Intro to Computer Science Arrays

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Intro to Computer Science Arrays

• Instructor Ms. Catherine Stocker
• Teaching Assistants Alex, Katie, Siraaj,
  Isaiah, Allison, Thibault
• University of Pennsylvania
• 26 March 2008

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Todays Agenda

• Discussion (Midterm and Quizzes)
• Arrays
• Arrays Lab
• Implementing Arrays

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Discussion/Demo

• Lets talk about the Midterm and Quizzes

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What if we wanted to store the names of all
players on a team?

• public class Team
• public String player1
• public String player2
• public String player3
• public String player4
• public String player5
• public String player6
• public String player7
• public String player8
• public String player9
• public String player10
• public String player11
• . . .

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... or to store the grades of all students in a
course?

• public class Grades
• public String className
• public int grade1
• public int grade2
• public int grade3
• public int grade4
• public int grade5
• public int grade6
• public int grade7
• public int grade8
• public int grade9
• public int grade10
• public int grade11
• public int grade12
• public int grade13
• public int grade14
• public int grade15
• . . .

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... or to keep track of all 2,496 of your
facebook friends?

• public class FacebookAccount
• public String name
• public Person friend1
• public Person friend2
• public Person friend3
• public Person friend4
• public Person friend5
• public Person friend6
• public Person friend7
• public Person friend8
• public Person friend9
• public Person friend10
• public Person friend11
• public Person friend12
• public Person friend13
• public Person friend14
• public Person friend15
• public Person friend16
• public Person friend17

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Arrays

• An array is a container object that holds a fixed
  number of values of a single type.
• The length of an array is established when the
  array is created. After creation, its length is
  fixed.
• Each item in an array is called an element, and
  each element is accessed by its numerical index.

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Counting in Java

• Array indices always start at 0!
• There is a such thing as a 0th element.
• You can only count up to array length - 1.
• Anything else is Out of Bounds.
• While using arrays, you will see a lot of these
• ArrayIndexOutOfBoundsException at
  java.lang.reflect.Array.get(Native Method)
• An Array of size 15 has what indices?

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Declaring a Variable to Refer to an Array

• An array's type is written as type, where type
  is the data type of the contained elements.
• int data // declares an array of integers.
• Similarly, you can declare arrays of other types
• double prices
• boolean verdicts
• char grades
• String names

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Creating, Initializing, and Accessing

• One way to create an Array is with the new
  operator.
• int data new int5
• creates an array of size 5, that contains
  integers
• These lines assign values to each element of the
  array
• data0 100
• data1 200
• data2 300
• data3 400
• data4 500
• NOTE An Array of Size 5 has indices 0 through
  4.
• This syntax creates and initializes an array all
  at once.
• int data 100, 200, 300, 400, 500

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Using Array Elements

• An element of an array of ints can be used just
  like other integers.
• The same goes for arrays of other types.
• What is the value of z after we execute these
  lines?int data 100, 200, 300, 400,
  500int x data0 data1int y data4
  - data3data2 data3int z x y

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Accessing an Arrays Length

• int datadata new int6
  data.length is 6
• Why is length important?for(int i 0 i lt
  data.length i) datai i 2

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Arrays Of Objects

• Remember the Student Array Game!
• PennStudents and SLAStudents are both allowed
  into an Array of Students.
• But SLAStudents are not allowed into an Array of
  PennStudents (and vice versa)
• You can perform operations and methods on the
  elements of the array.
• Changes to the state of an Object while it is in
  the array remain even if the Object is removed.
• The default value of an Object is null!

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Example Student, PennStudent, SLAStudent

• public abstract class Student abstract
  String getName() abstract String
  getColor() abstract boolean
  changeColor()
• public class PennStudent extends Student
  ...
• public class SLAStudent extends Student
  ...

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Sample Interactions

• gt PennStudent isaiah new PennStudent("Isaiah
  Greene", "green")
• gt SLAStudent itamar new SLAStudent("Itamar
  Ben-Amos", "blue")
• gt Student students new Student2
• gt students0 isaiah
• Isaiah Greene is a Penn Student.
• gt students0.getName()
• "Isaiah Greene"
• gt students1 itamar
• Itamar Ben-Amos is an SLA Student.
• gt students1.getName()
• "Itamar Ben-Amos"
• gt PennStudent pennStudents new PennStudent2
• gt pennStudents0 isaiah
• Isaiah Greene is a Penn Student.
• gt pennStudents0.getName()
• "Isaiah Greene"
• gt pennStudents1 itamar
• Error Bad types in assignment

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Sample Interactions cont.

• gt pennStudents0.changeColor("purple")
• true
• gt pennStudents0 null
• null
• gt pennStudents0
• null
• gt isaiah.getColor()
• "purple"

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2D Array Tic Tac Toe
0 1 2
0 X O X
1 X O O
2 O X X
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2D Arrays
int anArray new int33 anArray00
100 anArray01 200 anArray02
300 anArray10 400 anArray11
500 anArray12 600 anArray20
700 anArray21 800 anArray22 900