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List Data Structure
What is List?
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A list is a sequential data
structure.
It differs from the stack and
queue data, structures in that
additions and removals can be
made at any position in the list.
List operations
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Add
: adds a new node
Set
: update the contents of a
node
Remove : removes a node
IsEmpty : reports whether the list is
empty
IsFull
: reports whether the list is
full
Initialize : creates/initializes the list
Destroy : deletes the contents of the
list (may be implemented by reinitializing the list)
Illustration/example
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Initialize(L)
Create a new empty List named L
Add(1,X,L)
adds the value X to list L at position 1 (the start of the list
is position 0), shifting subsequent elements up
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Set(2,Z,L)
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Remove(Z,L) removes the node with value Z
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Get(2,L)
returns the value of the third node, i.e. C
IndexOf(X,L)
returns the index of the node with value X, i.e. 1
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updates the values at position 2 to be Z
Illustration/example
Operation
List’s contents
Return value
1. Initialiaze(L)
<empty>
-
2. Add(0,A,L)
3. Add(0,B,L)
4. Add(1,C,L)
5. Set(1,N,L)
6. Add(1,D,L)
7. Remove(A,L)
8. Set(3,I,L)
9. Remove(D,L)
A
B A
B C A
B N A
B D N A
B D N
B D N I
BN I
A
D
10. Remove(N,L)
BI
N
Exercise: List Operation
What would the contents of a list be after the following
operations?
Initialise(L)
Add(0,A,L)
Add(0,F,L)
Add(1,X,L)
Add(1,G,L)
Add(3,P,L)
Add(2,V,L)
Set(3,K,L)
Set(0,H,L)
Remove(V,L)
Remove(P,L)

HGKA
What values would be returned by the following
operations?
IndexOf(A,L)
IndexOf(H,L)
Get(3,L)
Storing a list in a static data
structure (Array List)
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This implementation stores the list in an array.
The position of each element is given by an index
from 0 to n-1, where n is the number of elements.
Given any index, the element with that index can
be accessed in constant time – i.e. the time to
access does not depend on the size of the list.
To add an element at the end of the list, the time
taken does not depend on the size of the list.
However, the time taken to add an element at any
other point in the list does depend on the size of
the list, as all subsequent elements must be
shifted up. Additions near the start of the list take
longer than additions near the middle or end.
When an element is removed, subsequent
elements must be shifted down, so removals near
the start of the list take longer than removals near
the middle or end.
Storing a list in a dynamic data
structure (Linked List)
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The Link List is stored as a sequence of linked nodes.
As in the case of the stack and the queue, each node in a
linked list contains data AND a reference to the next node.
The list can grow and shrink as needed
The position of each element is given by an index from 0 to n1, where n is the number of elements.
Given any index, the time taken to access an element with
that index depends on the index. This is because each
element of the list must be traversed until the required index is
found.
The time taken to add an element at any point in the list does
not depend on the size of the list, as no shifts are required. It
does, however, depend on the index. Additions near the end
of the list take longer than additions near the middle or start.
The same applies to the time taken to remove an element.
The first node is accessed using the name LinkedList.Head
Its data is accessed using LinkedList.Head.DataItem
The second node is accessed using
LinkedList.Head.NextNode
Adding a node
The new node is to be added at a specified index in the list
A special case is that the list is empty. In this case, the first
node is set to be the new node.
Another special case is that the specified index is 0 (the
node is to be added at the front of the list).
Adding a node (2)
In the general case, an object reference Probe moves
through the list until the node
before the required index is reached. Here, the node is to be
added at index 2.
If the new node is to be added at the end of the list, then the
NextNode of the last element is set to refer to the new
node.
Adding a node (3)
A possible algorithm (pseudocode) for the Add operation
Add(index, Data, LinkedList)
Declare NewNode and initialise NewNode.DataItem with data
If(List.isEmpty)
Set NewNode.NextNode to NULL
Copy NewNode to LinkedList.Head
Else
If (index is 0)
Copy LinkedList.Head to NewNode.NextNode
Copy NewNode to LinkedList.Head
Else
Copy LinkedList.Head to Probe
i=0
While i < index-1 And end of list not reached
Copy Probe.NextNode to Probe
Increment i
End While
Copy Probe.NextNode to NewNode.NextNode
Copy NewNode to Probe.NextNode
End If
End If
Set NewNode to NULL
Set Probe to NULL
Additional operations can also be implemented to add to the
beginning and end of the list.
AddFirst(Data, LinkedList) calls Add(0, Data, LinkedList)
AddLast(Data, LinkedList) calls Add(Size, LinkedList), where
Size is itself a call to the Size operation of the list.
Removing a node
A TargetNode object is created.
There is a special case when TargetNode is equal to the first
node. In this case, LinkedList is set to point to the
second node.
In the general case, an object reference Probe moves
through the list until the required node is reached – at
each node the current node (Probe) is compared with
TargetNode.
A Previous object reference is also required to keep track of
the predecessor of the node to be removed. A special
case is when both references point to the same node –
this happens when the first node is to be deleted.
To delete the current node we can set Previous.NextNode to
be equal to Probe.NextNode.
Removing a node (2)
Accessing nodes (Get, Set,
IndexOf, View)
Get and Set work in a similar way to
Add – Probe moves through the
list until the required index is
reached, and the DataItem is
returned (Get) or updated (Set).
IndexOf works in a similar way to
Remove – Probe moves through
the list until the target node is
found, and returns the index.
View – Probe moves through the list
from beginning to end and the
DataItem is returned at each node.
Variations on Linked Lists
Circularly linked lists
The tail of the list always points to the head of the list
Doubly linked lists
These permit scanning or searching of the list in both
directions. (To go backwards in a simple list, it is
necessary to go back to the start and scan forwards.) In
this case, the node structure is altered to have two links:
Sorted lists
Lists can be designed to be maintained in a given order. In this
case, the Add method will search for the correct place in
the list to insert a new data item.
List Implementation
List Implementation in Java
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The Java Collections Framework in the most recent
version of Java now includes list classes.
As you did for the stack & queue, you will create
your own List class in order to learn how a list is
implemented.
Your class will again be a bit simpler than the
Collections Framework one but it will do essentially
the same job
In this case you will look at Java implementations of
an ArrayList and a LinkedList. Both lists have the
same operations, so we can define a class List
which includes minimal implementations of the
operations. The ArrayList and LinkedList classes
will be subclasses of List.
The List class will define the set of operations which
a list class must have. This is sometimes known as
the interface of a list. Both kinds of list will have the
same interface.
List Implementation in Java
Notice that the LinkedList class makes use of
a Node class which is exactly the same as
the one used for the dynamic queue.