linked list1.ppt linked list ppts and notes

Why linked lists
• Disadvantages of arrays as storage data structures:
– slow insertion in ordered array
– Expensive to insert or delete elements
– Fixed size

Linked lists
•A linked list, or one way list, is a linear collection of data elements, called
nodes, where the linear order is given by means of pointers.
•Each node is divided into two parts:
•The first part contains the information of the element, and
•The second part, called the link field or next pointer field, contains the
address of the next node in the list.
•The pointer of the last node contains a special value,called the null pointer.
•A pointer variable – called START which contains the address of the first
node.
•A special case is the list that has no nodes, such a list is called the null list
or empty list and is denoted by the null pointer in the variable START.
Linked list with 3 nodes
Start
Data Next
node node
Data
Data Next
node
node
A
data pointer

linked lists
•A linked list organizes a collection of data items (elements ) such that elements
can easily be added to and deleted from any position in the list.
•Only references To next elements are updated in insertion and deletion
operations.
•There is no need to copy or move large blocks of data to facilitate insertion and
deletion of elements.
•Lists grow dynamically.

Representation of Linked lists in memory
1
2
3
4
5
6
7
8
67
45
80
75
90
3
5
2
7
4
0
START START=3, INFO[3]=45
LINK[3]=2, INFO[2]=67
LINK[2]=5, INFO[5]=75
LINK[5]=4, INFO[4]=80
LINK[4]=7, INFO[7]=90
LINK[7]=0, NULL value, So the list has ended
INFO LINK

Traversing a linked lists
LIST be a linked list in memory stored in linear arrays INFO and LINK with START
pointing to the first element and NULL indicating the end of LIST.
We want to traverse LIST in order to process each node exactly once.
Pointer variable PTR points to the node that is currently being processed.
LINK[PTR] points to the next node to be processed.
Thus update PTR by the assignment
PTR : =LINK[PTR]
START
X
INFO LINK
PTR
Fig : PTR : = LINK[PTR]

For printing the information at each node of a linked list, must traverse the list.
Algorithm 5.1 : TRAVERSE(INFO, LINK, START)
Algorithm Prints the information at each node of the list.
1. Set PTR : =START.
2. Repeat steps 3 and 4 while PTR : ≠ NULL:
3. Write : INFO[PTR].
4. Set PTR : =LINK[PTR].
5. Exit.

For Finding the number NUM of elements in a linked list, must traverse the list.
Algorithm 5.1 : COUNT(INFO, LINK, START, NUM)
1. Set NUM: =0.
2. . Set PTR : =START.
3. Repeat steps 4 and 5 while PTR : ≠ NULL:
4. Set NUM : =NUM+1.
5. Set PTR : =LINK[PTR].
6. Exit.

Searching a linked lists
List is unsorted
Algorithm 5.2 : SEARCH(INFO, LINK, START, ITEM,LOC)
It finds the location LOC of the node where ITEM first appears in LIST,
or sets LOC=NULL.
1. Set PTR: =START.
2. Repeat step 3 while PTR !=NULL:
3. If ITEM=INFO[PTR] then:
Set LOC:=PTR and Exit.
Else
Set PTR := LINK[PTR].
4 Set LOC:= NULL.
5 Exit.

Searching a linked lists
List is sorted
Algorithm 5.3 : SEARCHSL(INFO, LINK, START, ITEM,LOC)
It finds the location LOC of the node where ITEM first appears in LIST,
or sets LOC=NULL.
1. Set PTR: =START.
2. Repeat step 3 while PTR !=NULL:
3. If ITEM>INFO[PTR] then:
Set PTR:= LINK[PTR].
Else If ITEM=INFO[PTR] then:
Set LOC:=PTR and Exit.
Else
Set LOC:= NULL. And Exit.
4 Set LOC:= NULL.
5 Exit.

Memory allocation: Garbage collection
• Memory space can be reused if a node is deleted from a list
– i.e deleted node can be made available for future use
• The operating system of a computer may periodically collect all the deleted space
on to the free storage list. Any technique which does this collection called garbage
collection
Sent to
avail list
Garbage
(Deleted Space)
Computer
programs
Periodic
Collector
…
Avail List (Free space)
Takes
space
avail list

Overflow and Underflow
• Overflow:
– Sometimes data are inserted into a data structure but there is no
available space.
– This situation is called overflow
– Example: In linked list overflow occurs when
• AVAIL= NULL and
• There is an insertion operation
• Underflow:
– Situation:
• Want to delete data from data structure that is empty.
– Example: In linked list overflow occurs when
• START = NULL and
• There is an deletion operation

Insertion into a linked list
• Node N is to be inserted in to the list between nodes A and B
• Three pointer fields are changed as follows:
1. The next pointer field of node A now points to the new node N, to which AVAIL previously
pointed.
2. AVAIL now point to the second node in the free pool, to which node N previously pointed.
3. The next pointer field of node N now points to node B, to which node A previously pointed.
START
Node
A
Node
B
(a) Before insertion
START
Node
A
Node
B
(a) After insertion
AVAIL
Node N
Free storage list

Inserting a new node
• Possible cases of Insert Node
1. Insert into an empty list
2. Insert in front
3. Insert at back
4. Insert in middle
• But, in fact, only need to handle two cases:
1. Insert as the first node (Case 1 and Case 2)
2. Insert in the middle or at the end of the list (Case 3 and Case 4)

INSLOC(INFO, LINK, START, AVAIL, LOC, ITEM)
1. [OVERFLOW?] If AVAIL=NULL, then print OVERFLOW and exit
2. Set NEW= AVAIL and AVAIL=LINK[AVAIL]
3. Set INFO[NEW]= ITEM
4. IF LOC = NULL then [Insert as first Node]
Set LINK[NEW]= START and START=NEW.
Else: [Insert after node with location LOC]
Set LINK[NEW]= LINK [LOC] and LINK[LOC]= NEW
5. Exit.
Check for available
memory

Set LINK[]= START and START=NEW.
5. Exit.
Create a new node

5. Exit.
Insert as first element
START
NEW

5. Exit.
Insert after currNode
LOC
NEW

linked list1.ppt linked list ppts and notes

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