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C program-Linked Lists
Posted Date: 11 Jun 2008 Resource Type: Articles/Knowledge Sharing Category: Computer & Technology
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Posted By: Girish Patil Member Level: Diamond
Rating:  Points: 4
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Concept
An array is represented in memory using sequential mapping, which has the property that elements are fixed distance apart. But this has the following disadvantage: It makes insertion or deletion at any arbitrary position in an array a costly operation, because this involves the movement of some of the existing elements.
When we want to represent several lists by using arrays of varying size, either we have to represent each list using a separate array of maximum size or we have to represent each of the lists using one single array. The first one will lead to wastage of storage, and the second will involve a lot of data movement.
So we have to use an alternative representation to overcome these disadvantages. One alternative is a linked representation. In a linked representation, it is not necessary that the elements be at a fixed distance apart. Instead, we can place elements anywhere in memory, but to make it a part of the same list, an element is required to be linked with a previous element of the list. This can be done by storing the address of the next element in the previous element itself. This requires that every element be capable of holding the data as well as the address of the next element. Thus every element must be a structure with a minimum of two fields, one for holding the data value, which we call a data field, and the other for holding the address of the next element, which we call link field.
Therefore, a linked list is a list of elements in which the elements of the list can be placed anywhere in memory, and these elements are linked with each other using an explicit link field, that is, by storing the address of the next element in the link field of the previous element.
Program
Here is a program for building and printing the elements of the linked list:
# include
# include
struct node
{
int data;
struct node *link;
};
struct node *insert(struct node *p, int n)
{
struct node *temp;
/* if the existing list is empty then insert a new node as the
starting node */
if(p==NULL)
{
p=(struct node *)malloc(sizeof(struct node)); /* creates new node
data value passes
as parameter */
if(p==NULL)
{
printf("Error\n");
exit(0);
}
p-> data = n;
p-> link = p; /* makes the pointer pointing to itself because it
is a circular list*/
}
else
{
temp = p;
/* traverses the existing list to get the pointer to the last node of
it */
while (temp-> link != p)
temp = temp-> link;
temp-> link = (struct node *)malloc(sizeof(struct node)); /*
creates new node using
data value passes as
parameter and puts its
address in the link field
of last node of the
existing list*/
if(temp -> link == NULL)
{
printf("Error\n");
exit(0);
}
temp = temp-> link;
temp-> data = n;
temp-> link = p;
}
return (p);
}
void printlist ( struct node *p )
{
struct node *temp;
temp = p;
printf("The data values in the list are\n");
if(p!= NULL)
{
do
{
printf("%d\t",temp->data);
temp=temp->link;
} while (temp!= p);
}
else
printf("The list is empty\n");
}
void main()
{
int n;
int x;
struct node *start = NULL ;
printf("Enter the nodes to be created \n");
scanf("%d",&n);
while ( n -- > 0 )
{
printf( "Enter the data values to be placed in a node\n");
scanf("%d",&x);
start = insert ( start, x );
}
printf("The created list is\n");
printlist ( start );
}
Explanation
This program uses a strategy of inserting a node in an existing list to get the list created. An insert function is used for this.
The insert function takes a pointer to an existing list as the first parameter, and a data value with which the new node is to be created as a second parameter, creates a new node by using the data value, appends it to the end of the list, and returns a pointer to the first node of the list.
Initially the list is empty, so the pointer to the starting node is NULL. Therefore, when insert is called first time, the new node created by the insert becomes the start node.
Subsequently, the insert traverses the list to get the pointer to the last node of the existing list, and puts the address of the newly created node in the link field of the last node, thereby appending the new node to the existing list.
The main function reads the value of the number of nodes in the list. Calls iterate that many times by going in a while loop to create the links with the specified number of nodes.
Points to Remember
Linked lists are used when the quantity of data is not known prior to execution.
In linked lists, data is stored in the form of nodes and at runtime, memory is allocated for creating nodes.
Due to overhead in memory allocation and deallocation, the speed of the program is lower.
The data is accessed using the starting pointer of the list.
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