A Wide Array of Possibilities

1
A Wide Array of Possibilities
Javas Central Casting

• CS 102

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Whats an Array

• An array is a group of values of the same type
  (usually stored in consecutive memory locations)
• Arrays are objects
• Can be assigned to variables of type Object
• Dynamically created

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An Array of Examples

• From Figure 5.3
• int n // declare an array of integers
• // initialize instance variables
• public void init()
• n new int 10 //dynamic allocation

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The Declaration of Array

• Array declarations
• int ai // array of int
• short as // array of array of short
• Object ao, // array of Object
• otherAo // array of Object

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Declaring Arrays

• Cant specify the length in a declaration
• int test new int 100 // Okay
• int test2 100 // No good
• int test3 100new int 100 // No good

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Some Array Definitions

• These create array objects
• Exception ae new Exception3
• Object aao new Exception23
• int factorial 1, 1, 2, 6, 24
• String aas "array", "of", "String"

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An Array Example
int face int frequency // initialize
instance variables public void init()
frequency new int 7 for (int roll1
roll (Math.random() 6 ) frequency face

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An Array of Observations

• Arrays know their own length
• test.length 100
• What kind of loops do we usually use with arrays?
• Arrays start at index 0
• Last index ??
• Arrays must be indexed by int values (short,
  byte, or char are okay, but long is no good)

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Operating on an Array

• Square brackets are an operator
• Brackets have very high precedence
• Same as ()s
• Example
• -17studentGrades12 2

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"Can he do that?"

• Whats wrong with the following code?
• int scores 98, 76, 84, 97, 101, 78
• int counter0
• for ( counter
• System.out.println("Test " counter
• " " scorescounter)

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Array Initializers

• Use s to set initial values
• int grades 87, 92, 76, 94

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Finally, a Constant Variable

• Whats wrong with the following code?
• // Figure 5.6
• // This Applet wont compile
• import java.applet.Applet
• public class FinalTest extends Applet
• final int x

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Pass the Salt, and an Array of int

• From last time primitive types and reference
  types
• Parameter passing offering data to a method in
  the method call
• public void paint(Graphics graphObject)
• When you call paint, give it a Graphics object
• How does Java pass data around?

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Call Me Ishmael, But Don't Call Me Late for Dinner

• Call-by-value
• The called method gets a copy of the actual data
  (i.e., its value)
• Called method can't corrupt the original data,
  because it's only got a copy
• Call-by-reference
• The called method gets a reference to the data

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Arrays Call-by-Reference

• Reference types are always passed
  call-by-reference
• Pro Dont need to copy lots of data
• Con Since you can access the original object,
  you can change it
• Arrays are a reference type
• Whole arrays are passed call-by-reference

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What About Array Elements?

• If an array is an array of a reference type, then
  individual elements are passed as
  call-by-____________
• Graphics graphicsArray new Graphics10
• If an array is an array of a reference type, then
  individual elements are passed as
  call-by-____________
• int scores new int10

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Searching Sorting Data

• The Second Law of Thermodynamics
• Sorting restores order to the world
• Sorting in everyday life
• Bank checks by account
• Searching is easier when sorted
• Reverse directories of phone numbers

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Sorting in Computers

• Usually start with an array of items to be sorted
• Integers
• Social Security numbers
• Last names and first names
• Lots of data
• All the SSNs in the U.S.
• All of GMs employees

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Different Ways to Sort

• Bubble sort
• Compare two neighbors, swap if out of order
• 1 2 5 4 7 8 6
• 1 2 4 5 7 8 6
• 1 2 4 5 7 6 8
• 1 2 4 5 6 7 8

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The Code for Bubble Sort

• public void sort()
• int hold // temporary holding area for swap
• // passes
• for ( int pass 1 pass
• // one pass
• for ( int i 0 i
• // one comparison
• if ( a i a i 1 )
• // one swap
• hold ai a i a i 1
  a i 1 hold

Nested for loops
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Theres More Than One Way to Sort a Cat

• Other sorts
• Quicksort
• Heapsort
• Bucket sort
• Different sorts have different characteristics
• Memory usage
• Speed

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Searching Here, There Everywhere

• Need to find a value? Search for it!
• Linear search
• Is it the next one?
• No, then look again
• Yes, stick a fork in me, cuz Im done
• Works on unsorted items
• Binary search
• Cut in half every time

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Youre a Detective in a Small Town

• The killer called the victim
• Get the LUDs
• Look up the number in a reverse directory

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Arrays of Arrays

• Arrays in multiple dimensions
• Not just a list
• A table (an array of arrays)
• A cube (an array of arrays of arrays)

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An Example

• An array of arrays
• int b 1,2, 3, 4, 5
• Row 0 1 2
• Row 1 3 4 5
• A 3x3 array
• int b
• b new int33