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Gentle Introduction to Programming

Session 5: Memory Model, Object Oriented Programming

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Review

• Recursive vs. Iterative • Arrays

• Arrays in memory• Initialization and usage• foreach, filter • Arrays as functions arguments• Multi-dimensional arrays• References to array

• Sorting, searching and time-complexity analysis• Binary search• Bubble sort, Merge sort

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Today• Home work review• Scala memory model• Guest lecture by Prof. Ronitt Rubinfeld 11:10• Object-oriented programming (OOP)

• Classes and Objects• Functional Objects (Rational Numbers example)

• Home work

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Exercise 1

Write a program that gets 10 numbers from the user.It then accepts another number and checks to see ifthat number was one of the previous ones.

Example 1:Please enter 10 numbers:1 2 3 4 5 6 7 8 9 10Please enter a number to search for: 8Found it!

Example 2:Please enter 10 numbers:1 2 3 4 5 6 7 8 9 10Please enter a number to search for: 30Sorry, it’s not there

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Solution FindNumber.scala

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Exercise 2

• Implement a function that accepts two integer arrays and returns true if they are equal, false otherwise. The arrays are of the same size

• Write a program that accepts two arrays of integers from the user and checks for equality

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SolutionCompareArrays.scala

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Solution (main)CompareArrays.scala

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Exercise 3

• Read, understand and implement selection/insertion sort algorithm• http://en.wikipedia.org/wiki/Selection_sort• http://en.wikipedia.org/wiki/Insertion_sort

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Selection Sort

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Solution

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Today• Home work review• Scala memory model• Guest lecture by Prof. Ronitt Rubinfeld• Object-oriented programming (OOP)

• Classes and Objects• Functional Objects (Rational Numbers example)

• Home work

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Passing Arguments to Functions

• When a function is called, arguments’ values are attached to function’s formal parameters by order, and an assignment occurs before execution

• Values are copied to formal parameters

• “Call by value”

• Function’s parameters are defined as vals

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Passing Arguments to Functions

• A reference is also passed by value• Example: arrays• Objects (?)• This explains why we can change an array’s

content within a function

4 5 6 7 8 9a

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Local Names

• Arguments names do not matter!

• Local variable name hides in-scope variables with the same name

Different x!

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Everything is an Object (in Scala)

• In Java: primitives vs. objects• In Scala everything is an Object• But: special treatment for primitives• Why do we care?

val x = 5

var y = x

y = 6

val ar1 = Array(1,2,3)

val ar2 = ar1

ar2(0) = 4?

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?

val x = 5

var y = x

y = 6

val ar1 = Array(1,2,3)

val ar2 = ar1

ar2(0) = 4

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Memory Image 1

val x = 5

var y = x

y = 6

x 5

y 5y

6

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Memory Image 2

val ar1 = Array(1,2,3)

val ar2 = ar1

ar2(0) = 4

ar1 1 2 3

ar2

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Scala Memory Model

• Based on Java…• Stack: local variables and arguments, every

function uses a certain part of the stack• Stack variables “disappear” when scope ends

• Heap: global variables and object, scope independent• Garbage Collector

• Partial description

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How to Change a Variable via Functions?

• The arguments are passed as vals thus can not be changed

• So how can a method change an outer variable?• By its return value• By accessing heap-based memory (e.g., arrays)

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Today• Home work review• Scala memory model• Guest lecture by Prof. Ronitt Rubinfeld • Object-oriented programming (OOP)

• Classes and Objects• Functional Objects (Rational Numbers example)

• Home work

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Programming in Scala

Chapter 4: Classes and Objects Chapter 6: Functional Objects

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Singletone Objects

• All programs written so far in this course are Signletone objects

• File start with the reserved word object

• Contain functions that can be used elsewhere

• Application: singeltone object with a main function

• (Actually singeltone objects are more then that)• (Java programmers: think of it as a holder of static

methods)

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A Car

• How would you represent a car?• Parts / features: 4 wheels, steering wheel, horn,

color,…• Functionality: drive, turn left, honk, repaint,…

• In Scala???

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Object-Oriented Programming (OOP)

• Represent problem-domain entities using a computer language

• When building a software in a specific domain, describe the different components of the domain as types and variables

• Thus we can take another step up in abstraction

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Class as a BlueprintA class is a blueprint of objects

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Class as a BlueprintA class is a blueprint of objects

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Classes as Data Types

• Classes define types that are a composition of other types and have unique functionality

• An instance of a class is named an object• Every instance may contain:

• Data members / fields• Methods• Constructors

• Instances are accessed only through reference

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Examples

• String• Members: all private• Methods: length, replace, startsWith, substring,…• Constructors: String(), String(String),…• http://java.sun.com/j2se/1.5.0/docs/api/java/lang/String.html

• Array• Members: all private• Methods: length, filter, update,…• Constructors: initiate with 1-9 dimensions• http://www.scala-lang.org/docu/files/api/scala/Array.html

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Car Example

• Members: 4 wheels, steering wheel, horn, color,…

• Every car instance has its own

• Methods: drive, turn left, honk, repaint,…

• Constructors: Car(String color), Car(Array[Wheels], Engine,…), …

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Today• Home work review• Scala memory model• Guest lecture by Prof. Ronitt Rubinfeld• Object-oriented programming (OOP)

• Classes and Objects• Functional Objects (Rational Numbers example)

• Home work

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Rational Numbers

• A ration number is a number that can be expressed as a ration n/d (n,d integers, d not 0)

• Examples: 1/2, 2/3, 112/239, 2/1

• Not an approximation

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Specification• Add, subtract, multiply, divide

• println should work smoothly• Immutable (result of an operation is a new rational number)

• It should feel like native language support

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Constructing a Rational

• How client programmer will create a new Rational object?

Class parameters

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Constructing a Rational• The Scala compiler will compile any code placed in

the class body, which isn’t part of a field or a method definition, into the primary constructor

?

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Reimplementing toString

• toString method• A more useful implementation of toString would

print out the values of the Rational’s numerator and denominator

• override the default implementation

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Usage

• Now we can remove the debug println…

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Checking Preconditions

• Ensure the data is valid when the object is constructed

• Use require

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Define “add” Method

• Immutable• Define add:

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Add Fields• n, d are in scope in the add method• Access then only on the object on which add

was invoked

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Test Add, Access Fields

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Self Reference (this)

• Define method lessThan:

• Define method max:

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Auxiliary Constructors• Constructors other then the primary

• Example: a rational number with a denominator of 1 (e.g., 5/1 5)

• We would like to do: new Rational(5)• Auxiliary constructor first action: invoke

another constructor of the same class

• The primary constructor is thus the single point of entry of a class

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Revised Rational

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Private Fields and Methods

• 66/42 = 11/7• To normalize divide the numerator and

denominator by their greatest common divisor (gcd)• gcd(66,42) = 6 (66/6)/(42/6) = 11/7• No need for Rational clients to be aware of this• Encapsulation

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Off Topic: Calculate gcd• gcd(a,b) = g

• a = n * g• b = m * g• gcd(n,m)=1(otherwise g is not the gcd)• a = t * b + r = t * m * g + r g is a divisor of r

• gcd(a,b) = gcd(b,a%b)• The Euclidean algorithm: repeat iteratively:

if (b == 0) return aelse repeat using a b, b a%b

• http://en.wikipedia.org/wiki/Euclidean_algorithm

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Correctness• Example:

gcd(40,24) gcd(24,16) gcd(16,8) gcd(8,0) 8

• Prove: g = gcd(a,b) = gcd(b,a%b)= g1• g1 is a divisor of a ( g1 ≤ g)• There is no larger divisor of a ( g1 ≥ g)

• ≤ : a = t * b + r a = t * h * g1 + v * g1 g1 is a divisor of a

• ≥ : assume g > g1 a = t * b + r g is a divisor of b and r contradiction

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Implementation

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Revised Rational

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Defining Operators• Why not use natural arithmetic operators?

• Replace add by the usual mathematical symbol

• Operator precedence will be kept

• All operations are method calls

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Revised Rational

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Usage

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Method Overloading• Now we can add and multiply rational numbers!• What about mixed arithmetic?

• r * 2 won’t work • r * new Rational(2) is not nice

• Add new methods for mixed addition and multiplication

• Method overloading• The compiler picks the correct overloaded

method

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Usage

• The * method invoked is determined in each case by the type of the right operand

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Revised Rational

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Implicit Conversions

• 2 * r 2.*(r) method call on 2 (Int) Int class contains no multiplication method that takes a Rational argument

• Create an implicit conversion that automatically converts integers to rational numbers when needed

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Companion Object

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Revised Rational

• Define implicit conversion in Rational.scala, after defining object Rational

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In Eclipse

• In Rational.scala:• Companion object

(object Rational)• Rational class (class

Rational)

• Place the main method in another file

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Summary• Customize classes so that they are natural

to use• fields, methods, primary constructor• Method overriding• Self reference (this)• Define several constructors• Encapsulation• Define operators as method• Method overloading• Implicit conversions, companion object

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Today• Home work review• Scala memory model• Guest lecture by Prof. Ronitt Rubinfeld• Object-oriented programming (OOP)

• Classes and Objects• Functional Objects (Rational Numbers example)

• Home work

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Exercise 1• Implement class Complex so it is natural to use

complex numbers• Examples: