.00;

Assignments– taxAmount = salePrice * taxPercent/100.0;– finalCost = salePrice + taxAmount;

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MoneyDemo.cpp

// FILE: MoneyDemo.cpp// ILLUSTRATES MONEY OPERATIONS

#include <iostream>#include "money.h" using namespace std;

int main (){

// Local declarations const float taxPercent = 6.25; money creditLimit = 5000.00;

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MoneyDemo.cpp

money salePrice; money taxAmount; money finalCost; cout << "Enter item price: ";cin >> salePrice;

// Compute tax amount taxAmount = salePrice * taxPercent / 100.0;

// Compute final cost finalCost = salePrice + taxAmount;

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MoneyDemo.cpp

// Display sales receipt cout << "Sales receipt" << endl; cout << "Price " << salePrice << endl; cout << "Tax " << taxAmount << endl; cout << "Paid " << finalCost << endl <<

endl; // Compute new credit limit

cout << "Your initial credit limit is " << creditLimit << endl;

creditLimit = creditLimit - finalCost; cout << "Your new credit limit is " <<

creditLimit << endl;return 0;

}

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MoneyDemo.cpp

Program outputEnter item price: 345.77

Sales Receipt

Price $345.77

Tax $21.61

Paid $367.38

Your initial credit limit is $5,000.00

Your new credit limit is $4,632.62

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Header and Implementation Files

Header file (.h)– File containing information that C++ needs to

compile a program that accesses a user-defined class

Implementation file (.cpp)– File containing the C++ code for the operators

of a user defined class

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3.8 Common Programming Errors

– Semicolon in Function Prototype (Declaration)– Inconsistencies in Number of Arguments

• Too few arguments in a call

• Too many arguments in a call

• Incorrect number of arguments in call

• Extra argument in call

– Argument Mismatch• Correct position (formal & actual params)

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Common Programming Errors

– Function Prototype & Definition Mismatches• Both are the same except for the ;

– Return Statements• “Return value expected”

• “Function should return value”

– Missing Object Name in Call to Member Function

– Missing #include– Type Mismatches