Linked List Concept

Introduction

When dealing with many problems we need a dynamic list, dynamic in the sense that the size requirement need not be known at compile time. Thus, the list may grow or shrink during runtime. A linked list is a data structure that is used to model such a dynamic list of data items, so the study of the linked lists as one of the data structures is important.
 

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.

# include <stdio.h>
# include <stdlib.h>
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");
}

int 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);
	getchar();
	return 0;
}

Explanation

1.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.
2.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.
3.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.
4.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.
5.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

1.Linked lists are used when the quantity of data is not known prior to execution.
2.In linked lists, data is stored in the form of nodes and at runtime, memory is allocated for creating nodes.
3.Due to overhead in memory allocation and deallocation, the speed of the program is lower.
4.The data is accessed using the starting pointer of the list.