Arrays

1
Arrays

• Arrays are objects that help us organize large
  amounts of information
• Chapter 6 focuses on
• array declaration and use
• passing arrays and array elements as parameters
• arrays of objects
• sorting elements in an array
• multidimensional arrays
• the ArrayList class

2
Arrays

• An array is an ordered list of values

0 1 2 3 4 5 6 7 8
9
An array of size N is indexed from zero to N-1
This array holds 10 values that are indexed from
0 to 9
3
Arrays

• A particular value in an array is referenced
  using the array name followed by the index in
  brackets
• For example, the expression
• scores2
• refers to the value 94 (the 3rd value in the
  array)
• That expression represents a place to store a
  single integer and can be used wherever an
  integer variable can be used

4
Arrays

• For example, an array element can be assigned a
  value, printed, or used in a calculation
• scores2 89
• scoresfirst scoresfirst 2
• mean (scores0 scores1)/2
• System.out.println ("Top " scores5)

5
Arrays

• The values held in an array are called array
  elements
• An array stores multiple values of the same type
  (the element type)
• The element type can be a primitive type or an
  object reference
• Therefore, we can create an array of integers, or
  an array of characters, or an array of String
  objects, etc.
• In Java, the array itself is an object
• Therefore the name of the array is a object
  reference variable, and the array itself must be
  instantiated

6
Declaring Arrays

• The scores array could be declared as follows
• int scores new int10
• The type of the variable scores is int (an
  array of integers)
• Note that the type of the array does not specify
  its size, but each object of that type has a
  specific size
• The reference variable scores is set to a new
  array object that can hold 10 integers
• See BasicArray.java (page 322)

7
BasicArray.java

• public class BasicArray
• final static int LIMIT 15
• final static int MULTIPLE 10
• public static void main (String args)
• int list new intLIMIT
• for (int index 0 index
• listindex index MULTIPLE
• list5 999 // change one array value
• for (int index 0 index
• System.out.print (listindex " ")
• System.out.println ()

8
Declaring Arrays

• Some examples of array declarations
• float prices new float500
• boolean flags
• flags new boolean20
• char codes new char1750

9
Bounds Checking

• Once an array is created, it has a fixed size
• An index used in an array reference must specify
  a valid element
• That is, the index value must be in bounds (0 to
  N-1)
• The Java interpreter throws an ArrayIndexOutOfBoun
  dsException if an array index is out of bounds
• This is called automatic bounds checking

10
Bounds Checking

• For example, if the array codes can hold 100
  values, it can be indexed using only the numbers
  0 to 99
• If count has the value 100, then the following
  reference will cause an exception to be thrown
• System.out.println (codescount)
• Its common to introduce off-by-one errors when
  using arrays

for (int index0 index index) codesindex index50 epsilon
11
Bounds Checking

• Each array object has a public constant called
  length that stores the size of the array
• It is referenced using the array name
• scores.length
• Note that length holds the number of elements,
  not the largest index
• See ReverseOrder.java (page 324)
• See LetterCount.java (page 326)

12
ReverseOrder.java

• import cs1.Keyboard
• public class ReverseOrder
• public static void main (String args)
• double numbers new double10
• System.out.println ("The size of the array
  " numbers.length)
• for (int index 0 index index)
• System.out.print ("Enter number "
  (index1) " ")
• numbersindex Keyboard.readDouble()
  
• System.out.println ("The numbers in reverse
  order")
• for (int index numbers.length-1 index
  0 index--)
• System.out.print (numbersindex "
  ")
• System.out.println ()

13
LetterCount.java

• import cs1.Keyboard
• public class LetterCount
• public static void main (String args)
  
• final int NUMCHARS 26
• int upper new intNUMCHARS
• int lower new intNUMCHARS
• char current // the current character being
  processed
• int other 0 // counter for non-alphabetics
  
• System.out.println ("Enter a sentence")
• String line Keyboard.readString()
• for (int ch 0 ch
• current line.charAt(ch)
• if (current 'A' current uppercurrent-'A'
• else if (current 'a' current lowercurrent-'a'
• else other // Print the
  results
• System.out.println ()
• for (int letter0 letter letter)
• System.out.print ( (char) (letter 'A') )
  
• System.out.print (" " upperletter)
  

14
Alternate Array Syntax

• The brackets of the array type can be associated
  with the element type or with the name of the
  array
• Therefore the following declarations are
  equivalent
• float prices
• float prices
• The first format generally is more readable

15
Initializer Lists

• An initializer list can be used to instantiate
  and initialize an array in one step
• The values are delimited by braces and separated
  by commas
• Examples
• int units 147, 323, 89, 933, 540,
• 269, 97, 114, 298, 476
• char letterGrades 'A', 'B', 'C', 'D', F'

16
Initializer Lists

• Note that when an initializer list is used
• the new operator is not used
• no size value is specified
• The size of the array is determined by the number
  of items in the initializer list
• An initializer list can only be used only in the
  array declaration
• See Primes.java (page 330)

17
Primes.java Author Lewis/Loftus

• public class Primes
• public static void main (String args)
• int primeNums 2, 3, 5, 7, 11, 13, 17,
  19
• System.out.println ("Array length "
  primeNums.length)
• System.out.println ("The first few prime
  numbers are")
• for (int scan 0 scan scan)
• System.out.print (primeNumsscan "
  ")
• System.out.println ()

18
Arrays as Parameters

• An entire array can be passed as a parameter to a
  method
• Like any other object, the reference to the array
  is passed, making the formal and actual
  parameters aliases of each other
• Changing an array element within the method
  changes the original
• An array element can be passed to a method as
  well, and follows the parameter passing rules of
  that element's type

19
Arrays of Objects

• The elements of an array can be object references
• The following declaration reserves space to store
  25 references to String objects
• String words new String25
• It does NOT create the String objects themselves
• Each object stored in an array must be
  instantiated separately
• See GradeRange.java (page 332)

20
GradeRange.java Author Lewis/Loftus

• public class GradeRange
• public static void main (String args)
• String grades "A", "A-", "B", "B",
  "B-", "C", "C", "C-",
• "D", "D", "D-", "F"
• int cutoff 95, 90, 87, 83, 80, 77, 73, 70,
  67, 63, 60, 0
• for (int level 0 level level)
• System.out.println (gradeslevel "\t"
  cutofflevel)

21
Command-Line Arguments

• The signature of the main method indicates that
  it takes an array of String objects as a
  parameter
• These values come from command-line arguments
  that are provided when the interpreter is invoked
• For example, the following invocation of the
  interpreter passes an array of three String
  objects into main
• java StateEval pennsylvania texas arizona
• These strings are stored at indexes 0-2 of the
  parameter
• See NameTag.java (page 334)

22
NameTag.java Author Lewis/Loftus

• public class NameTag
• public static void main (String args)
• System.out.println ()
• System.out.println (" " args0)
• System.out.println ("My name is "
  args1)
• System.out.println ()

23
Arrays of Objects

• Objects can have arrays as instance variables
• Many useful structures can be created with arrays
  and objects
• The software designer must determine carefully an
  organization of data and objects that makes sense
  for the situation
• See Tunes.java (page 335)
• See CDCollection.java (page 337)
• See CD.java (page 340)

24
Tunes.java Author Lewis/Loftus

• public class Tunes
• public static void main (String args)
• CDCollection music new CDCollection ()
• music.addCD ("Storm Front", "Billy Joel",
  14.95, 10)
• music.addCD ("Come On Over", "Shania
  Twain", 14.95, 16)
• music.addCD ("Soundtrack", "Les
  Miserables", 17.95, 33)
• music.addCD ("Graceland", "Paul Simon",
  13.90, 11)
• System.out.println (music)
• music.addCD ("Double Live", "Garth Brooks",
  19.99, 26)
• music.addCD ("Greatest Hits", "Jimmy
  Buffet", 15.95, 13)
• System.out.println (music)

25
CDCollection.java 1

• import java.text.NumberFormat
• public class CDCollection
• private CD collection
• private int count
• private double totalCost
• public CDCollection ()
• collection new CD100
• count 0
• totalCost 0.0
• public void addCD (String title, String
  artist, double cost, int tracks)
• if (count collection.length)
  increaseSize()
• collectioncount new CD (title, artist,
  cost, tracks)
• totalCost cost
• count

26
CDCollection.java 2

• public String toString ( )
• NumberFormat fmt NumberFormat.getCurrencyIn
  stance()
• String report "
  \n"
• report "My CD Collection\n\n"
• report "Number of CDs " count "\n"
• report "Total cost "
  fmt.format(totalCost) "\n"
• report "Average cost "
  fmt.format(totalCost/count)
• report "\n\nCD List\n\n"
• for (int cd 0 cd collectioncd.toString() "\n"
• return report
• private void increaseSize ( )
• CD temp new CDcollection.length 2
• for (int cd 0 cd cd) tempcd collectioncd
• collection temp

27
CD.java Author Lewis/Loftus

• import java.text.NumberFormat
• public class CD
• private String title, artist
• private double cost
• private int tracks
• public CD (String name, String singer, double
  price, int numTracks)
• title name
• artist singer
• cost price
• tracks numTracks
• public String toString()
• NumberFormat fmt NumberFormat.getCurrencyIn
  stance()
• String description
• description fmt.format(cost) "\t"
  tracks "\t"
• description title "\t" artist
• return description

28
Sorting

• Sorting is the process of arranging a list of
  items in a particular order
• The sorting process is based on specific value(s)
• sorting a list of test scores in ascending
  numeric order
• sorting a list of people alphabetically by last
  name
• There are many algorithms for sorting a list of
  items
• These algorithms vary in efficiency
• We will examine two specific algorithms
• Selection Sort
• Insertion Sort

29
Selection Sort

• The approach of Selection Sort
• select a value and put it in its final place into
  the list
• repeat for all other values
• In more detail
• find the smallest value in the list
• switch it with the value in the first position
• find the next smallest value in the list
• switch it with the value in the second position
• repeat until all values are in their proper places

30
Selection Sort

• An example
• original 3 9 6 1 2
• smallest is 1 1 9 6 3 2
• smallest is 2 1 2 6 3 9
• smallest is 3 1 2 3 6 9
• smallest is 6 1 2 3 6 9
• See SortGrades.java (page 342)
• See Sorts.java (page 343) -- the selectionSort
  method

31
SortGrades.java Author Lewis/Loftus

• public class SortGrades
• public static void main (String args)
• int grades 89, 94, 69, 80, 97, 85, 73,
  91, 77, 85, 93
• Sorts.selectionSort (grades)
• for (int index 0 index index)
• System.out.print (gradesindex "
  ")

32
Sorts.java 1

• public class Sorts
• public static void selectionSort (int
  numbers)
• int min, temp
• for (int index 0 index numbers.length-1 index)
• min index
• for (int scan index1 scan numbers.length scan)
• if (numbersscan min scan
• temp numbersmin
• numbersmin numbersindex
• numbersindex temp

33
Swapping

• Swapping is the process of exchanging two values
• Swapping requires three assignment statements
• temp first
• first second
• second temp

34
Insertion Sort

• The approach of Insertion Sort
• pick any item and insert it into its proper place
  in a sorted sublist
• repeat until all items have been inserted
• In more detail
• consider the first item to be a sorted sublist
  (of one item)
• insert the second item into the sorted sublist,
  shifting the first item as needed to make room to
  insert the new addition
• insert the third item into the sorted sublist (of
  two items), shifting items as necessary
• repeat until all values are inserted into their
  proper positions

35
Insertion Sort

• An example
• original 3 9 6 1 2
• insert 9 3 9 6 1 2
• insert 6 3 6 9 1 2
• insert 1 1 3 6 9 2
• insert 2 1 2 3 6 9
• See Sorts.java (page 343) -- the insertionSort
  method

36
insertionSort.java

• public static void insertionSort (int numbers)
• for (int index 1 index index)
• int key numbersindex
• int position index
• while (position 0
  numbersposition-1 key)
• numbersposition numbersposition-1
  
• position--
• numbersposition key

37
Sorting Objects

• Integers have an inherent order, but the ordering
  criteria of a collection of objects must be
  defined
• Recall that a Java interface can be used as a
  type name and guarantees that a particular class
  implements particular methods
• We can use the Comparable interface and the
  compareTo method to develop a generic sort for a
  set of objects
• See SortPhoneList.java (page 347)
• See Contact.java (page 348)
• See Sorts.java (page 343) the second
  insertionSort method

38
SortPhoneList.java

• public class SortPhoneList
• public static void main (String args)
• Contact friends new Contact7
• friends0 new Contact ("John", "Smith",
  "610-555-7384")
• friends1 new Contact ("Sarah",
  "Barnes", "215-555-3827")
• friends2 new Contact ("Mark", "Riley",
  "733-555-2969")
• friends3 new Contact ("Laura", "Getz",
  "663-555-3984")
• friends4 new Contact ("Larry", "Smith",
  "464-555-3489")
• friends5 new Contact ("Frank",
  "Phelps", "322-555-2284")
• friends6 new Contact ("Marsha",
  "Grant", "243-555-2837")
• Sorts.insertionSort(friends)
• for (int index 0 index index) System.out.println (friendsindex)

39
Contact.java Author Lewis/Loftus

• public class Contact implements Comparable
• private String firstName, lastName, phone
• public Contact (String first, String last,
  String telephone)
• firstName first
• lastName last
• phone telephone
• public String toString ( )
• return lastName ", " firstName "\t"
  phone
• public int compareTo (Object other)
• int result
• if (lastName.equals(((Contact)other).lastNam
  e))
• result firstName.compareTo(((Contact)ot
  her).firstName)
• else result lastName.compareTo(((Contact)
  other).lastName)
• return result

40

• public static void insertionSort (Comparable
  objects)
• for (int index 1 index index)
• Comparable key objectsindex
• int position index
• while (position 0
  objectsposition-1.compareTo(key) 0)
• objectsposition objectsposition-1
  
• position--
• objectsposition key

41
Comparing Sorts

• Both Selection and Insertion sorts are similar in
  efficiency
• They both have outer loops that scan all
  elements, and inner loops that compare the value
  of the outer loop with almost all values in the
  list
• Approximately n2 number of comparisons are made
  to sort a list of size n
• We therefore say that these sorts are of order n2
• Other sorts are more efficient order n log2 n

42
Two-Dimensional Arrays

• A one-dimensional array stores a list of elements
• A two-dimensional array can be thought of as a
  table of elements, with rows and columns

43
Two-Dimensional Arrays

• To be precise, a two-dimensional array in Java is
  an array of arrays
• A two-dimensional array is declared by specifying
  the size of each dimension separately
• int scores new int1250
• A two-dimensional array element is referenced
  using two index values
• value scores36
• The array stored in one row or column can be
  specified using one index

44
Two-Dimensional Arrays

• See TwoDArray.java (page 351)
• See SodaSurvey.java (page 352)

45
TwoDArray.java Author Lewis/Loftus

• public class TwoDArray
• public static void main (String args)
  
• int table new int510
• for (int row0 row
• for (int col0 col
• tablerowcol row 10 col
• for (int row0 row
• for (int col0 col
• System.out.print (tablerowcol "\t")
  
• System.out.println()

46
SodaSurvey.java

• import java.text.DecimalFormat
• public class SodaSurvey
• public static void main (String args)
• int scores 3, 4, 5, 2, 1, 4, 3, 2,
  4, 4,
• 2, 4, 3, 4, 3, 3, 2, 1,
  2, 2,
• 3, 5, 4, 5, 5, 3, 2, 5,
  5, 5,
• 1, 1, 1, 3, 1, 2, 1, 3,
  2, 4
• final int SODAS scores.length
• final int PEOPLE scores0.length
• int sodaSum new intSODAS
• int personSum new intPEOPLE
• for (int soda0 soda
• for (int person0 person person) sodaSumsoda scoressodaperson
  
• personSumperson
  scoressodaperson
• DecimalFormat fmt new DecimalFormat
  ("0.")
• System.out.println ("Averages\n")
• for (int soda0 soda
• System.out.println ("Soda " (soda1)
  " " fmt.format ((float)sodaSumsoda/PEOPLE)
  )

47
Multidimensional Arrays

• An array can have many dimensions
• If it has more than one dimension, it is called a
  multidimensional array
• Each dimension subdivides the previous one into
  the specified number of elements
• Each array dimension has its own length constant
• Because each dimension is an array of array
  references, the arrays within one dimension can
  be of different lengths
• these are sometimes called ragged arrays

48
The ArrayList Class

• The ArrayList class is part of the java.util
  package
• Like an array, it can store a list of values and
  reference them with an index
• Unlike an array, an ArrayList object grows and
  shrinks as needed
• Items can be inserted or removed with a single
  method invocation
• It stores references to the Object class, which
  allows it to store any kind of object
• See Beatles.java (page 357)

49
Beatles.java Author Lewis/Loftus

• import java.util.ArrayList
• public class Beatles
• public static void main (String args)
• ArrayList band new ArrayList()
• band.add ("Paul")
• band.add ("Pete")
• band.add ("John")
• band.add ("George")
• System.out.println (band)
• int location band.indexOf ("Pete")
• band.remove (location)
• System.out.println (band)
• System.out.println ("At index 1 "
  band.get(1))
• band.add (2, "Ringo")
• System.out.println (band)
• System.out.println ("Size of the band "
  band.size())

50
ArrayList Efficiency

• The ArrayList class is implemented using an array
• The code of the ArrayList class automatically
  expands the array's capacity to accommodate
  additional elements
• The array is manipulated so that indexes remain
  continuous as elements are added or removed
• If elements are added to and removed from the end
  of the list, this processing is fairly efficient
• If elements are inserted and removed from the
  middle of the list, the elements are constantly
  being shifted around

51
Summary

• Chapter 6 has focused on
• array declaration and use
• passing arrays and array elements as parameters
• arrays of objects
• sorting elements in an array
• multidimensional arrays
• the ArrayList class