y.j.f.cartwrightstaffs.ac.uk

1
Data Structures
Staffordshire University Faculty of
Computing, Engineering and Technology

• Arrays
• Multi-dimensional arrays
• Arrays of objects
• The List abstract data type

2
Arrays

• All programming languages have a simple way to
  store related elements of the same type - the
  array.
• Different languages do arrays in different ways.
• This is particularly true for web languages.
• In most 3GLs, an array has a fixed
• name
• type
• length
• All of these must be declared when it is created.
• C arrays follow the same syntax as C, C and
  Java.

3
Example array

• myArray has room for 8 elements.
• All elements are of type int.
• The elements are accessed by their index.
• In C, array indices start at 0.

4
Declaring arrays

• int myArray
• declares myArray to be an array of integers.
• myArray new int8
• sets up 8 integer-sized spaces in memory,
  labelled myArray0 to myArray7.
• int myArray new int8
• combines the two statements in one line.

5
Arrays

• We refer to the array elements by their index to
  store values in them
• myArray0 3
• myArray1 6
• myArray2 3 etc.
• We can create and initialise in one step
• int myArray 3, 6, 3, 1, 6, 3, 4, 1
• Although compact, there are very few practical
  uses of creating and initialising arrays in this
  way.

6
Looping through arrays

• for loops are useful when dealing with arrays
• for (int i 0 i lt myArray.Length i)
• Console.WriteLine("Please enter a number ")
• myArrayi Convert.ToInt32(Console.ReadLine())
  
• for (int i 0 i lt myArray.Length i)
• Console.WriteLine("Element " i " "
  myArrayi)

7
Multi-dimensional arrays

• So far, our arrays have one dimension.
• We have seen this pictorially in the previous
  slides as a row of slots or elements.
• For two-dimensional arrays, you have rows and
  columns of elements (a grid).
• For three-dimensional arrays, you have multiple
  layers of rows and columns (a 3D grid).
• For four-dimensional arrays? Well, you can have
  them but they are hard to picture!

8
Multi-dimensional arrays

• In C, there are actually two different ways of
  specifying a multi-dimensional array.
• For 2D, the standard approach is to do something
  like
• int myArray new int
• int myArray new int2 // array has 2
  rows
• int myArray new int3 // not valid!
• int myArray new int23 // 2 rows, 3
  columns
• This has a similar syntax to the one-dimensional
  array.
• This is known as a jagged array (or array of
  arrays) where each row can have a different
  number of elements.

9
Jagged arrays

• After you declare a jagged array, you have to
  then define how many elements a particular row
  has before you can use it
• int myArray int
• myArray0 new int7
• myArray1 new int5
• This would be a two-dimensional jagged array with
  2 rows, the first of which can store 7 integers
  and the second row stores 5 integers.
• Jagged arrays are very flexible but have a
  performance penalty if all you want is a simple
  2D grid of elements.

10
Rectangular arrays

• When all you want is a standard 'grid' of
  elements (in any arbitrary number of dimensions),
  you can use a rectangular array instead.
• int, myArray new int2,3
• This creates a two-dimensional array which has 2
  rows containing 3 elements each.
• myArray1,2 refers to the second row and third
  column along.
• These types of arrays are simpler to use and
  faster to run than jagged arrays but have no
  flexibility to their sizes.

11
Arrays of objects

• So far, we have looked at an array of primitive
  types
• integers.
• could also use doubles, floats, characters
• We often want to have an array of objects
• Students, Books, Loans etc.
• We need to follow 3 steps.
• Declare the array
• private Student studentArray
• this declares studentArray to be an array of
  Student objects

12
Arrays of objects

• Create the array
• studentArray new Student10
• this sets up 10 spaces in memory that can hold
  references to Student objects.
• Create Student objects and add them to the array
• studentArray0 new Student(Yvan",
  "Computing")

13
Arrays of objects

• Once we have declared, initialised and filled our
  array of objects, we can call the objects'
  methods as usual.
• Note we use the array index to identify which
  object we are invoking the method on.
• studentArray4.setProgram("Multimedia")
• studentArray6.incrementLevel( )
• Console.WriteLine(studentArray3.ToString())

14
Looping through arrays of objects

• for loops are also useful when dealing with
  arrays of objects.
• for (int i 0 i lt studentArray.Length i)
• Console.WriteLine("Name? ")
• String name Console.ReadLine()
• Console.WriteLine(Program? ")
• String program Console.ReadLine()
• studentArrayi new Student(name, program)
• for (int i 0 i lt studentArray.Length i)
• Console.WriteLine(studentArrayi.ToString())

15
List - An abstract data type

• In computing (and in real life) we often use
  lists.
• A list is a group of items arranged in some
  order
• groceries to buy
• names in a phone book
• students in tutorial group
• Lists are often modelled using an array
• there are other ways of modelling lists, such as
  trees and linked lists which we haven't covered
  yet
• or you could use paper!

16
List - An abstract data type

• There are certain operations we would like to be
  able to carry out on a list
• These operations should not depend on how the
  List is actually modelled in code form hence
  abstract.
• insert a new value at the end of the list
• output the list in order
• insert a new value into a given position, not
  necessarily at the end
• delete a data value
• search for a given data value
• sort the list by some criterion (for example,
  into alphabetical order)

17
Modelling a list with an array

• We need to create an array to hold all the list
  elements.
• This array will need to be
• big enough for the likely maximum size of the
  list
• but not so big that it wastes space
• We need to keep track of the next empty array
  element.
• this also equals the current number of list
  elements
• We need to store the maximum size of the array.
• this is already done for us as the Length
  attribute of the array class provides this.
• notice that this implies that an array (of any
  type) is really a C class.

18
Lists and arrays

• This array has room for 8 integers
• therefore, myArray.Length has the value 8
• It now contains 4 integers.
• The next available space is position 4

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Create a list with a given size

• We need a constructor that makes use of a
  user-defined parameter to set the size of the
  array.
• public IntegerList(int size)
• myArray new intsize
• next 0

20
Adding to the end of a list

• We need to
• store the new data value (8 in this example) in
  position next
• increase next by 1

21
Adding to the end of a list

• We need to check first that the array is not
  already full
• this is the case when next myArray.Length
• if the array is full
• display an appropriate message
• do not add the new element

22
Adding to the end of a list

• public bool add(int newData)
• if (next lt myArray.length) // make sure array
  not full
• myArraynext newData
• next
• return true // indicate success
• else
• return false // indicate new data not added

23
Printing out the list

• Remember the array might not be full!
• Loop from element 0 to next - 1
• public void printList()
• Console.WriteLine("List follows ")
• for (int i0 i lt next i)
• Console.WriteLine(i " " myArrayi)

24
Lists of objects

• You can write methods to deal with lists of
  objects similar to our examples of lists of
  integers
• public bool addStudent(Student s)
• public void printStudentList()
• Put these methods in a suitably named class (e.g.
  School).
• Store the list in an array, which is a private
  attribute of the class.
• private Student studentArray
• Be sure to create the array in the class
  constructor
• public School(int capacity)
• can then create an array of size capacity in this
  constructor

25
Tutorial work

• Implement a class which maintains a list of
  integers
• use methods from this lecture.
• write a main method to test.
• Implement a School class which has a list of
  Students
• adapt methods from this lecture.
• write a SchoolApp class with a main method for
  testing purposes.