A deque stands for Double-Ended Queue. It is a data structure that allows adding and removing elements from both ends efficiently. Unlike regular queues, which are typically operated on using FIFO (First In, First Out) principles, a deque supports both FIFO and LIFO (Last In, First Out) operations.
Example:
Python from collections import deque # Declaring deque de = deque(['name','age','DOB']) print(de)
Outputdeque(['name', 'age', 'DOB'])
Deque in Python
Types of Restricted Deque Input
- Input Restricted Deque: Input is limited at one end while deletion is permitted at both ends.
- Output Restricted Deque: output is limited at one end but insertion is permitted at both ends.
Operations on deque
Here’s a table listing built-in operations of a deque in Python with descriptions and their corresponding time complexities:
| Operation | Description | Time Complexity |
|---|
| append(x) | Adds x to the right end of the deque. | O(1) |
|---|
| appendleft(x) | Adds x to the left end of the deque. | O(1) |
|---|
| pop() | Removes and returns an element from the right end of the deque. | O(1) |
|---|
| popleft() | Removes and returns an element from the left end of the deque. | O(1) |
|---|
| extend(iterable) | Adds all elements from iterable to the right end of the deque. | O(k) |
|---|
| extendleft(iterable) | Adds all elements from iterable to the left end of the deque (reversed order). | O(k) |
|---|
| remove(value) | Removes the first occurrence of value from the deque. Raises ValueError if not found. | O(n) |
|---|
| rotate(n) | Rotates the deque n steps to the right. If n is negative, rotates to the left. | O(k) |
|---|
| clear() | Removes all elements from the deque. | O(n) |
|---|
| count(value) | Counts the number of occurrences of value in the deque. | O(n) |
|---|
| index(value) | Returns the index of the first occurrence of value in the deque. Raises ValueError if not found. | O(n) |
|---|
| reverse() | Reverses the elements of the deque in place. | O(n) |
|---|
Appending and Deleting Dequeue Items
- append(x): Adds x to the right end of the deque.
- appendleft(x): Adds x to the left end of the deque.
- extend(iterable): Adds all elements from the iterable to the right end.
- extendleft(iterable): Adds all elements from the iterable to the left end (in reverse order).
- remove(value): Removes the first occurrence of the specified value from the deque. If value is not found, it raises a ValueError.
- pop(): Removes and returns an element from the right end.
- popleft(): Removes and returns an element from the left end.
- clear(): Removes all elements from the deque.
Python from collections import deque dq = deque([10, 20, 30]) # Add elements to the right dq.append(40) # Add elements to the left dq.appendleft(5) # extend(iterable) dq.extend([50, 60, 70]) print("After extend([50, 60, 70]):", dq) # extendleft(iterable) dq.extendleft([0, 5]) print("After extendleft([0, 5]):", dq) # remove method dq.remove(20) print("After remove(20):", dq) # Remove elements from the right dq.pop() # Remove elements from the left dq.popleft() print("After pop and popleft:", dq) # clear() - Removes all elements from the deque dq.clear() # deque: [] print("After clear():", dq) Output:
After extend([50, 60, 70]): deque([5, 10, 20, 30, 40, 50, 60, 70])
After extendleft([0, 5]): deque([5, 0, 5, 10, 20, 30, 40, 50, 60, 70])
After remove(20): deque([5, 0, 5, 10, 30, 40, 50, 60, 70])
After pop and popleft: deque([0, 5, 10, 30, 40, 50, 60])
After clear(): deque([])
Accessing Item and length of deque
- Indexing: Access elements by position using positive or negative indices.
- len(): Returns the number of elements in the deque.
Python import collections dq = collections.deque([1, 2, 3, 3, 4, 2, 4]) # Accessing elements by index print(dq[0]) print(dq[-1]) # Finding the length of the deque print(len(dq))
Count, Rotation and Reversal of a deque
- count(value): This method counts the number of occurrences of a specific element in the deque.
- rotate(n): This method rotates the deque by n steps. Positive n rotates to the right and negative n rotates to the left.
- reverse(): This method reverses the order of elements in the deque.
Python from collections import deque # Create a deque dq = deque([10, 20, 30, 40, 50, 20, 30, 20]) # 1. Counting occurrences of a value print(dq.count(20)) # Occurrences of 20 print(dq.count(30)) # Occurrences of 30 # 2. Rotating the deque dq.rotate(2) # Rotate the deque 2 steps to the right print(dq) dq.rotate(-3) # Rotate the deque 3 steps to the left print(dq) # 3. Reversing the deque dq.reverse() # Reverse the deque print(dq)
Output3 2 deque([30, 20, 10, 20, 30, 40, 50, 20]) deque([20, 30, 40, 50, 20, 30, 20, 10]) deque([10, 20, 30, 20, 50, 40, 30, 20])
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