Introduction to arrays

1
Introduction to arrays

• Data in economy size packages

2
Array

• Data structure that contains a collection of data
  items
• All items are of the same data type
• Access to the entire collection via the array
  name
• Access to individual elements via their indexes
  (or subscripts)
• Behaves like a list of variables with a uniform
  naming mechanism

3
Example

• Suppose you needed to store 20 distinct
  whole-number values what can you do?
• Declare 20 distinct int variables, giving each a
  unique name
• Declare a single int array with the capacity to
  hold 20 elements
• Which is preferable? What if the problem
  involved 200 numbers? 2000? 2,000,000?

4
Array declaration

• The syntax for array declaration is
• data type identifier
• data type is any built-in, library or
  user-defined data type (int, double, char,
  String, etc.)
• identifier is a valid Java identifier
• Java supports a variation of this syntax, which
  is closer to C/C syntax
• data type identifier

5
Array creation

• An array is a reference data type, similar to a
  class
• Declaration doesnt allocate any memory this is
  done, as with objects, with the new operator
• Examples
• double averages new double10
• char alphabet
• alphabet new char26

6
Array creation

• The number in square brackets when an array is
  created indicates the number of elements that can
  be stored in the array
• Any positive int expression can be used to
  allocate an array
• Example
• public static final int LETTERS 26
• char asciiAlphabet new char LETTERS 2

7
Array indexing length

• The length of the array is determined when it is
  created the value is stored in a public instance
  variable, length
• asciiAlphabet.length has the value 52
• If an array is created with size n, the
  subscripts of its elements are numbered (in
  order) from 0 to n-1
• So, for example, the first element in the
  asciiAlphabet array is element 0, and the last
  element is element 51

8
One overloaded operator

• We have already seen two uses for the indexing
  operator ()
• Array declaration for example, int nums
• Array creation nums new int 20
• This operator has one more use access to
  individual array elements, or indexing

9
Accessing individual array values

• An element is accessed via its index or subscript
• The notation for accessing an element via its
  subscript is
• arrayNamesubscript where
• arrayName is the arrays identifier
• subscript is a number between 0 and size-1
• The first element has subscript 0, the second has
  subscript 1, the third has subscript 2, and so
  forth up to the last element, which has subscript
  length-1

10
Examples

• asciiAlpha0 A
• System.out.print( asciiAlpha8)
• averages7 0.5
• Note although these examples use literal values,
  a subscript can be any int expression
• It is up to the programmer to ensure that the
  subscript value is within the bounds of the array

11
Filling an array with initial values
char upperAlpha new char 26 for (char c
A c lt Z c) upperAlpha(int)(c A)
c
Trace c c A value stored A 0
A B 1 B C 2 C .
. . Z 25 Z 26
12
Initializing at declaration
List all values in a block of code, terminating
with a semicolon
char lowerAlpha a, b, c, d, e,
f, g, h, i, j, k, l, m, n,
o, p, q, r, s, t, u, v, w, x,
y, z
Compiler automatically sizes array to hold the
specified data array will remain this size, even
if different data assigned
for (int x 0 x lt 10 x) lowerAlphax!
13
What values are assigned?

• float temps float 5 // allocates
  memory for array
• int m
• for (m 0 m lt 5 m)
• temps m 100.0 m 0.2

7000 7004 7008 7012
7016
? ? ? ?
?
temps0 temps1 temps2 temps3
temps4
14
Now what values are printed?

• float temps float 5 // allocates
  memory for array
• int m
• . . . . .
• for (m 4 m gt 0 m-- )
• System.out.println( temps m )

7000 7004 7008 7012
7016
100.0 100.2 100.4 100.6
100.8
temps0 temps1 temps2 temps3
temps4
15
Variable Subscripts

• float temps new float 5 //allocates
  memory for array
• int m 3
• . . . . . .
• What is temps m 1 ?
• What is temps m 1 ?

7000 7004 7008 7012
7016
100.0 100.2 100.4 100.6
100.8
temps0 temps1 temps2 temps3
temps4
16
Arrays of objects

• Arrays can be collections of elements of any data
  type, including classes
• For example, suppose we wanted to create an array
  of Fraction objects
• Array declaration
• Fraction ratios new Fraction100
• This creates the array, but not the individual
  Fraction objects it will contain these have to
  be created individually, as shown on the next
  slide

17
Initializing an array of objects
for (int x 0 x lt ratios.length
x) ratiosx new Fraction() // initializes
entire array with values 1/1 Random rg new
Random() for (int y 0 y lt ratios.length
y) ratiosy new Fraction (rg.nextInt(10)
1, rg.nextInt(20) 1) // initializes
array with random values
18
Methods array parameters

• An array element can be passed to any method that
  takes its base type as an argument
• For example, in the Fraction class the plus
  method takes a Fraction argument elements of the
  array declared on the previous slide could serve
  as both calling object and argument
• ratios0 ratios1.plus(ratios2)

19
Methods array parameters

• An entire array can be passed as an argument to a
  method, if such a parameter is specified
  example
• public static void fillFractionArray (Fraction
  fracs)
• Random rg new Random()
• for (int x 0 x lt fracs.length x)
• fracsx new Fraction (rg.nextInt(10)1,rg.next
  Int(20)1)
• A call to this method
• Fraction ratios new Fraction25
• Fraction.fillFractionArray(ratios)
• Note that the square brackets are not used when
  an entire array is passed as an argument

20
public static void main (String args) What
is args?
A. A String B. An array of Strings C.
Nothing D. None of the above