OneDimensional Arrays

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One-Dimensional Arrays
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Arrays

• An array is an ordered list of values
• An array of size N is indexed from zero to N-1
• This array holds 4 values that are indexed from 0
  to 3

Entire array has A single name
Each value has A numeric index
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Processing Array Elements

• Access individual elements of an array using
• the name of the array
• a number (index or subscript) that tells which of
  the element of the array
• What value is stored in scores2?

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Arrays

• For example, an array element can be assigned a
  value, printed, or used in a calculation
• scores2 87
• scoresfirst scoresfirst 2
• Avg (scores0 scores1)/2
• System.out.println (Max " scores3)

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Arrays

• The values held in an array are called array
  elements
• Array elements are of the same type
• The element type can be a primitive type or an
  object reference.
• Therefore, we can create an array of integers, or
  an array of characters, or an array of String
  objects, etc.
• In Java, the array itself is an object
• Therefore the name of the array is a object
  reference variable, and the array itself must be
  instantiated

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Declaring an Array

• This declares the handle only
• Initialized to null
• Stores an address when arrays are created

• Defining an array
• Type name
• Where
• Type specifies the kind of values the array
  stores
• the brackets indicate this is an array
• name is the handle to access the array

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Declaring an Array

• The scores array could be declared as follows
• double scores new double 4
• The above statement will
• allocate block of memory to hold 4 doubles
• initialize block with zeros
• create a handle or a pointer or a reference
  called scores
• store address of the block in scores

0.0
0.0
0.0
0.0
Block
Handle
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Syntax

• Forms
• ElementType arrayNameElementType
  arrayName new ElementType sizeElementType
  arrayName array-literal
• Where
• ElementType is any type
• arrayName is the handle for the array
• array-literal is a list of literals enclosed in
  curly braces

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Declaring Arrays

• Some examples of array declarations
• float prices new float500
• boolean flags
• flags new boolean20
• char codes new char1750

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Bounds Checking

• Once an array is created, it has a fixed size
• An index used in an array reference must specify
  a valid element
• That is, the index value must be in bounds (0 to
  N-1)
• The Java interpreter throws an ArrayIndexOutOfBoun
  dsException if an array index is out of bounds
• This is called automatic bounds checking

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Bounds Checking

• For example, if the array scores can hold 100
  values, it can be indexed using only the numbers
  0 to 99
• If scores has the value 100, then the following
  reference will cause an exception to be thrown
• System.out.println (scorescount)
• Often for loop is used to process array. Its
  common to introduce off-by-one errors when using
  arrays
• for (int index0 index lt 100 index)
• scoresindex index50 epsilon

problem
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Bounds Checking

• Each array object has a public constant called
  length that stores the size of the array
• It is referenced using the array name
• scores.length
• Note that length holds the number of elements,
  not the largest index

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Initializer Lists

• An initializer list can be used to instantiate
  and initialize an array in one step
• The values are delimited by braces and separated
  by commas
• Examples
• int scores 98, 76, 54, 83, 87, 65, 99,
  66
• char letterGrades 'A', 'B', 'C', 'D',
  F'

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Initializer Lists Example 1

• Used when exact size and initial values of an
  array are known in advance
• int scores 98, 76, 54, 83
• Visualize the results of the above command

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Initializer Lists Example 2

• Consider the following 4 student names
• Note results

String STUDENTS Aref", Ali, Emad,
"Sami"
Aref
Ali
Emad
Sami
Array elements are handles for String values
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Initializer Lists

• Note that when an initializer list is used
• the new operator is not used
• no size value is specified
• The size of the array is determined by the number
  of items in the initializer list
• An initializer list can only be used only in the
  array declaration

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The Assignment Operation

• Java provides a few operations to use with
  arrays, including assignment
• Consider
• int alist 1, 12, 15, 7
• int blist
• blist alist
• Recall that alist and blist are handles
• alist contains an address of where the numbers
  are
• blist now contains that same address
• blist does not have a copy of alist

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Array Cloning

• To actually create another array with its own
  values, Java provides the .clone() method
• int alist 1, 12, 15, 7
• int blist
• blist alist.clone()
• Now there are two separate lists of numbers, one
  with handle alist, the other with handle blist

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Array Cloning with Reference Types

• Recall previous declaration
• Consider
• String s_list STUDENTS.clone()
• This will create another list of handles, also
  pointing to the names

Aref
Ali
Emad
Sami
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Array Cloning with Reference Types

• We can write our own "deep copy"
• String original Aref,Ali, Emad,
  Sami
• String result new Stringoriginal.length
  for (i 0 i lt original.length i)
  resulti originali.clone()

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Array Equality

• Java has an equals() method for classes
• if (a1.equals(a2))
• If a1 and a2 are arrays, the equals() method just
  looks at the addresses the handles point to
• They could be pointing to different addresses but
  the contents of the array still be equal
• It is the contents that we really wish to compare

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Array Equality

• We must write our own method to compare the
  arrays
• they both must have the same length
• then use a for( ) loop to compare element by
  element for equality
• Boolean areEqual true
• if (list1.length ! list2.length) areEqual
  false
• else
• for (int i 0 i lt list1.length i) if
  (list1i ! list2i) areEqual false
• if(areEqual) System.out.println(They are
  equal)
• else System.out.println(They are NOT equal)

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THE END