Why do we need arrays?

1
Why do we need arrays?

• Problem - Input 5 test scores gt
• int test1,test2,test3,test4,test5
• 100 test scores? 10,000 employees?
• A structured data type is a type that
• Stores a collection of individual components
• The whole collection has just one variable name
• Allows individual components from the collection
  to be stored and retrieved

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Array

• structured collection of components (called array
  elements)
• all elements are of the same data type
• Array has one single name
• Array elements stored in adjacent memory
  locations.
• access individual components by using the array
  name together with index in square brackets
• The index indicates the position of the component
  within the collection.

3
Declaring an arraystep 1. dataType
arrayNamestep 2. arrayName new dataTypesize

• dataType -type of data stored in each component
  of the array
• int, float, char, etc
• arrayName - the name of the array
• An array is an object, arrayName is a reference
  variable
• size - number of components/members in the array
• Integer expression,must have a value gt 0.
• can be a variable
• int scores
• scores new int10

4
Declaring an array (two steps combined)dataType
arrayName new dataTypeintExp

• int scores new int10
• Initializes its components to their default
  values
• Numeric arrays are initialized to 0

5
Zero-Equivalent Auto-Initialization Values
Type Value
int 0
double 0.0
char \0
boolean false
objects null
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int scores new int10

• scores

7
Specifying Array Size during Program Execution

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

8
Accessing Array ComponentsarrayNameindexExp

• indexExp nonnegative integer
• reference array elements by using the subscript
  or the index
• array subscripting operator
• Zero based indexing

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Accessing Array Components

• new intn gt subscripts 0 thru n-1
• int scores10
• scores0 - is the first element of array test
• scores9 - last element of test
• scores10 1000 //writes beyond
  array-Exception
• scores4 88

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Valid subscriptsgt can use variables,
expression, or constant as subscripts

• scores0 95
• scoresi2 scoresi scoresi 1
• x scoresi
• System.out.println(scoresi 2 )

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Each array element is treated exactly as a
variable

• buf5 30
• scores1
• i buf8 92
• System.out.println(buff80)
• if (ai lt 14)
• scores3 console.nextInt()

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Array Initialization during Declaration

• by declaration
• int age 23, 34, 18, 44, 50
• double temps 0.0, 112.37, 98.6
• compiler will allocate space for each initialized
  item
• new is not required here

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length

• You can use the built-in length property to
  determine the size of any array.
• System.out.println(anArray.length)
• will print the array's size
• length contains the size of the array
• int list 10,20,30,40,50,60
• list.length is 6

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Array traversal

• Processing each array sequentially from the first
  to the last
• for (int i 0 i lt ltarraygt.length i)
• ltdo something with arrayigt

15
Array limitations

• You cant
• change the size of an array in the middle of
  program execution (ArrayList class)
• compare arrays for equality using
• print an array with
• System.out.println(arrayName)

16
Initializing an array to a specific value

• double sales new double10
• for (int index 0 index lt sales.length
  index)
• salesindex 10.00

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Input data into an array

• double sales new double10
• for (int index 0 index lt sales.length
  index)
• Sysmt.out.println(Enter value)
• salesindex console.nextDouble()

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Printing an array

• double sales new double10
• for (int index 0 index lt sales.length
  index)
• System.out.println(salesindex)

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Printing an array

• System.out.println(Arrays.toString(list))
• Yields
• 17, -3, 42, 8, 12, 2, 103

20
Finding the sum and average of an array

• double sales new double10
• double Sum 0
• for (int index 0 index lt sales.length
  index)
• sum salesindex
• double avg sum/sales.length

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Determining the largest element of an array

• double sales new double10
• int maxIndex 0
• for (int index 1 index lt sales.length
  index)
• if (salesmaxIndex lt salesindex)
• maxIndex index
• double largest salesmaxIndex

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Printing an array in reverse order

• double sales new double10
• for (int index sales.length - 1 index gt 0
  index--)
• System.out.println(salesindex

23
Array Index out of Bounds Exception

• double num new double 10
• int i//i can be 0,1, 2, 3, 4, 5, 6, 7, 8, 9
• index out of bounds i lt 0 or i gt 9
• ArrayIndexOutOfBoundsException
• for (int i 0 i lt 10 i)
• numi 0

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MethodsArrays as Formal Parametersdatatype
arrayName

• Size not necessary
• Because arrays are reference variables, the array
  can be changed
• public static void processArrays(int listA,
• 
  double listB)

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Arrays as Actual Parameterspass name of array
in callmethod(arrayName)

• formal
• public static void processArrays (int listA,
• 
  double listB)
• actual
• int intList new int10
• double doubleList new double15
• processArrays(intList,doubleList)
  

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Method print an array

• public static void printArray(int list)
• for (int index 0 index lt list.length
  index)
• System.out.println(listindex)

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Method read an array

• public static void readArray(int list)
• for (int index 0 index lt list.length
  index)
• System.out.println(Enter value)
• listindex console.nextInt()

28
Method read an array

• public static int readArray(int list,
• 
  Scanner infile)
• index 0
• while (infile.hasNext() index lt
  list.length)
• listindex infile.nextInt()
• index
• return index // number of elements read

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Method read an array

• public static int readArray()
• for (int index 0 index lt list.length
  index)
• System.out.println(Enter value)
• listindex console.nextInt()
• return list

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Method sum an array

• public static int sumArray(int list)
• int sum 0
• for (int index 0 index lt list.length
  index)
• sum listindex
• return sum

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Method return index of largest element

• public static int indexLargestElement(int list)
• int maxIndex 0
• for (int index 1 index lt list.length
  index)
• if (listindex gt listmaxIndex)
• maxIndex index
• return maxIndex

32
Method copy array

• public static void copyArray(int list1, int
  list2)
• for (int index 0 index lt list.length
  index)
• list2index list1index

33
Arrays as parameters to methods

• static final int ARRAY_SIZE 10
• int listA new intARRAY_SIZE
• int listB new intARRAY_SIZE
• printArray(listA)
• readArray(listA)
• copyArray(listA,listB)
• printArray(listA)
• printArray(listB)

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Passing an array

• arrayName ? base address
• arrayName ? address of arrayName0
• When you pass an array as a parameter, the base
  address of the actual array is passed to the
  formal parameter

35
Linear Search

• public static int find(int list, int
  searchItem)
• for (int ix 0 ix lt list.length() ix)
• if (listi searchItem)
• return ix
• return -1

36
Linear Search(also called Sequential Search)

• public static boolean find(int list, int
  searchItem)
• int ix 0
• boolean found false
• while (!found ix lt list.length)
• if (listix searchItem)
• found true
• else
• ix
• return found

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BINARY SEARCH

• requires sorted list
• (Ex phone books, dictionaries).
• keep dividing list in half, compare against
  middle argument
• eliminates one half of the elements after each
  comparison.
• efficient

38
Binary Search

• public static int binSearch(int list, int
  searchItem)
• int first 0 // lower bound on
  list
• int last length - 1 // upper bound on
  list
• int middle // middle index
• boolean found false
• while (!found last gt first)
• middle (first last)/2
• if (item lt listmiddle
• last middle - 1
• else if (item gt listmiddle)
• first middle 1
• else
• found true
• if (!found)
• return -1
• return middle

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Average Number of Iterations
40
For-Each Loop

• Java 5 Enhanced for loop
• for (lttypegt ltindexNamegt ltarrayNamegt)
• statement
• for (int index 0 index lt list.length index)
• System.out.println(listindex)
• for (int index list)
• System.out.println(index)
• Usually only applicable for examining each value
  in sequence

41
Caution using arrays with

• arrayA arrayB
• both will refer to same array
• Shallow copy
• use the for loop for a deep copy

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Caution using arrays with

• if(arrayA arrayB)
• Returns true if they refer to the same array
• arrays.equals provided
• if (arrays.equals(list1, list2))
• System.out.println(The arrays are equal)

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Comparing two 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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Parallel Arrays

• Two or more arrays
• Corresponding components hold related information
• int studentId new int50
• char courseGrade new char50
• int numStudents 0
• while (infile.hasNext() numStudents lt 50)
• studentsnumStudents infile.nextInt ()
• courseGradenumStudents infile.next().charAt(
  0)
• numStudents

45
Array of String objects

• String nameList new String5
• nameList0 Joe Smith
• Use for loop to output nameList
• for (int I 0 I lt nameList.lengthi)
• System.out.println(namelisti )
• Also use String methods
• nameList4.substring(0,3)

46
Command line arguments

• public static void main(String args)
• Java DoSomething
• Java DoSomething temps.txt temps.out
• args0 temps.txt
• args1 temps.out

47
Two Dimensional Arrays

• table with rows and columns
• all items of the same data type
• access by row and column indices
• Example tables or graphs

48
Declaring a two-dimensional array

• dataType arrayName
• arrayName new dataTypeintExp1intExp2
• dataType arrayName new dataTypeintExp1int
  Exp2

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int A new int34
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(No Transcript)
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A00
A01
A02
A03
A10
A11
A12
A13
A20
A21
A22
A23

• int A34
• Virtually Stored

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Accessing Array Components

• arrayNameindexExp1indexExp2
• A23 25
• Indices can be variables
• Aij

53
Two-dimensional arrays and the instance variable
length

• Length can be used for number of rows and columns
• int matrix new int2015
• Number of rows matrix.length is 20
• Number of columns - matrix0.length is 15

54
Two dimensional arrays special cases

• Can have different number of columns for each row
• int board
• board new int5 //board.length is 5
• board0 new int6 // board0.length is 6
• board1 new int2
• board2 new int5
• board3 new int3
• board4 new int4

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Two-dimensional array initialization during
declaration

• Elements of each row are enclosed within braces
  and separated by commas
• All rows are enclosed in braces
• int board1 2, 3, 1,
• 15, 25, 13,
• 20, 4, 7,
• 11, 18, 14
• int board2 2, 3, 1, 5,
• 15, 25,
• 11, 18, 10

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Initialize two-dimensional arrays

• int matrix new intROWSCOLS
• for (int row 0 row lt matrix.length row)
• for (col 0 col lt matrixrow.length
  col)
• matrixrowcol 10

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Print two-dimensional arrays

• int matrix new intROWSCOLS
• for (int row 0 row lt matrix.length row)
• for (col 0 col lt matrixrow.length
  col)
• System.out.printf(7d,matrixrowcol)
• System.out.println()

58
Input two-dimensional arrays

• int matrix new intROWSCOLS
• for (int row 0 row lt matrix.length row)
• for (col 0 col lt matrixrow.length
  col)
• matrixrowcol console.nextInt()

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Sum by row two-dimensional arrays

• int matrix new intROWSCOLS
• for (int row 0 row lt matrix.length row)
• sum 0
• for (col 0 col lt matrixrow.length
  col)
• sum matrixrowcol
• System.out.println(The sum of the elements of
  row
• (row 1)
  sum)

60
Sum by column two-dimensional arrays

• int matrix new intROWSCOLS
• for (int col 0 col lt matrix0.length col)
• sum 0
• for (row 0 row lt matrix.length row)
• sum matrixrowcol
• System.out.println(The sum of the elements of
  column
• (col 1)
  sum)

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Passing Two-Dimensional Arrays as Parameters

• public static void printMatrix(int matrix)
• for (int row 0 row lt matrix.length row)
• for (col 0 col lt matrixrow.length
  col)
• System.out.printf(7d,matrixrowcol)
• System.out.println()
• Call printMatrix(board)

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Multidimensional arrays

• arrays can have any number of dimensions
• dataType.. arrayName new
  dataTypeintExp1intExp2..intRxpn
• To access
• arrayNameindexExp1indexExp2indexExpn
• Requires multiple loops to process

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Multidimensional arrays

• Example
• double graph new double303030
• graph325 27.68