Top-Down Design with Functions and Classes

By Dr. Awad Khalil

Computer Science & Engineering Department

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3.1 Building Programs from Existing Information

Reuse of existing programsDevelop program in stagesCompile along the wayKeep it smallComments 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 -

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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 ProgrammingError free code

ReusabilityDon’t reinvent the wheel

C ++ provides collection of functionsOrganized in Libraries

Library examples Table 3.1

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C++ Math Library

Functions in the Math librarysqrt cos sin powExamples Table 3.1

Function use in Assignmentsy = sqrt (x);sqrt is function namex 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 Designmain() is just a function

called by OSC++ program is a collection of Functions

top level function is called the main()lower level functions

User Defined or LibrariesExample 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

bodyexecutable-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 projectSimplify tasksEach Function is a separate unitTop-down approachProcedural abstraction Information hidingReuse (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 blocksBuild systems one functions at a time

Stereo ComponentsUse function return values and arguments

to communicate between functionsDiscuss 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 argCorrespondence 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

scopeCall anywhere in program

Functions declared globally

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Scope of Names

Positional correspondence Type consistent is key because of positional

correspondenceArgument typesReturn 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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void one (ont anArg, double second); // prototype 1int funTwo (int one, char anArg); // prototype 2//const int MAX = 950;const int limit = 200;//int main (){

int localVar. . . . . . . . .

} // end main//void one(int anArg, double second} // header one{

int oneLocal; // local variable …………….} // end one//int funTwo (int one, char anArg) // header

funTwo{

int localVar; // local variable…………….

} // end funTwo

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3.7 Extending C++ through Classes

Discuss two classesString class part of compilerMoney 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 >> and the stream

cin for inputcin >> firstName;

Use insertion operator << and the stream cout for outputcout << 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 namewholeName = 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 meaningsOperator OverloadingC++ 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 wholeNameFunction returns the objects lengthTable 3.3 lists some additional functions

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Money Class

Process money objects Two attributes

dollarscents

Why do this ?? Why not float ??

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Money Class

Allocate a money objectmoney 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 callToo many arguments in a callIncorrect number of arguments in callExtra argument in call

Argument MismatchCorrect position (formal & actual params)

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Common Programming Errors

Function Prototype & Definition MismatchesBoth 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