Queues
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This document discusses queues as an abstract data structure. It explains that queues are opened at both ends, with one end for insertion (enqueue) and the other for removal (dequeue), following a First-In-First-Out (FIFO) approach. Real-world examples of queues include lines at ticket windows and bus stops. The document then covers queue representation using arrays or linked lists, basic queue operations like enqueue, dequeue, peek, isfull and isempty, and provides pseudocode algorithms for implementing these operations.
![peek
Like stacks, this function helps to see the data at the front of the queue. Algorithm of peek
function −
begin procedure peek
return queue[front]
end procedure
Implementation of peek function in C programming language −
int peek() {
return queue[front];
}
isfull
As we are using single dimension array to implement queue, we just check for the rear pointer to
reach at MAXSIZE to determine that queue is full. In case we maintain queue in a circular linked-
list, the algorithm will differ. Algorithm of isfull function −
begin procedure isfull
if rear equals to MAXSIZE
return true
else
return false
endif
end procedure
Implementation of isfull function in C programming language −
bool isfull() {
if(rear == MAXSIZE - 1)
return true;
else
return false;
}
isempty
Algorithm of isempty function −
begin procedure isempty
if front is less than MIN OR front is greater than rear
return true
else
return false
endif
end procedure
If value of front is less than MIN or 0, it tells that queue is not yet initialized, hence empty.
Here's the C programming code −
bool isempty() {
if(front < 0 || front > rear)
return true;
else](https://image.slidesharecdn.com/queue-161223152010/85/Queues-2-320.jpg)
![return false;
}
Enqueue Operation
As queue maintains two data pointers, front and rear, its operations are comparatively more
difficult to implement than stack.
The following steps should be taken to enqueue insert data into a queue −
Step 1 − Check if queue is full.
Step 2 − If queue is full, produce overflow error and exit.
Step 3 − If queue is not full, increment rear pointer to point next empty space.
Step 4 − Add data element to the queue location, where rear is pointing.
Step 5 − return success.
Sometimes, we also check that if queue is initialized or not to handle any unforeseen situations.
Algorithm for enqueue operation
procedure enqueue(data)
if queue is full
return overflow
endif
rear ← rear + 1
queue[rear] ← data
return true
end procedure
Implementation of enqueue in C programming language −
int enqueue(int data)
if(isfull())
return 0;
rear = rear + 1;
queue[rear] = data;](https://image.slidesharecdn.com/queue-161223152010/85/Queues-3-320.jpg)
![return 1;
end procedure
Dequeue Operation
Accessing data from queue is a process of two tasks − access the data where front is pointing and
remove the data after access. The following steps are taken to perform dequeue operation −
Step 1 − Check if queue is empty.
Step 2 − If queue is empty, produce underflow error and exit.
Step 3 − If queue is not empty, access data where front is pointing.
Step 3 − Increment front pointer to point next available data element.
Step 5 − return success.
Algorithm for dequeue operation −
procedure dequeue
if queue is empty
return underflow
end if
data = queue[front]
front ← front - 1
return true
end procedure
Implementation of dequeue in C programming language −
int dequeue() {
if(isempty())
return 0;
int data = queue[front];
front = front + 1;
return data;
}](https://image.slidesharecdn.com/queue-161223152010/85/Queues-4-320.jpg)
![For a complete stack program in C programming language, please click here.
Loading [MathJax]/jax/output/HTML-CSS/jax.js](https://image.slidesharecdn.com/queue-161223152010/85/Queues-5-320.jpg)
Queues
- 1. http://www.tutorialspoint.com/data_structures_algorithms/dsa_queue.htm Copyright © tutorialspoint.com DATA STRUCTURE - QUEUEDATA STRUCTURE - QUEUE Queue is an abstract data structure, somewhat similar to stack. In contrast to stack, queue is opened at both end. One end is always used to insert data enqueue and the other is used to remove data dequeue. Queue follows First-In-First-Out methodology, i.e., the data item stored first will be accessed first. A real world example of queue can be a single-lane one-way road, where the vehicle enters first, exits first. More real-world example can be seen as queues at ticket windows & bus-stops. Queue Representation As we now understand that in queue, we access both ends for different reasons, a diagram given below tries to explain queue representation as data structure − Same as stack, queue can also be implemented using Array, Linked-list, Pointer and Structures. For the sake of simplicity we shall implement queue using one-dimensional array. Basic Operations Queue operations may involve initializing or defining the queue, utilizing it and then completing erasing it from memory. Here we shall try to understand basic operations associated with queues − enqueue − add store an item to the queue. dequeue − remove access an item from the queue. Few more functions are required to make above mentioned queue operation efficient. These are − peek − get the element at front of the queue without removing it. isfull − checks if queue is full. isempty − checks if queue is empty. In queue, we always dequeue oraccess data, pointed by front pointer and while enqueing orstoring data in queue we take help of rear pointer. Let's first learn about supportive functions of a queue −
- 2. peek Like stacks, this function helps to see the data at the front of the queue. Algorithm of peek function − begin procedure peek return queue[front] end procedure Implementation of peek function in C programming language − int peek() { return queue[front]; } isfull As we are using single dimension array to implement queue, we just check for the rear pointer to reach at MAXSIZE to determine that queue is full. In case we maintain queue in a circular linked- list, the algorithm will differ. Algorithm of isfull function − begin procedure isfull if rear equals to MAXSIZE return true else return false endif end procedure Implementation of isfull function in C programming language − bool isfull() { if(rear == MAXSIZE - 1) return true; else return false; } isempty Algorithm of isempty function − begin procedure isempty if front is less than MIN OR front is greater than rear return true else return false endif end procedure If value of front is less than MIN or 0, it tells that queue is not yet initialized, hence empty. Here's the C programming code − bool isempty() { if(front < 0 || front > rear) return true; else
- 3. return false; } Enqueue Operation As queue maintains two data pointers, front and rear, its operations are comparatively more difficult to implement than stack. The following steps should be taken to enqueue insert data into a queue − Step 1 − Check if queue is full. Step 2 − If queue is full, produce overflow error and exit. Step 3 − If queue is not full, increment rear pointer to point next empty space. Step 4 − Add data element to the queue location, where rear is pointing. Step 5 − return success. Sometimes, we also check that if queue is initialized or not to handle any unforeseen situations. Algorithm for enqueue operation procedure enqueue(data) if queue is full return overflow endif rear ← rear + 1 queue[rear] ← data return true end procedure Implementation of enqueue in C programming language − int enqueue(int data) if(isfull()) return 0; rear = rear + 1; queue[rear] = data;
- 4. return 1; end procedure Dequeue Operation Accessing data from queue is a process of two tasks − access the data where front is pointing and remove the data after access. The following steps are taken to perform dequeue operation − Step 1 − Check if queue is empty. Step 2 − If queue is empty, produce underflow error and exit. Step 3 − If queue is not empty, access data where front is pointing. Step 3 − Increment front pointer to point next available data element. Step 5 − return success. Algorithm for dequeue operation − procedure dequeue if queue is empty return underflow end if data = queue[front] front ← front - 1 return true end procedure Implementation of dequeue in C programming language − int dequeue() { if(isempty()) return 0; int data = queue[front]; front = front + 1; return data; }
- 5. For a complete stack program in C programming language, please click here. Loading [MathJax]/jax/output/HTML-CSS/jax.js