Arrays

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Arrays

• Chapter 6

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The Very Large Array
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The Very Large Array

• National Radio Astronomy Observatory, NM
• The array consists of 27 independent radio
  antennas
• The array acts like a single antenna with a
  diameter of 36km.
• The antennas are arranged along three arms of
  Y-shape (each of which measures 21km)
• The antennas can be physically relocated on the
  railroad tracks along the Y-shape.
• Stands at 2124m above sea level
• The dish diameter is 25m and it weighs 230 tons.

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The Very Large Array
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Introduction to Arrays

• An array is an object used to store a (possibly
  large) collection of data.
• All the data stored in the array must be of the
  same type.
• An array object has a small number of predefined
  methods.

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Introduction to Arrays, cont.

• Sometimes you want to know several things about a
  collection of data
• the average
• the number of items below the average
• the number of items above the average
• etc.
• This requires all the items to be stored as
  variables of one kind or another.

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Creating and Accessing Arrays

• example
• double temperature new double7
• temperature3 32.0
• temperature6 temperature3 5
• System.out.println(temperature6)
• temperatureindex1 66.5
• The integer expression within the square brackets
  is called the index (or subscript).

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Creating and Accessing Arrays
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Array Base Type

• The type of the elements is called the base type.
• The base type of an array can be any type
  including a class type.
• for example,
• Species entry new Species3
• The number of elements is the length or size of
  the array.

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The length Instance Variable

• An array has only one public instance variable,
  namely length.
• The length variable stores the number of elements
  the array can hold.
• Using Array_Name.length typically produces
  clearer code than using an integer literal.

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Indices and length

• The indices of an array start with 0 and end with
  Array_Name.length-1.
• example
• for (lcv 0 lcv lt temperature.length
  index)
• Otherwise, the index is said to be out of bounds
  or invalid.

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Initializing Arrays

• double reading 3,3, 15.8, 9.7
• The size of the array is determined by the number
  of values in the initializer list.
• int count new int100
• for (int i 0, i lt count.length, i)
• ai 0

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Arrays in Classes and Methods

• Arrays can be used as instance variables in
  classes.
• Both an indexed variable of an array and an
  entire array can be a argument of a method.
• Methods can return an indexed variable of an
  array or an entire array.

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Case Study Array as an Instance Variable

• Prepare a sales report showing
• which sales associate (or associates) has the
  highest sales
• how each associates sales compare to the
  average.
• needed for each associate
• name
• sales figures

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Case Study Using an Array as an Instance
Variable, cont.

• tasks
• obtain the data (getFigures)
• update the instance variables (update)
• display the results (displayResults)

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Pseudocode

• An outline of a program, written in a form that
  can easily be converted into real programming
  statements.
• Pseudocode cannot be compiled executed.
• There are no real formatting or syntax rules.
• It is simply one step - an important one - in
  producing the final code.
• It enables the programmer to concentrate on the
  algorithm without worrying about all the
  syntactic details of a particular programming
  language.

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Pseudocode - an Example

• pseudocode for a bubble sort routine
• while not at end of list compare adjacent
  elementsif second is greater than first
  switch them get next two elementsif elements
  were switched repeat for entire list

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Indexed Variables as Method Arguments
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Arrays as Method Arguments

• An entire array can be used as a single argument
  passed to a method.
• example
• double a new double10
• SampleClass.change(a)
• ...
• public static void change(double d)
• No brackets accompany the argument.
• Method change accepts an array of any size.

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Arguments for the Method main

• Recall the heading for method main
• public static void main(String args)
• Method main takes an array of String values as
  its argument.
• java TestProgram Mary Lou
• args0 is set to Mary
• args1 is set to Lou

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Use of and with Arrays

• The assignment operator and the equality
  operator , when used with arrays, behave the
  same as when used with other objects.
• The assignment operator creates an alias, not a
  copy of the array.
• The equality operator determines if two
  references contain the same memory address, not
  if two arrays contain the same values.

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Making a Copy of an Array

• example
• int a new int50
• int b new int50
• ...
• for (int j 0 j lt a.length j)
• bj aj

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Determining the Equality of Two Arrays

• To determine if two arrays at different memory
  locations contain the same elements in the same
  order, define an equals method which determines
  if
• both arrays have the same number of elements
• each element in the first array is the same as
  the corresponding element in the second array.

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• class TestEquals

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• class ReturnArrayDemo

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Programming Example A Specialized List Class

• An array can be an instance variable of a class
  that is accessible only through the class
  methods.
• We will define a class whose objects store lists
  of items, such as a grocery list or a list of
  things to do.
• Well name the class OneWayNoRepeatsList.

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Programming Example, cont.

• some features of the class
• a method for adding an item to the end of the
  list unless it is on the list already
• an array of strings to hold the items
• numbering starting with 1 rather than 0
• a maximum list size
• accessor and mutator methods, but no methods for
  changing or deleting items
• a method to erase the entire list

A well-encapsulated class
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• class ListDemo

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More Features of the Class

• We need a means of determining that the end of
  the list has been reached.
• Let toDoList.getEntryAt(position) return the
  value null.

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• class OneWayNoRepeatsList

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• class OneWayNoRepeatsList, contd.

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Partially Filled Arrays
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Sorting Arrays

• Sometime we want numbers in an array sorted from
  smallest to largest, or from largest to smallest.
• Sometimes we want the strings referenced by an
  array to be in alphabetical order.
• Sorting techniques typically are easy to adapt to
  sort any type that can be ordered.

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Selection Sort
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• class SelectionSort

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Sorting Algorithms

• Heap Sort
• Bubble Sort
• Quick Sort
• .

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Bubble Sort
Programming Project 4
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Multidimensional Arrays
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Multidimensional-Array Basics

• example declaration
• int table new int 106
• or
• int table
• table new int106
• SomeClass entry new SomeClass10080

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• class InterestTable

P P0(1r)
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• multidimentional array as a parameter

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Multidimensional Arrays

• Multidimensional arrays are implemented in Java
  using one-dimensional arrays.
• Consider
• int table new int106
• The array table is a one-dimensional array of
  length 10.
• Its base type is int.
• Therefore, it is an array of arrays.

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Multidimensional Arrays, cont.

• This permits us to use the length instance
  variable, instead of an integer literal, to
  control a for loop used to initialize or print
  the values of the elements of an array.
• example
• for (r 0 r lt table.length r)
• for (c 0 c lt tabler.length c)

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Ragged Arrays

• Since a two-dimensional array in Java is an array
  of arrays, each row can have a different number
  of elements (columns).
• Arrays in which rows have different numbers of
  elements are called ragged arrays.
• Example
• int b new int3
• b0 new int5
• b1 new int7
• b2 new int4