Arrays Vectores

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Arrays (Vectores)
OutlineDeclaring ArraysCreating
ArraysInitializing ArraysManipulating Arrays
Passing Arrays to MethodsMultidimensional
Arrays
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Declaring and Creating Arrays

• Created dynamically with keyword new
• int c new int 12
• or
• int c new int 12
• Equivalent to int c // declare array
  variable c new int 12 // create array
• We can create arrays of objects too
• String b new String 100

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c 0
Name of array (note that all elements of this
array have the same name, c)
c 1
c 2
c 3
c 4
c 5
c 6
c 7
c 8
c 9
c 10
Index (or subscript) of the element in array c
c 11
Fig. 7.1 A 12-element array.
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Initializing Arrays

• Using an array initializer
• Use initializer list
• Items enclosed in braces ()
• Items in list separated by commas
• int n 10, 20, 30, 40, 50
• Creates a five-element array
• Index values of 0, 1, 2, 3, 4
• Do not need keyword new

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Manipulating Arrays

• Examine array c
• c is the array name
• c.length accesses array cs length
• c has 12 elements ( c0, c1, c11 )
• The value of c0 is 45

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• 1 // Fig. 7.2 InitArray.java
• 2 // Creating an array.
• 3 import javax.swing.
• 4
• 5 public class InitArray
• 6
• 7 public static void main( String args
  )
• 8
• 9 int array // declare
  reference to an array
• 10
• 11 array new int 10 // create
  array (initialized with 0's by default)
• 12
• 13 String output "Index\tValue\n"
• 14
• 15 // append each array element's value
  to String output
• 16 for ( int counter 0 counter lt
  array.length counter )
• 17 output counter "\t" array
  counter "\n"
• 18
• 19 JTextArea outputArea new
  JTextArea()

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• 1 // Fig. 7.3 InitArray.java
• 2 // Initializing an array with a
  declaration.
• 3 import javax.swing.
• 4
• 5 public class InitArray
• 6
• 7 public static void main( String args
  )
• 8
• 9 // array initializer specifies
  number of elements and
• 10 // value for each element
  
• 11 int array 32, 27, 64, 18, 95,
  14, 90, 70, 60, 37
• 12
• 13 String output "Index\tValue\n"
• 14
• 15 // append each array element's value
  to String output
• 16 for ( int counter 0 counter lt
  array.length counter )
• 17 output counter "\t" array
  counter "\n"
• 18
• 19 JTextArea outputArea new
  JTextArea()

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• 1 // Fig. 7.4 InitArray.java
• 2 // Initialize array with the even integers
  from 2 to 20.
• 3 import javax.swing.
• 4
• 5 public class InitArray
• 6
• 7 public static void main( String args
  )
• 8
• 9 final int ARRAY_LENGTH 10 //
  constant
• 10 int array //
  reference to int array
• 11
• 12 array new int ARRAY_LENGTH //
  create array
• 13
• 14 // calculate value for each array
  element
• 15 for ( int counter 0 counter lt
  array.length counter )
• 16 array counter 2 2
  counter
• 17
• 18 String output "Index\tValue\n"
• 19

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• 1 // Fig. 7.7 RollDie.java
• 2 // Roll a six-sided die 6000 times.
• 3 import javax.swing.
• 4
• 5 public class RollDie
• 6
• 7 public static void main( String args
  )
• 8
• 9 int frequency new int 7
• 10
• 11 // roll die 6000 times use die value
  as frequency index
• 12 for ( int roll 1 roll lt 6000
  roll )
• 13 frequency 1 ( int ) (
  Math.random() 6 )
• 14
• 15 String output "Face\tFrequency"
• 16
• 17 // append frequencies to String
  output
• 18 for ( int face 1 face lt
  frequency.length face )
• 19 output "\n" face "\t"
  frequency face

Increment frequency values at index associated
with random number
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Passing Arrays to Methods (1)

• Two ways to pass arguments to methods
• Pass-by-value
• Copy of arguments value is passed to called
  method
• In Java, every primitive is pass-by-value
• Pass-by-reference
• Caller gives called method direct access to
  callers data
• Called method can manipulate this data
• Improved performance over pass-by-value
• In Java, every object is pass-by-reference
• In Java, arrays are objects
• Therefore, arrays are passed to methods by
  reference

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Passing Arrays to Methods (2)

• Example Sorting
• Bubble sort
• Smaller values bubble their way to top of array
• Larger values sink to bottom of array
• Use nested loops to make several passes through
  array
• Each pass compares successive pairs of elements
• Pairs are left along if increasing order (or
  equal)
• Pairs are swapped if decreasing order

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• 1 // Fig. 7.10 BubbleSort.java
• 2 // Sort an array's values into ascending
  order.
• 3 import java.awt.
• 4 import javax.swing.
• 5
• 6 public class BubbleSort extends JApplet
• 7
• 8 // initialize applet
• 9 public void init()
• 10
• 11 JTextArea outputArea new
  JTextArea()
• 12 Container container
  getContentPane()
• 13 container.add( outputArea )
• 14
• 15 int array 2, 6, 4, 8, 10, 12,
  89, 68, 45, 37
• 16
• 17 String output "Data items in
  original order\n"
• 18
• 19 // append original array values to
  String output

Pass array by reference to method bubbleSort to
sort array
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• 35 // sort elements of array with bubble
  sort
• 36 public void bubbleSort( int array2 )
  
• 37
  
• 38 // loop to control number of passes
  
• 39 for ( int pass 1 pass lt
  array2.length pass )
• 40
  
• 41 // loop to control number of
  comparisons
• 42 for ( int element 0
  
• 43 element lt array2.length - 1
  
• 44 element )
  
• 45
  
• 46 // compare side-by-side
  elements and swap them if
• 47 // first element is greater
  than second element
• 48 if ( array2 element gt
  array2 element 1 )
• 49 swap( array2, element,
  element 1 )
• 50
  
• 51 // end loop to control
  comparisons
• 52
  
• 53 // end loop to control passes
  

Method swap swaps two values in array reference
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Multidimensional Arrays

• Multidimensional arrays
• Tables with rows and columns
• Two-dimensional array
• Declaring two-dimensional array b22
• int b 1, 2 , 3, 4
• 1 and 2 initialize b00 and b01
• 3 and 4 initialize b10 and b11
• int b 1, 2 , 3, 4, 5
• row 0 contains elements 1 and 2
• row 1 contains elements 3, 4 and 5

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Multidimensional Arrays (Cont.)

• Creating multidimensional arrays
• Can be allocated dynamically
• 3-by-4 array
• int b b new int 3 4
• Rows can have different number of columns
• int b b new int 2 // allocate
  rows b 0 new int 5 // allocate row 0
  b 1 new int 3 // allocate row 1

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Column 0
Column 1
Column 2
Column 3
Row 0
a 0 0
a 0 1
a 0 2
a 0 3
Row 1
a 1 0
a 1 1
a 1 2
a 1 3
Row 2
a 2 0
a 2 1
a 2 2
a 2 3
Column index
Row index
Array name
Fig. 7.13 Two-dimensional array with three rows
and four columns.
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• 1 // Fig. 7.14 InitArray.java
• 2 // Initializing two-dimensional arrays.
• 3 import java.awt.Container
• 4 import javax.swing.
• 5
• 6 public class InitArray extends JApplet
• 7 JTextArea outputArea
• 8
• 9 // set up GUI and initialize applet
• 10 public void init()
• 11
• 12 outputArea new JTextArea()
• 13 Container container
  getContentPane()
• 14 container.add( outputArea )
• 15
• 16 int array1 1, 2, 3 , 4,
  5, 6
• 17 int array2 1, 2 , 3 ,
  4, 5, 6
• 18
• 19 outputArea.setText( "Values in array1
  by row are\n" )

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• 27 // append rows and columns of an array
  to outputArea
• 28 public void buildOutput( int array )
• 29
• 30 // loop through array's rows
  
• 31 for ( int row 0 row lt
  array.length row )
• 32
  
• 33 // loop through columns of current
  row
• 34 for ( int column 0 column lt
  array row .length column )
• 35 outputArea.append( array row
  column " " )
• 36
  
• 37 outputArea.append( "\n" )
  
• 38
  
• 39
• 40 // end method buildOutput
• 41
• 42 // end class InitArray

arrayrow.length returns number of columns
associated with row subscript
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Exercícios

• Escrever um programa que gera uma aposta
  aleatória do totoloto
• Nota o método Math.random() gera um número real
  aleatório entre 0 (inclusivé) e 1 (exclusivé)
• 2) Escrever um programa que gera N apostas
  aleatórias do totoloto, todas diferentes entre
  si, com N indicado pelo utilizador