Array Basics

1
Array Basics
2
Arrays

• An array is a special kind of object used to
  store a collection of data.
• Arrays are different from other objects in the
  following two ways
• All the data stored in an array must be of the
  same type.
• The only methods for an array object are a small
  number of predefined methods which are invoked
  using a special notation.

3
Example Showing How to Compute the Average of
Seven Values

• int count
• double next, sum, average
• Scanner keyboard new Scanner(System.in)
• System.out.println(Enter 7 values)
• sum 0
• for (count 0 count lt 7 count)
• next keyboard.next.Double()
• sum sum next
• average sum/7

4
Comments on Previous Example

• It works fine if all you want know is the average
  but suppose you want to know how many values are
  above average.
• In order to do this you must save all the values
  and compare them to the average.
• You could do this by creating seven variables,
  one for each value, but what if there were 1000
  values? That would be tedious.

5
Creating and Accessing Arrays

• An array is something like a list of variables,
  but it handles the naming of the variables in a
  nice compact way.
• In Java, an array is a special kind of object,
  but it is often more useful to think of an array
  as a collection of variables all of the same
  type.
• For example, an array to serve as a collection of
  seven variables of type double can be created as
  follows
• double value new double7

6
Creating and Accessing Arrays (contd)

• This is like declaring the following to be seven
  variables of type double
• value0, value1, value2, value3,
  value4, value5, value6
• Note that the number begins with 0 not 1.
• Each of these variables can be used like any
  other variable of type double, for example
• value3 32
• value6 value3 5
• System.out.println(value6)

7
Creating and Accessing Arrays (contd)

• However these seven variables are more than just
  seven plain old variables of type double.
• The number in the square brackets does not have
  to be an integer constant it can be any integer
  expression.
• That is, you can compute its value.

8
Creating and Accessing Arrays (contd)

• For example, you could write
• Scanner keyboard new Scanner(System.in)
• System.out.println(Enter day number (0-6))
• int index keyboard.nextInt()
• System.out.println(Enter temperature for day
  index)
• temperatureindex keyboard.nextDouble()

9
Creating and Accessing Arrays (contd)

• These variables with an integer expression in
  square brackets are referred to by many names,
  e.g., indexed variables, elements, or subscripted
  variables.
• The integer expression in the square brackets is
  called the index or subscript.
• When we think of these indexed variables grouped
  together into one collective item, we will call
  them an array.

10
Original Example Written Using an Array

• for (index 0 index lt 7 index)
• valueindex keyboard.nextDouble()
• sum sum valueindex

11
Array Details

• The syntax for creating an array of elements of
  Base_Type is as follows
• Base_Type Array_Name new Base_TypeLength
• For example, the following creates an array named
  pressure that is equivalent to 100 variables of
  type int
• int pressure new int100

12
Array Details (contd)

• The base type of an array can be of any type.
• In particular, it can be a class type.
• For example, we can create an array of type
  Species.
• Species entry new Species3

13
Example Use of an Array

• import java.util.public class
  ArrayOfTemperatures / Reads in 7
  temperatures and shows which are above and
  which are below the average of the 7
  temperatures. / public static void
  main(String args) double
  temperature new double7 int index
  double sum, average Scanner
  keyboard new Scanner(System.in)
  System.out.println("Enter 7 temperatures")
  sum 0 for (index 0 index lt 7
  index) temperatureindex
  keyboard.nextDouble( ) sum sum
  temperatureindex average
  sum/7 System.out.println("The average
  temperature is "
  average)
  System.out.println("The temperatures are")
  for (index 0 index lt 7 index)
  if (temperatureindex lt average)
  System.out.println(
  temperatureindex " below average.")
  else if (temperatureindex gt average)
  System.out.println(
  temperatureindex " above average.")
  else //temperatureindex average
  System.out.println(
  temperatureindex " the average.")
  System.out.println("Have a nice week.")
  

14
The length Instance Variable

• An array has one public instance variable, called
  length which is equal to the length of the array.
• For example, for the following array
• Species entry new Species20
• entry.length has the value of 20.
• The use of this variable can help to make you
  program clearer.
• The length instance variable cannot be changed by
  your program.

15
Array Index Out of Bounds

• If you try to use a value for an index that is
  outside the range of legal values allowed for
  your array an error will occur.
• The compiler will not catch the error, but it
  will be discovered when you try to execute the
  statement containing an invalid value for the
  index of an array element.
• When this happens we say that the index is out of
  bounds.

16
Array Index Out of Bounds (contd)

• This can happen when you try to fill an array by
  reading in values from the keyboard, for example
• System.out.println(enter a list of nonnegative
  integers.)
• System.out.println(Place a negative integer at
  the end.)
• int a new int10
• Scanner keyboard new Scanner(System.in)
• int number keyboard.nextInt()
• int i 0
• while (number gt 0)
• ai number
• i
• number keyboard.nextInt()

17
Initializing Arrays

• An array can be initialized at the time that it
  is declared. For example
• double reading 3.3, 15.8, 9.7
• The size of this array is automatically set to 3.
• The above statement is equivalent to double
  reading new double3
• reading0 3.3
• reading1 15.8
• reading2 9.7

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Array Index Out of Bounds (contd)

• A better version of the while loop is as follows
• while ( (i lt a.length) (number gt 0))
• ai number
• i
• number keyboard.nextInt()
• if (number gt 0)
• System.out.println(Could not read in all the
  numbers.)
• System.out.println(Only read in a.length
  numbers.)