46-699 Object-oriented Programmingwww.andrew.cmu.edu/course/46-699/

Ananda GunaMay 17, 1999 - Lecture #1

guna@ cs.cmu.eduSchool of Computer ScienceCarnegie Mellon University

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Administrivia

Web page, schedule, text TA’s: Ramu Arunachalam(Pgh) and TBA (NYC) Due dates Final exam Recitation hours Software

– MSVC++ (5.0)– Metrowerks Codewarrior – Any other compiler e.g. Borland– g++

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C++: Language Background

Designed “on top of” C– C plus OOP features, hence the name C++

Implications– Almost all of the C features still work– Some features were simplified, notably the I/O– Many people claim to program in C++, but never

really use its OOP features - also called “a better C”

– Most compilers support C/C++ The challenge of learning C++ is in the paradigm

shift Java is based on C++

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Review

Variables Data types

– Primitives: int, double, float, char... Conditionals (if, if-else, switch) Repetition (for, while loops) General structure of a C program Functions

– Prototypes– Arguments/parameters– Call by value

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Review

All variables must be declared before use– Must specify the type of your variable – Compiler allocates memory – e.g. int count=0; float num1= 0.0, num2 = 0.0;– Always initialize your variables or they contain

garbage Conditionals: Used to control execution

– if– if-else, nested if– switch

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Conditionals if ( boolean condition true){ if(( boolean condition true){

statements; if (boolean condition true){

} ...

}

} if ( boolean condition true){ if ( boolean condition true){

statements; statement

} else { } else if (boolean condition true){

statements; statements;

} } else if (...)

switch(varName){ //int varName

case 1: statements;

break; // REQUIRED

case 2,3,4: statements;

break; // REQUIRED

default: debug statement;

}

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Repetition

Loops– for: Used when we know the # iterations– while: We don’t know the exact # iterations, but we

know the exit condition– do while: Executes the loop body at least once, test is at the

bottom

for ( i=0; i< MAX; i++){

//loop body statements;

} while (i < MAX){ // assume i is initialized

//loop body statements; //Be sure to update loop control variable

} do while{

//loop body statements; // Be sure to update loop control variable

} while (i < MAX);

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Functions

Units or modules within a program Each performs a specific task Allow for cleaner programs, promote reuse. May be

– system-defined. E.g. the math library functions– User defined tailored to the application

Must be called or invoked to be executed Prototypes are optional if the function implementations

appear before the function call. However, it is good style to use function prototypes

Can pass arguments to each other Can return a value upon exit(Single return functions)

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Scope of a variable

The segment of the program where it can be “seen” A local variable’s scope is the block within which it

is declared plus any nested blocks A global variable’s scope is the entire program Lifetime of variables:

– Locals die upon exit of a function– Globals are “alive” throughout the “life” of the

program It is bad style to use global varibles. Why? Global constants are fine. Why?

– E.g. const int MAX = 10; // same as #define in C

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Passing Arguments/parameters

Call by value:– A copy of the variable is passed

Call by reference:– The address of the variable is passed– The called function has direct access to the

variable Arguments

– Must match in number– Must match in type– The names do not matter, what matters is the

order of the arguments, i.e. the first is mapped to the first and so on

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Call by value example

void print ( int x, float y){

cout << x << “ “ << y << endl;

}

int

main(){

int x = 10;

float y= 22.7;

print(x, y);

return 0;

}

// What will be the output for print(y,x)?

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22.7

10 22.7

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Call by reference example

void swap ( int& a, int& b){ // the addresses are passed

int temp;

temp = a;

a = b; // change a ==> change x

b = temp; // change b ==> change y

cout << a << “ “ << b << endl;

}

int

main(){

int x = 7;

int y= 55;

swap(x, y); // Note: NO & is required here unlike C

cout << x << “ “ << y << endl;

return 0;

}

x y557

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Arrays

Group/collection of memory cells Each cell must be of the same type Declaring an array in C++:

– const int MAX = 30; // #define in C– float stocksPrices[MAX];

[0] [1] [MAX - 1]

- Subscript/index begins at zero- stocksPrices[0] is the first element- Every element is of type float

98.30 51.5

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Passing arrays as arguments

void multArray(float arr[], int count){

// no & for array arguments - as in C

int i;

for ( i = 0; i < count; i++){

arr[i] *= 10.2; // change to the original array

}

return sum;

} //multArray

// function call

multArray(arr, count); // assume arr has been loaded

// count is the # items in the arraycout << "\narr[0] ” << setiosflags(ios :: fixed|ios ::showpoint)

<< setw(10) << setprecision(2)<< arr[0];

//output with two decimal places

// need #include <iomanip>

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const array arguments

void multArray(const float arr[], int count){

// no & for array arguments - as in C

int i;

for ( i = 0; i < count; i++){

arr[i] *= 10.2; // change to the original array

}

} //multArray

const means the function can use, but may not change the array

The above will throw a compile error - at which line? Use const to protect against accidently changing the array

values– Printing the contents of an array– Using the array to do some computation

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Input/output in C++

To read from the console: cin– cin >> varName; // read into this variable– cin >> varName1 >> varName2;

– >> is called the stream extraction operator To print to the console

– cout << varName; // dump contents of varName– cout << varName1 << varName2 << endl;– cout << varName1 + varName2 << “\n”;

– << is called the stream insertion operator– endl: end of line (newline plus flush the output buffer)

Much simpler and cleaner than C! Sample program

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File I/O in C++

Sequential Access Files - input– programmer must create/recognize structure– ifstream infile(filename, ios::in)– if (!infile) { cerr<<“File cannot be open”;– exit(1);}– infile >> date >> maxprice >> minprice >> close;

File output– ofstream outfile(filename, ios::out)– outfile << varName1 << varName2 << endl;– if no file then a file is created

– << is called the stream insertion operator– >> is called the stream extraction operator

Much simpler and cleaner than C!

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File I/O in C++ ctd...

ifstream infile(“infile.txt”); is ok too ofstream outfile(“outfile.out”) is ok too In each case object constructor is called logical file physical file alternative style

– ofstream outfile;– outfile.open(“filename”,ios::out);

Reading a file of data to eof– while (infile>>var1>>var2…) { process}– while (!infile.eof()) { infile>>var1>>var2>>…}

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File I/O in C++ ctd...

You may use #include “Openfile.h” utility ifstream infile; OpenFileForReading(“Enter input file “ , infile); ofstream outfile; OpenFileForWriting(“Outfile ? “ , outfile); infile.close(); outfile.close(); // explicit close File stream is also destroyed upon leaving the

scope.

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Problem Solving Process

Review:– Algorithm– Top-down design, stepwise refinement– Pseudocode– Flow Chart

Phases of software development– Planning: figuring out the problem specs– Design: the algorithm, use pseudocode etc – Code: Translate the design into C++/Java syntax– Code review: “Spell-check” your code. Simulate the

compiler looking for syntax errors– Compile: With a thorough code review, compile time is small– Test/debug

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System Costs: Hardware vs. Software*

100%

1950 2000

Software

Hardware

PercentSystemCost

Year

* courtesy R.Pattis

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Motivation for Design before Coding

Analogy: Think about how you write a paper– Problem Statement? Outline? Abstract? Fill in details

based on the outline? Refine sections/paragraphs? How do you proof your paper? Do you just type something and watch the spell-checker or do you print it out and read it for errors?

Your solution is only as good as your design Coding should be low priority compared to design If you take short cuts, you will end up spending a lot

more time on the program. Bottom Line: There is nothing mysterious about

coding!!!

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OOP Versus Non-OOP

Procedural programming OOP

- Identify tasks/sub-tasks - Design the classes- Write procedures - Design the methods- Procedures act upon data - Objects communicate to solve to solve the problem the problem- Start with “verbs” - Start with “nouns”- Data is secondary - Data is primary

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Problem Solving - OO Design

Identify the objects Identify the operations Design the algorithm/decompose into smaller chunks E.g: Compute the amount of tuition to be paid Name the objects

– # credit hours– cost per credit hour– Other costs, such as student activity fee etc– Any discounts

Name the operations to be performed:– Addition, subtraction, multiplication

Algorithm

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Key Elements of OOP

Abstraction: Focus on the important, ignore the details– e.g. Driving a car

» The interface of the car is what the driver sees on the outside, i.e. the dash board, the gas pedal and so on.

» The implementation is a detail, e.g. what exactly happens when I step on the gas pedal.

Encapsulation: Information hiding– The details of an object are hidden. – Users are not allowed to make changes to the

implementation.– If there is any change in the implementation, it does not

affect the users(clients) of the interface. – E.g. A Stack Class has as its interface: init, push, pop. The

implementation may be changed from an array to a linked list

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Key Elements of OOP

Inheritance– Building upon what is already defined– A class can inherit from another class much like a

child inherits from its parent

Polymorphism– Greek for “many shapes”– Same name for different things– Two forms in OOP

» Overloading: Functions with the same name» Over-riding: A child class redefining its parent’s

functions

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OOP Terminology

Program = objects cooperating to solve a problem

Properties of an object:

» State ==> data ==> nouns» Operations ==> behavior ==> verbs» Identity

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Object Examples

Example 1: An electronic mailbox

» States: full or empty» Operations: add, delete ...» Identity: any 2 mailboxes will be different

e.g. hou and aj29 each have a mail box

Example 2: traffic signal

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Object Components

An object is composed of both data and Operations which can act upon the data

Data

Operations

In a C++ object: Operations ==> member functions (methods) Data ==> data members

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Classes

Class:– A collection of related objects. – Instance of a class = object– Factory with blueprints/instructions to build gadgets– Objects: the gadgets the factory makes.

Example:– Class : checkingaccount– Objects: minnieCheckacct, mickeyCheckacct

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Example: OOP Design

Problem: write a program to simulate an ATM Data (nouns):

– Account name– Account balance– ...

Operations /behavior(verbs):

– Deposit

– Withdraw

– Inquire balance

– ...

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Problem ( Elevator Control Problem)

Simulation of an elevator control system. 20 floors , 4 elevators1. Identify the data objects2. Identify the data operations3. Solve the problem

Read Chapter 6-7 (Dietel Text) Any questions Email : guna@cs.cmu.edu