Chapter 8 OneDimensional Arrays

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Chapter 8One-Dimensional Arrays

• Lecture Slides to Accompany
• An Introduction to Computer Science Using Java
  (2nd Edition)
• by
• S.N. Kamin, D. Mickunas, E. Reingold

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Chapter Preview

• In this chapter we will
• introduce the array as a structure for storing
  large amounts of data
• discuss common array operations
• introduce algorithms for searching and sorting
  arrays
• show how multiple images can be painted from an
  array to use in programming simple animations

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Array Declarations

• Arrays contain a fixed number of variables of
  identical type
• Array declaration and allocation are separate
  operations
• Declaration examples
• int counts
• double scores
• String studentNames

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Array Allocation

• Arrays are allocated using the Java new operator
• The syntax is
• new typesize
• Examples
• counts new int10
• scores new double15
• studentNames new String10

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Array Organization

• 0 1 2 3 4 5 6 7 8 9
• counts
• Each box is an int variable
• The numbers on top are each variables subscript
  or index
• An array of size 10 has subscripts 0 to 9

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Array Subscripts

• Arrays can contain any one type of value (either
  primitive values or references)
• Subscripts are used to access specific array
  values
• Examples
• counts0 // first variable in counts
• counts1 // second variable in counts
• counts9 // last variable in counts
• counts10 // error trying to access
• // variable outside counts

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Expressions as Subscripts

• Array subscripts do not have to be constants
• Array subscripts do need to be integer
  expressions that evaluate to valid subscript
  values for the current array allocation
• Examples
• countsi
• counts2i
• countsI/2

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Array Initialization

• Arrays can be initialized by giving a list of
  their elements
• If your list contains n elements the subscripts
  will range from 0 to n 1
• You do not need to allocate the array explicitly
  after it is initialized
• Example
• int primes
• 2, 3, 5, 7, 11, 13, 17, 19, 23, 29

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Initializing an Array of Strings

• final Sring NAME
• Sunday, Monday, Tuesday,
• Wednesday, Thursday, Friday,
• Saturday
• // procedure that prints the day of week
• public void printName (int day,
• OutputBox out)
• out.print(NAMEday 1)

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Aliases

• It is possible to have two different variables
  refer to the same array
• When this happens these variables are called
  aliases
• Creating aliases is not a good programming
  practice
• Example of how it can happen
• int A, B
• B new int 10
• A B

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Loops and Array Processing

• Initializes counts to 0, 10, 20, , 90
• for (int i0 i lt 10 i)
• countsi i 10
• Prints the contents of counts using length
• for (int i0 i lt counts.length i)
• out.println(countsi)

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Extra Capacity Array

• Arrays cannot grow after they have been allocated
• You can allocate more space than you believe your
  application will need
• If you guess too low, you will still run out of
  space
• You do not need to use all the elements in an
  array (but total computer memory is finite)

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Searching

• This loop terminates as soon as it finds the
  value 90 stored in grades
• bool found false
• int i 0
• while (i lt size !found)
• // 90 is not in grades0..gradesi - 1
• if (gradesi 90)
• found true
• else
• i

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Processing Parallel Arrays

• This loop counts the number of students whose
  performance improved from the first test to the
  second
• int improved 0
• for (int i 0 i lt size i)
• if (grades1i lt grades2i)
• improved

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Arrays of Objects

• Arrays of objects are declared in the same manner
  as arrays of primitive variables
• Assuming that a class Student was declared
  elsewhere a client application could declare and
  allocate an array of 10 students using
• Student students
• students new Student10

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Passing Arrays as Arguments

• When an array is passed as an argument to a
  method, what is passed is a pointer to the array,
  not a new array (arrays are passed by reference)
• This means that if the method makes changes to
  the array, these changes are still in effect when
  the method returns to its caller
• This is not true for primitive values which are
  passed by value and not by reference
• Method header example
• public void read(Student students)

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Selection Sort

• Find the smallest element among the elements
  A0..An-1 and call it Amin
• Swap A0 and Amin so A0 contains the
  smallest element and A1..An-1 not sorted
• Now the smallest element among the elements
  A1..An-1 and call it Amin
• Swap A1 and Amin so A1 contains the second
  smallest element and A2..An-1 not sorted
• Proceed similarly for A3, A4, and so on

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SelectionSort Class part 1

• public class SelectionSort
• public void selectionSort (double A, int
  size)
• for (int i0 i lt size i)
• // elements in A0..Ai1 are less than
• // elements in Ai..Asize-1 and
• // A0..Ai-1 are sorted
• int min findMinimum(A, i, size)
• swap(A, I, min)

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SelectionSort Class part 2

• int findMinimum (double A, int i, int size)
  
• int j, min 1
• for (j i 1 j lt size j)
• // Amin lt all elements in A0..Aj1
• if (Aj lt Amin) min j
• return min
• void swap (double A, int I, int j)
• double temp Ai
• Ai Aj
• Aj temp

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Insertion Sort

• Iterate over subscripts 1 to n-1
• At the ith step, shift the elements A0..Ai so
  that this part of the array is sorted
• Note when the ith iteration begins elements
  A0..Ai 1 are already sorted, so the only
  shifting is that required to insert Ai into
  A0..Ai-1

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InsertionSort Class

• class InsertionSort
• public void InsertionSort (double A, int
  size)
• int i,j
• for (int i1 i lt size i)
• double Ai Ai
• j j - 1
• while (j gt 0 Aj gt Ai)
• Aj 1 Aj
• j--
• Aj 1 Ai

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Linear Search

• int linearSearch (int A, int key)
• int i
• for (i 0 i lt A.length i)
• // key not in A0..Ai - 1
• if (Ai key)
• return i
• // key not in A
• return 1

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Searching

• When searching for int values you can test for
  exact matches using (Ai key)
• Comparing two double values for equality will not
  always give a correct result
• A better comparison for doubles would be
  (Math.abs(Ai key) lt epsilon)
• Epsilon should be as small a value as is
  acceptable for your application

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Using Arrays in Animation

• Image mouse new ImageNUMBER
• Int sleepTime 1540, 240, 240, 240, 240, 240,
  240, 240,
• 240, 240, 240, 240, 240,
  240, 240
• public void show( )
• for (int i 0 i lt NUMBER i)
• mousei Toolkit.getDeafaultTool().
• getImage(images/T(i1)gif)
• ticker 0
• do
• g.drawImage(mouseticker, 70, 70)
• Timer.pause(sleepTimeticker)
• ticker (ticker 1) NUMBER
• while (true)

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