Arrays

1
Arrays

• Chapter 7

2
"All students to receive arrays!" reports Dr.
Austin.
scores 85 79 92 57 68 80 . . .
0 1 2 3 4 5 98 99
3
Declaring and Creating Arrays

• Recall that an array is a collection of elements
  all of the same type
• Array objects in Java must be created with the
  key word new
• Syntaxint c new int 12
• Results
• 12 integer locations are allocated
• They are initialized to 0(null for reference
  variables, false for boolean)

4
Using Arrays

• View example Figure 7.2
• Note
• Declaration of the array
• Allocation of memory with new
• Initialization of array elements
• Use of array attribute (length)
• Printing of array contents

5
The Array Initializer

• A comma separated list of expressions
• The initializer list
• Enclosed in braces
• View the Example
• Note again, how the array knows its own
  lengthfor (int count 0 count lt array.length
  count) . . .

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Processing Elements of An Array

• Finding and using the arithmetic mean
• Declare the array, create
• Read in values, count how many
• Declare a summation variable, initialize
• Sum the array, calculate average
• Sort the array
• Print results, accessing the array
• See sample program

7
Histogram Program

• Use the value in the array to
• Print the value
• Print a string of asterisks for visualization of
  size of the value
• View Figure 7.6,

8
Using Array Elements as Counters

• Recall program which tested random number
  generator (Figure 6.8)
• Used 6 different variables
• Used switch statement to increment
• At the time we noted the inefficiency
• Consider a new version of the program
• Uses array elements to total the number of rolls
  of each number 1 6
• No switch statement needed
• View Figure 7.7

9
Enhanced for Statement

• Enhanced for statement
• New feature of J2SE 5.0
• Allows iterates through elements of an array or a
  collection without using a counter
• Syntax
• for ( parameter arrayName )
• statement
• View example, Figure 7.12

10
References and Reference Parameters

• In Java, primitive-type variables are always
  passed by value
• Incoming data only
• Objects are not passed to methods
• References to objects are passed
• The reference is passed by value
• With a reference to the object the method can
  manipulate the object directly

Address of object
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References and Reference Parameters

• Passing arrays by reference
• Improves performance
• Saves time
• Initialization of large array not required
• Saves memory
• Use memory already occupied by array
• No new memory allocation required
• View example program, Figure 7.13

12
Using Arrays

• Sorting Arrays
• Recall previous example
• Searching Arrays
• Linear search (see example)
• Step through array until desired value located
• Binary search
• Array must be sorted
• Look in middle, then above or below
• Look in middle of sub-section then above or below
• Etc.

13
Multidimensional Arrays

• Java does not support multidimensional arrays
  directly
• Does allow declaration of arrays whose elements
  are arrays!

int bb new int 2 // allocate
rowsb 0 new int 5 // allocate columns
for row 0b 1 new int 3 // allocate
columns for row 1
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Fig. 7.16 Two-dimensional array with three rows
and four columns.
Multidimensional Arrays
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Creating Two-dimensional Arrays

• Can be created 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 // create 2 rows b 0
  new int 5 // create 5 columns for row 0
  b 1 new int 3 // create 3 columns for
  row

View ExampleFigure 7.17
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Variable-Length Argument Lists

• New feature in J2SE 5.0
• Unspecified number of arguments
• Use ellipsis () in methods parameter list
• Can occur only once in parameter list
• Must be placed at the end of parameter list
• Array whose elements are all of the same type
• View example, Fig. 7.20

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Using Command-Line Arguments

• Pass arguments from the command line
• String args
• Appear after the class name in the java command
• java MyClass a b
• Number of arguments passed in from command line
• args.length
• First command-line argument
• args 0

View ExampleFigure 7.21