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Why do we need arrays?

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Why do we need arrays? Problem - Input 5 test scores = int test1,test2,test3,test4,test5 100 test scores? 10,000 employees? A structured data type is a type that: – PowerPoint PPT presentation

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Title: 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

2
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
6
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

9
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

10
Valid subscriptsgt can use variables,
expression, or constant as subscripts
  • scores0 95
  • scoresi2 scoresi scoresi 1
  • x scoresi
  • System.out.println(scoresi 2 )

11
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()

12
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

13
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

14
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

17
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()

18
Printing an array
  • double sales new double10
  • for (int index 0 index lt sales.length
    index)
  • System.out.println(salesindex)

19
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

21
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

22
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

24
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)

25
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)

26
Method print an array
  • public static void printArray(int list)
  • for (int index 0 index lt list.length
    index)
  • System.out.println(listindex)

27
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

29
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

30
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

31
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)

34
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

37
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

39
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

42
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)

43
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)

44
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

49
int A new int34
50
(No Transcript)
51
A00
A01
A02
A03
A10
A11
A12
A13
A20
A21
A22
A23
  • int A34
  • Virtually Stored

52
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

55
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

56
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

57
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()

59
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)

61
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)

62
Multidimensional arrays
  • arrays can have any number of dimensions
  • dataType.. arrayName new
    dataTypeintExp1intExp2..intRxpn
  • To access
  • arrayNameindexExp1indexExp2indexExpn
  • Requires multiple loops to process

63
Multidimensional arrays
  • Example
  • double graph new double303030
  • graph325 27.68
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