Lecture 9: Top-Down Design with Functions COS120 Software Development Using C++ AUBG, COS dept

Lecture 9: Top-Down Design with Functions

COS120 Software Development Using C++ AUBG, COS dept

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Lecture Contents:

Top-Down Design and step-wise refinement with structure charts

Basics of functions. General form of a function definition

Functions topics: prototype, call, definition Systems defined and user defined functions The minimal C++ function Functions with multiple arguments

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Top-Down design

There exist problems whose solving algorithms are too complicated, more complex than those already solved. Then developer breaks up problem into sub problems to solve and develop program solution. If necessary some sub problems are to be broken up into sub sub problems and so on.

The process described is called Top-Down Design or step-wise refinement and is being illustrated using structure charts.

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Basics of functions:

“Properly designed functions permit to ignore how a job’s done. Knowing what is done is sufficient.”

B.Kernighan & D.Ritchie

Most important reasons to use functions:

1. Dividing a program into functions is one of the major principles of structured programming.

2. Using functions results in reduced program size.

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Basics of functions:

“A function provides a convenient way to encapsulate some computation, which can then be used without worrying about its implementation. ”

B.Kernighan & D.Ritchie

Usually functions are specified to be called many times.

You can see often a short function defined and called only once, just because it clarifies some piece of code.

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Details on functions

Three important topics when dealing with functions:

        prototype (signature) statement        function call statement         function definition (includes function

head or function title followed by a function body – compound statement surrounded in curly braces)

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Syntax: functions with no argument(s) and no return value

Prototype statement: void fname(void);

Call statement: fname( );

Function Definition: void fname(void){ <local declarations> <executable

statements>}

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Syntax: functions with argument(s) and no return value

Prototype: void fname(<formal parameters declaration list>);

Call: fname(<list of actual arguments>);

Definition:void fname(<formal parameters declaration list>){

<local variable declarations><executable statements>

}

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Syntax: functions with argument(s) and return value

Prototype: ftype fname(<formal parameters declaration list>);

Call: res = fname(<list of actual arguments>);cout << fname(<list of actual arguments>);

Definition:ftype fname(<formal parameters declaration list>){

<local variable declarations><executable statements>return (<expression>);

}

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The minimal C/C++ function

Minimal C++ function in 3 versions: it does nothing and returns “almost” nothing.

dummy() { }

dummy() { return 0; }

void dummy(void) { }

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Functions with multiple arguments

The accent:Formal parameter: variableFormal parameters declaration list: parameters, i.e. variables,

separated by comma

Actual argument: literal, variable, fun. call, expressionList of actual arguments: arguments, separated by comma.

Example: function to compute (return) the average of two integers on next slide

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Function to return average of two integers

Prototype in two versions:

double ave(int num1, int num2);

double ave(int, int);

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Function to return average of two integers

Function definition in two versions:

// 1st versiondouble ave(int num1, int num2){ double result; result = (num1 + num2) / 2.; return result;}

OR

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Function to return average of two integers

Function definition in two versions:

OR

// 2nd versiondouble ave(int num1, int num2){ return (num1 + num2) / 2.;}

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Function to return average of two integers

Function definition in two versions:double ave(int num1, int num2) // 1st version { double result; result = (num1 + num2) / 2.; return result;}

ORdouble ave(int num1, int num2) // 2nd version{ return (num1 + num2) / 2.;}

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Function to return average of two integers

Function call in variety of versions: Actual arguments: two literals

// value returned from ave directly displayed cout << endl << ave(30, 40);

// value returned from ave assigned to variable for use

double res; res = ave(50, 60); cout<<‘\n’<<“Average value is = “<< res;

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Function to return average of two integers

Function call in variety of versions: Actual arguments: two variables initialized// value returned from ave directly displayed int a=60, b=40; cout << endl << ave(a, b);

// value returned from ave assigned to variable for use

double res; int a=650, b=420; res = ave(a, b); cout << ‘\n’ << “Average value is = “ << res;

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Function to return average of two integers

Function call in variety of versions: Actual arguments: two variables loaded at runtime// value returned from ave directly displayed int a, b; cout <<“\nEnter 2 integers:”; cin >> a >> b; cout << endl << ave(a, b);

// value returned from ave assigned to variable for use

double res; int a, b; cout <<“\nEnter data:”; cin >> a >> b; res = ave(a, b); cout << ‘\n’ << “Average value is = “ << res;

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Task to train functions

Create and test a function to return average of three integers

Create and test a function to return average of five integers

Create and test a function to return average of seven integers– Solution as a single function with 7 parameters– Solution based on a function with 2 only parameters – Reuse

functions ave2(p1,p2) and ave5(p1,p2,p3,p4,p5)

Attention: Next slide of high importanceRead carefully, think, understand

&

Do not forget what is written!!!

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The Moral on function calling statement

If a function does not return a value, it’s called in the following context as a statement:

<fname>( <optional list of arguments> );

DrawHouse( );NumInBox(35.68);

If a function does return a value, it’s called in two other contexts: – As an operand of cout statement to display the value returned

cout << endl << ave(30,50);– As on operand in the right side of assignment statement

res = ave(60,70)*5 + pol/ave(56,88);

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Function to return average of two integers

Function call in variety of versions:Actual arguments:

– Literal and variable– ave(30, x);– Expression composed of literal and variables– ave(x+y, b-4);– Run-time library function calls– ave(int(sqrt(6+p)),(int)exp(var));

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Previous lecture reminder

Title:

Top-Down Design Using Functions

Source: Friedman/Koffman, Chapter 03

Have a quick look at next approx 60 slides to refresh your knowledge on functions

Top-Down Design with Functions and Classes

Chapter 3

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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 sinpow

• 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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How to continue?

Next 35 slides (37-70) contain general purpose info on functions already discussed in previous two lectures

You may read them as a refreshing reminder OR

You may skip them jumping to slide titled “3.7 Extending C++ through Classes”

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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 USER OF AREA/CIRCUMFERENCE PROGRAM

void instruct (){ cout << "This program computes the area and " << endl; cout << "circumference of a circle. " << endl << endl; cout << "To use this program, enter radius of the " << endl; cout << "circle after the prompt" << endl; cout << "Enter the circle radius: " << endl << endl; cout << "The circumference will be computed in the ” << endl; cout << "units of measurement as radius. The area " << endl; cout << "will be computed in thesame units squared." << 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 Parameter

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 Parameter

x + 2.0 x

y y

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Argument / Parameter List Correspondence

Function call scale (y, x);

Actual Argument Formal Parameter

y x

x y

Watch for type matches in formal parameters 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 classes• String class part of compiler

– #include <string>

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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 input–cin >> firstName;

Use insertion operator << and 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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Reading & Displaying string Objects

Other valid calls:

getline (cin, lastName);

getline (cin, lastName, ‘ ’);

getline (cin, lastName, ‘\t’);

getline (cin, lastName, ‘e’);

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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– >> and << are overloaded operators, see next slide– * / - == = all can be overloaded

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Operator Overloading

<< originally means shift left >> originally means shift right Here are more details on SHIFT operators:

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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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Header Files 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 params & actual args)

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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

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Exercise 9.1

Define a function and Build a program: to compute the volume of a pool

Design should include:– Function name– Return value data type– Number of parameters/arguments– Data type of parameters/arguments– Algorithm implementation

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Exercise 9.2

Define a function and Build a program: To return the product of a real and an

integer using the formula m*10n

Design should include:– Function name– Return value data type– Number of parameters/arguments– Data type of parameters/arguments– Algorithm implementation

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Exercise 9.3

Define a function and Build a program: to solve the Finding dollar value of coins

problem

Design should include:– Function name– Return value data type– Number of parameters/arguments– Data type of parameters/arguments– Algorithm implementation

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Exercise 9.4

Define a function and Build a program: to solve the Feet/Inches to Meters

conversion problem

Design should include:– Function name– Return value data type– Number of parameters/arguments– Data type of parameters/arguments– Algorithm implementation

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Homework 2

Paper record to be presented before the end of next class

Tasks:– See .doc file.

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reminder

Quiz #2

on

Skills in C++ Functions

will take place next lesson

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Before lecture end

Lecture:

Top-Down Design using Functions

More to read:

Friedman/Koffman, Chapter 03

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Thank You

For

Your Attention!