Chapter 9: Arrays

1
Chapter 9 Arrays

• Java Programming
• From Problem Analysis to Program Design,
• Second Edition

2
Chapter Objectives

• Learn about arrays.
• Explore how to declare and manipulate data into
  arrays.
• Understand the meaning of array index out of
  bounds.
• Become familiar with the restrictions on array
  processing.

3
Chapter Objectives

• Discover how to pass an array as a parameter to a
  method.
• Discover how to manipulate data in a
  two-dimensional array.
• Learn about multidimensional arrays.

4
Array

• A structured data type with a fixed number of
  components.
• Every component is of the same type.
• Components are accessed using their relative
  positions in the array.

5
One-Dimensional Arrays

• Syntax to instantiate an array
• dataType arrayName
• arrayName new dataTypeintExp
• dataType arrayName new dataTypeintExp
• dataType arrayName1, arrayName2
• Syntax to access an array component
• arrayNameindexExp
• intExp number of components in array gt 0
• 0 lt indexExp lt intExp

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int num new int5
Array num
7
Array list
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Specifying Array Size During Program Execution

• int arraySize
• System.out.print("Enter the size of "
• "the array ")
• arraySize console.nextInt()
• System.out.println()
• int list new intarraySize

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Array Initialization During Declaration

• double sales 12.25, 32.50, 16.90, 23,
• 45.68
• The values, called initial values, are placed
  between braces and separated by commas.
• Here, sales0 12.25, sales1 32.50, sales2
  16.90, sales3 23.00, and sales4 45.68.
• When declaring and initializing arrays, the size
  of the array is determined by the number of
  initial values within the braces.
• If an array is declared and initialized
  simultaneously, we do not use the operator new to
  instantiate the array object.

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Arrays and the Instance Variable length

• A public instance variable length is associated
  with each array that has been instantiated.
• The variable length contains the size of the
  array.
• The variable length can be directly accessed in a
  program using the array name and the dot
  operator.
• This statement creates the array list of six
  components and initializes the components using
  the values given. Here list.length is 6.
• int list 10, 20, 30, 40, 50, 60

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Arrays and the Instance Variable length

• This statement creates the array numList of 10
  components and initializes each component to 0.
• int numList new int10
• The value of numList.length is 10.
• These statements store 5, 10, 15, and 20,
  respectively, in the first four components of
  numList.
• numList0 5
• numList1 10
• numList2 15
• numList3 20
• You can store the number of filled elements, that
  is, the actual number of elements, in the array
  in a variable, say noOfElement. It is a common
  practice for a program to keep track of the
  number of filled elements in an array.

12
Processing One-Dimensional Arrays

• Loops used to step through elements in array and
  perform operations.
• int list new int100
• int i
• for (i 0 i lt list.length i)
• //process listi, the (i 1)th
• //element of list
• for (i 0 i lt list.length i)
• listi console.nextInt()
• for (i 0 i lt list.length i)
• System.out.print(listi " ")

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Arrays

• Some operations on arrays
• Initialize
• Input data
• Output stored data
• Find largest/smallest/sum/average of elements

double sales new double10 int
index double largestSale, sum, average
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Code to Initialize Array to Specific Value (10.00)
for (index 0 index lt sales.length
index) salesindex 10.00
Code to Read Data into Array
for (index 0 index lt sales.length
index) salesindex
console.nextDouble()
Code to Print Array
for (index 0 index lt sales.length
index) System.out.print(salesindex
" ")
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Code to Find Sum and Average of Array
sum 0 for (index 0 index lt sales.length
index) sum sum
salesindex if (sales.length ! 0) average
sum / sales.length else average 0.0
Determining Largest Element in Array
maxIndex 0 for (index 1 index lt
sales.length index) if
(salesmaxIndex lt salesindex) maxIndex
index largestSale salesmaxIndex
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Determining Largest Element in Array
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• / program to create an array called cpsc,
  instantiate the array, enter some data into
• // the array, compute the average of the data,
  and look for the largest member of the array
• import java.util.
• public class array1
• public static void main (String mike)
• Scanner console
• console new Scanner(System.in)
• int cpsc //create the array
• cpsc new int10 // instantiate the array
  to contain 10 elements and value 0 each
• // to enter new values into the array
• int i
• System.out.println("enter ten integer numbers
  separated by spaces")
• for (i0 iltcpsc.lengthi)
• cpsci console.nextInt()

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• // to find sum and average
• double sum
• double avg
• sum0.00
• for(i0 iltcpsc.length i)
• sum sum cpsci
• avg sum/cpsc.length
• System.out.println("Sum" "" sum "and" " "
  "Average" "" avg)
• // to find largest element of the array
• int maxind0
• int largestval0
• for (i1 iltcpsc.length ii1)
• if (cpscmaxindltcpsci)
• maxindi
• largestvalcpscmaxind
• System.out.println("Largest value"largestval)

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Array Index Out of Bounds

• An array is in bounds if
• 0 lt index lt arraySize 1
• If index lt 0 or index gt arraySize
• ArrayIndexOutOfBoundsException exception is
  thrown.
• Base address Memory location of the first
  component in an array.

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Declaring Arrays as Formal Parameters to Methods

• General syntax to declare an array as a formal
  parameter
• dataType arrayName
• public static void arraysAsFormalParameter(int
  listA,
• double listB, int num)
• //...
• int intList new int10
• double doubleNumList new double15
• int number
• arraysAsFormalParameter(intList, doubleNumList,
  number)

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Methods for Array Processing

• public static void fillArray(int list,
• int noOfElements)
• int index
• for (index 0 index lt noOfElements
• index)
• listindex console.nextInt()

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The Assignment Operators and Arrays
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The Assignment Operators and Arrays
25
The Assignment Operators and Arrays

• for (int index 0 index lt listA.length
• index)
• listBindex listAindex

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Relational Operators Arrays

• if (listA listB)
• ...
• The expression listA listB determines if the
  values of listA and listB are the same, thus
  determining whether listA and listB refer to the
  same array.
• To determine whether listA and listB contain the
  same elements, you need to compare them component
  by component.
• You can write a method that returns true if two
  int arrays contain the same elements.

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Relational Operators and Arrays
boolean isEqualArrays(int firstArray,
int secondArray) if
(firstArray.length ! secondArray.length)
return false for (int index 0 index lt
firstArray.length
index) if (firstArrayindex !
secondArrayindex) return false
return true if (isEqualArrays(listA,
listB)) ...
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• //to determine if the values of two arrays are
  equal
• import java.util.
• public class array2
• public static boolean mike (double faith1,
  double faith2)
• boolean anstrue
• int j
• if(faith1.length ! faith2.length)
• ansfalse
• for(j0 jltfaith1.length j)
• if (faith1j ! faith2j)
• ansfalse
• else
• anstrue
• return ans
• public static void main(String args)

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• System.out.println("enter four decimal numbers
  separated by spaces for list1")
• for (i0 iltlist1.lengthi)
• list1i console.nextDouble()
• System.out.println("enter four decimal numbers
  separated by spaces for list2")
• for (i0 iltlist2.lengthi)
• list2i console.nextDouble()
• // to print out the values of list1 and list2
• int k
• for(k0 kltlist1.length k)
• System.out.println(list1k)
• System.out.println()
• for(k0 kltlist2.length k)
• System.out.println(list2k)
• // to print the two arrays in two columns

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Arrays of Objects

• Can use arrays to manipulate objects.
• Example Create an array named array1 with N
  objects of type T
• T array1 new TN
• Can instantiate array1 as follows
• for(int j0 j ltarray1.length j)
• array1j new T()

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Array of String Objects

• String nameList new String5
• nameList0 "Amanda Green"
• nameList1 "Vijay Arora"
• nameList2 "Sheila Mann"
• nameList3 "Rohit Sharma"
• nameList4 "Mandy Johnson"

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Array of String Objects

• Some String methods to work on array
• nameList0.equals(Amanda Green) returns true
• nameList3.substring(0,3) returns Rohi

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Clock arrivalTimeEmp new Clock100
Arrays of Objects
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Instantiating Array Objects
for (int j 0 j lt arrivalTimeEmp.length j)
arrivalTimeEmpj new Clock()
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Instantiating Array Objects
arrivalTimeEmp49.setTime(8, 5, 10)
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Parallel Arrays

• Two (or more) arrays are said to be parallel if
  their corresponding components hold related
  information.
• For example, we can have arrays that hold
  students ID, and another that holds GPA.
• Each component of the array corresponds to a
  given students information

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• import java.util.
• public class array3
• public static void main(Stringme1)
• // to read the id, and gpa and get degree class
  of 5 students
• int numb5
• Stringid "14562","15628", "17829","24561","25
  678"
• double gpa 3.89,4.91,3.89,4.23,2.98
• String dclass
• dclassnew String5
• int i
• // compute the grade based on gpa
• Upper"
• else if (gpaigt 2.50 gpailt3.50)
  dclassi"Second Class Lower"
• else if (gpaigt 1.50 gpailt2.50)
  dclassi"Third Class"

Program Output
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Arrays and Variable Length Parameter List

• We may use same name for different methods
  (polymorphism) but with different parameter
  lists.
• Example public static int faith(int x, int y)
• public static int
  faith(int x, int y, int z)
• public static int
  faith (int w, int x, int y, int z)
• We may intend that the list of parameters could
  be variable (ie be two, three, four, etc). In
  that case, the syntax to declare a variable
  length formal parameter (list) is
• dataType ... Identifier
• E.g. public static int faith(intnumList)
• Where numList is a formal parameter (an array
  whose components are each integers)
• In calling the method, we specify the actual
  parameters of type integer.

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Arrays and Variable Length Parameter List

• A method can also have both variable length
  formal parameter and other formal parameters
• E.g. public static void faith(String name,
  double gpa, intnumList)
• A method can only have one variable length formal
  parameter
• If a method has both variable length formal
  parameter and other formal parameters, then the
  variable length formal parameter should be the
  last to be listed on the parameter list

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Arrays and Variable Length Parameter List

• public static double largest(double ... numList)
• double max
• int index
• if (numList.length ! 0)
• max list0
• for (index 1 index lt numList.length
• index)
• if (max lt numList index)
• max numList index
• return max
• return 0.0

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Arrays and Variable Length Parameter List

• double num1 largest(34, 56)
• double num2 largest(12.56, 84, 92)
• double num3 largest(98.32, 77, 64.67, 56)
• System.out.println(largest(22.50, 67.78,
• 92.58, 45, 34, 56))
• double numberList 18. 50, 44, 56.23, 17.89
• 92.34, 112.0, 77, 11, 22,
• 86.62
• System.out.println(largest(numberList))

43
foreach loop

• The syntax to use this for loop to process the
  elements of an array is
• for (dataType identifier arrayName)
• statements
• identifier is a variable, and the data type of
  identifier is the same as the data type of the
  array components.

44
foreach loop

• sum 0
• for (double num list)
• sum sum num
• The for statement in Line 2 is read for each num
  in list. The identifier num is initialized to
  list0. In the next iteration, the value of num
  is list1, and so on.
• for (double num numList)
• if (max lt num)
• max num

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• // example on variable length formal parameter
  and alternative looping
• //
• public class array4
• private String firstName
• private String lastName
• private double testScores // array to store
  test scores
• private char grade
• // default constructor
• public array4()
• firstName""
• lastName""
• grade ''
• testScores new double5
• // constructor with variable length parameter
  list
• // testscores vary in number from one student to
  another

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• // determine grade
• if (averagegt90)
• return 'A'
• else if (averagegt80)
• return 'B'
• else if (average gt 70)
• return 'C'
• else if (average gt60)
• return 'D'
• else
• return 'F'
• // method to return a student's name, scores, and
  grade as string
• public String toString()
• String str
• str String.format("-10s -10s", firstName,
  lastName)
• for (double score testScores)
• strstr " " score

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Two-Dimensional Arrays

• Data is sometimes in table form (difficult to
  represent using a one-dimensional array).
• To declare/instantiate a two-dimensional array
• dataType arrayName newdataTypeintExp1
  intExp2
• To access a component of a two-dimensional array
• arrayNameindexExp1indexExp2
• intExp1, intExp2 gt 0
• indexExp1 row position
• indexExp2 column position

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Two-Dimensional Arrays

• Can specify different number of columns for each
  row (ragged arrays).
• Three ways to process two-dimensional arrays
• Entire array.
• Particular row of array (row processing).
• Particular column of array (column processing).
• Processing algorithms is similar to processing
  algorithms of one-dimensional arrays.

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doublesales new double105
Two-Dimensional Arrays
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Accessing Two-Dimensional Array Components
52
Two-Dimensional Arrays Special Cases
Java allows us to specify a different number of
columns in each row
Each row must be instantiated separately. E.g. int
faithnew int 5 - 5 rows faith0new
int 6 faith1 new int2 faith2 new
int5 and so on.
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Two-Dimensional Arrays Processing
Initialization for (row 0 row lt matrix.length
row) for (col 0 col lt
matrixrow.length col)
matrixrowcol 10
Print for (row 0 row lt matrix.length
row) for (col 0 col lt
matrixrow.length col)
System.out.printf("7d",
matrixrowcol) System.out.println() I
nput for (row 0 row lt matrix.length row)
for (col 0 col lt matrixrow.length
col) matrixrowcol
console.nextInt()
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• // to manipulate two dimensional arrays
• import java.util.
• import java.io.
• public class array5
• public static void main (String ala) throws
  FileNotFoundException
• Scanner inputF new Scanner(new
  FileReader("array5.txt"))
• PrintWriter outputF new PrintWriter("array5.out
  ")
• // to declare two dimensional array
• int boys
• // to instantiate the array
• boys new int 56
• // to read data from the file into the array
• int row, col
• for(row0 rowltboys.length row)

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Two-Dimensional Arrays Processing
Sum by row // to sum by row, eg. row number
4, i.e 5th row row4 int
sum0 for(col0 colltboysrow.length
col) sum sum boysrowcol
System.out.println() System.out.println()
System.out.println("The sum of
row"(row1)"" sum)
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Two-Dimensional Arrays Processing

• Sum by Column
• // to sum by column, eg. column number 3, i.e
  4th column
• col3
• sum0
• for(row0 rowltboys.length row)
• sum sum boysrowcol
• System.out.println()
• System.out.println("The sum of
  column"(col1)"" sum)

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Two-Dimensional Arrays Processing

• Largest Element of a Row
• // to find the largest element of a row (say row
  2)
• row2
• lgestboys20 // assumes that the
  first element of the row is the largest
• for(col0 colltboys0.length col)
• if(boysrowcolgtlgest)
• lgestboysrowcol
• System.out.println()
• System.out.println("The largest element of
  row"(row1)"" lgest)

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Two-Dimensional Arrays Processing

• Largest Element of a Column
• / to find the largest element of a column 3
• col3
• int lgest
• lgestboys03 // assumes that the
  first element of the column is the largest
• for(row0 rowltboys.length row)
• if(boysrowcolgtlgest)
• lgestboysrowcol
• System.out.println()
• System.out.println("The largest element of
  column"(col1)"" lgest)

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Two-Dimensional Arrays Processing

• Largest Element of each Row
• // to find the largest element in each row
• for(row0 rowltboys.length row)
• lgestboysrow0
• for(col0 colltboysrow.length col)
• if(lgestltboysrowcol)
• lgestboysrowcol
• System.out.println()
• System.out.println("The largest element of
  row"(row1)"" lgest)

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Two-Dimensional Arrays Processing

• Largest Element of each Column
• // to find the largest element in each column
• for(col0 colltboys0.length col)
• lgestboys0col
• for(row0 rowltboys.length row)
• if(lgestltboysrowcol)
• lgestboysrowcol
• System.out.println()
• System.out.println("The largest element of
  column"(col1)"" lgest)

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• // new method to print the contents of a matrix
• public static void toprint(int boys)
• int row, col
• int presrow0
• for(row0 rowltboys.length row)
• for(col0colltboys0.length col)
• if (row presrow)
• System.out.printf("6d",boysrowcol )
• else if (rowgtpresrow)
• System.out.println()
• System.out.printf("6d",boysrowcol )
• presrowrow
• System.out.println()

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• // new method to find the largest element in each
  row
• public static void rowlarge(int boys)
• int row, col, lgest
• for(row0 rowltboys.length row)
• lgestboysrow0
• for(col0 colltboysrow.length col)
• if(lgestltboysrowcol)
• lgestboysrowcol
• System.out.println()
• System.out.println("The largest element of
  row"(row1)"" lgest)
• // method to find the largest element in each
  column
• public static void colarge(int boys)

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• //main method that calls the other methods
• import java.util.
• import java.io.
• public class array6
• public static void main (String ala) throws
  FileNotFoundException
• Scanner inputF new Scanner(new
  FileReader("array5.txt"))
• PrintWriter outputF new PrintWriter("array5.out
  ")
• int mike
• mike new int 56
• // to read data from the file into the array
• int row, col
• for(row0 rowltmike.length row)
• for(col0 colltmike0.length col)
• mikerowcol inputF.nextInt()
• // to call the methods
• toprint(mike)

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Multidimensional Arrays

• Can define three-dimensional arrays or
  n-dimensional arrays (n can be any number).
• Syntax to declare and instantiate array
• dataType arrayName new
• dataTypeintExp1intExp2intExpn
• Syntax to access component
• arrayNameindexExp1indexExp2indexExpn
• intExp1, intExp2, ..., intExpn positive
  integers
• indexExp1,indexExp2, ..., indexExpn
  non-negative integers

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Loops to Process Multidimensional Arrays
double carDealers new double1057
For (i 0 i lt 10 i) for (j 0 j lt 5
j) for (k 0 k lt 7 k)
carDealersijk 10.00
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• //to handle the input data as three dimensional
  array (2 blocks)
• import java.util.
• import java.io.
• public class array7
• public static void main (String ala) throws
  FileNotFoundException
• Scanner inputF new Scanner(new
  FileReader("array7.txt"))
• PrintWriter outputF new PrintWriter("array5.out
  ")
• int boys
• boys new int 253
• // to read data from the file into the array
• int i,j,k
• for(i0 ilt2 i)
• for(j0 jlt5 j)
• for(k0 klt3 k)
• boysijk inputF.nextInt()
• // to print

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Chapter Summary

• Arrays
• Definition
• Uses
• Different arrays
• One-dimensional
• Two-dimensional
• Multidimensional (n-dimensional)
• Arrays of objects
• Parallel arrays

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Chapter Summary

• Declaring arrays
• Instantiating arrays
• Processing arrays
• Entire array
• Row processing
• Column processing
• Common operations and methods performed on arrays
• Manipulating data in arrays