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QUEUE for Data Structure in C Programming

Queue is also a special type of linear data structure resulting after restriction of insertion and deletion operations on linear data structure. So, it can be implemented using either one-dimensional array or linked list. When the insertion operation is restricted to the other end in a linear data structure, then the resulting data structure is called as QUEUE. Insertion or addition of items is done at the end called REAR end and deletion of items is done from the other end called as FRONT end.
                         Assume a service center, with one serving window. The person comes for the service first is served first. If another person comes for the service, while the first person is being served, he has to wait. He will stand behind the first person. If any other persons come for the service, they will also stand one by one behind each other. The waiting persons frame a QUEUE. The persons are added into the QUEUE from end called REAR end. One person who enters the QUEUE  first is served first. The deletion is done from the other end FRONT end. As an item is added to QUEUE at the REAR end and it is deletion from the other end, FRONT end, the item added first comes out first. So the data structure QUEUE is also called as FIFO, First In First Out structure. QUEUE has many applications in computer Field.
                   In a multiprocessing environment, the processes waiting for the CPU service are placed in a QUEUE called Ready Queue. In multiprocessing environment the CPU executes more than one process.

Representation of Queue: 
          The representation of QUEUE  depends on the type of QUEUE. The QUEUE can be implemented either using a one-dimensional array or using Linked List. As there are different type of QUEUE depending on storage and the way the ADD and DELETE operations are done on the QUEUE.

Type of QUEUE:

1 Linear QUEUE: The elements are arranged in a QUEUE in linear order.

2 Circular QUEUE: The elements are arranged in a QUEUE by means of circular array.
3 DEQUE-Double Ended QUEUE: The ADD and DELETE operations are done from both the ends.
4 Priority QUEUE: The elements are stored in QUEUE with a characteristic called priority and DELETE  operation depends on priority, high priority elements are deleted first (served first).


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Binary Search Linear Search Works only on sorted items. such as  1,2,3,4,5,6  etc
Works on sorted as well as unsorted items. 12,4,5,3,2,1 etc Very efficient if the items are sorted Very efficient if the items are less and present in the beginning of the list. such as Suppose your list items are : 12,3,4,5,1 and you want to search 12 number then you get beginning in the list. Works well with arrays and not on linked lists. Works with arrays and linked lists.
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Memory representation of Linked List Data Structures in C Language

Memory representation of Linked List

             In memory the linked list is stored in scattered cells (locations).The memory for each node is allocated dynamically means as and when required. So the Linked List can increase as per the user wish and the size is not fixed, it can vary.

               Suppose first node of linked list is allocated with an address 1008. Its graphical representation looks like the figure shown below:

      Suppose next node is allocated at an address 506, so the list becomes,

  Suppose next node is allocated with an address with an address 10,s the list become,

The other way to represent the linked list is as shown below:

 In the above representation the data stored in the linked list is “INDIA”, the information part of each node contains one character. The external pointer root points to first node’s address 1005. The link part of the node containing information I contains 1007, the address of next node. The last node …