Deque in Python
Deque (Doubly Ended Queue) in Python is implemented using the module “collections“. Deque is preferred over a list in the cases where we need quicker append and pop operations from both the ends of the container, as deque provides an O(1) time complexity for append and pop operations as compared to a list that provides O(n) time complexity.
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.
Example: Python code to demonstrate deque
Python3
from collections import deque # Declaring dequequeue = deque(['name','age','DOB']) print(queue) |
Output
deque(['name', 'age', 'DOB'])
Operations on deque
Example 1: Appending Items Efficiently
- append():- This function is used to insert the value in its argument to the right end of the deque.
- appendleft():- This function is used to insert the value in its argument to the left end of the deque.
Python3
# importing "collections" for deque operationsimport collections# initializing dequede = collections.deque([1, 2, 3])print("deque: ", de)# using append() to insert element at right end# inserts 4 at the end of dequede.append(4)# printing modified dequeprint("\nThe deque after appending at right is : ")print(de)# using appendleft() to insert element at left end# inserts 6 at the beginning of dequede.appendleft(6)# printing modified dequeprint("\nThe deque after appending at left is : ")print(de) |
Output
deque: deque([1, 2, 3]) The deque after appending at right is : deque([1, 2, 3, 4]) The deque after appending at left is : deque([6, 1, 2, 3, 4])
Refer end for complexity analysis.
Example 2: Popping Items Efficiently
- pop():- This function is used to delete an argument from the right end of the deque.
- popleft():- This function is used to delete an argument from the left end of the deque.
Python3
# importing "collections" for deque operationsimport collections# initializing dequede = collections.deque([6, 1, 2, 3, 4])print("deque: ", de)# using pop() to delete element from right end# deletes 4 from the right end of dequede.pop()# printing modified dequeprint("\nThe deque after deleting from right is : ")print(de)# using popleft() to delete element from left end# deletes 6 from the left end of dequede.popleft()# printing modified dequeprint("\nThe deque after deleting from left is : ")print(de) |
Output
deque: deque([6, 1, 2, 3, 4]) The deque after deleting from right is : deque([6, 1, 2, 3]) The deque after deleting from left is : deque([1, 2, 3])
Refer end for complexity analysis.
Example 3: Accessing Items in a deque
- index(ele, beg, end):- This function returns the first index of the value mentioned in arguments, starting searching from beg till end index.
- insert(i, a) :- This function inserts the value mentioned in arguments(a) at index(i) specified in arguments.
- remove():- This function removes the first occurrence of the value mentioned in arguments.
- count():- This function counts the number of occurrences of value mentioned in arguments.
Python3
# Python code to demonstrate working of# insert(), index(), remove(), count()# importing "collections" for deque operationsimport collections# initializing dequede = collections.deque([1, 2, 3, 3, 4, 2, 4])# using index() to print the first occurrence of 4print ("The number 4 first occurs at a position : ")print (de.index(4,2,5))# using insert() to insert the value 3 at 5th positionde.insert(4,3)# printing modified dequeprint ("The deque after inserting 3 at 5th position is : ")print (de)# using count() to count the occurrences of 3print ("The count of 3 in deque is : ")print (de.count(3))# using remove() to remove the first occurrence of 3de.remove(3)# printing modified dequeprint ("The deque after deleting first occurrence of 3 is : ")print (de) |
Output
The number 4 first occurs at a position : 4 The deque after inserting 3 at 5th position is : deque([1, 2, 3, 3, 3, 4, 2, 4]) The count of 3 in deque is : 3 The deque after deleting first occurrence of 3 is : deque([1, 2, 3, 3, 4, 2, 4])
Refer end for complexity analysis.
Example 4: Size of a deque
- len(dequeue):- Return the current size of the dequeue.
Python3
# Python Program to demonstrate# how to find size of a Dequeuefrom collections import deque# initializing dequede = deque([1, 2, 3, 4, 5, 6])print("Current Deque: ", de)# printing current size of dequeprint(f"Size of Deque: {len(de)}")# using pop() to delete element from right end# deletes 6 from the right end of dequede.pop()# printing modified dequeprint("\nThe deque after deleting from right is: ", end = '')print(de)# printing current size of dequeprint(f"Size of Deque: {len(de)}")# This code is contributed by Susobhan Akhuli |
Output
Current Deque: deque([1, 2, 3, 4, 5, 6]) Size of Deque: 6 The deque after deleting from right is: deque([1, 2, 3, 4, 5]) Size of Deque: 5
Refer end for complexity analysis.
Example 5: Front and Back of a deque
- Deque[0] :- We can access the front element of the deque using indexing with de[0].
- Deque[-1] :- We can access the back element of the deque using indexing with de[-1].
Python3
# Python Program to demonstrate# accessing the front and back of a Dequefrom collections import deque# initializing dequede = deque([1, 2, 3, 4, 5, 6])print("Current Deque: ", de)# Accessing the front element of the dequeprint("Front element of the deque:", de[0])# Accessing the back element of the dequeprint("Back element of the deque:", de[-1])# This code is contributed by Susobhan Akhuli |
Output
Current Deque: deque([1, 2, 3, 4, 5, 6]) Front element of the deque: 1 Back element of the deque: 6
Refer end for complexity analysis.
Example 6: Different operations on deque
- extend(iterable):- This function is used to add multiple values at the right end of the deque. The argument passed is iterable.
- extendleft(iterable):- This function is used to add multiple values at the left end of the deque. The argument passed is iterable. Order is reversed as a result of left appends.
- reverse():- This function is used to reverse the order of deque elements.
- rotate():- This function rotates the deque by the number specified in arguments. If the number specified is negative, rotation occurs to the left. Else rotation is to right.
Python3
# Python code to demonstrate working of# extend(), extendleft(), rotate(), reverse()# importing "collections" for deque operationsimport collections# initializing dequede = collections.deque([1, 2, 3,])# using extend() to add numbers to right end# adds 4,5,6 to right endde.extend([4,5,6])# printing modified dequeprint ("The deque after extending deque at end is : ")print (de)# using extendleft() to add numbers to left end# adds 7,8,9 to left endde.extendleft([7,8,9])# printing modified dequeprint ("The deque after extending deque at beginning is : ")print (de)# using rotate() to rotate the deque# rotates by 3 to leftde.rotate(-3)# printing modified dequeprint ("The deque after rotating deque is : ")print (de)# using reverse() to reverse the dequede.reverse()# printing modified dequeprint ("The deque after reversing deque is : ")print (de) |
Output
The deque after extending deque at end is : deque([1, 2, 3, 4, 5, 6]) The deque after extending deque at beginning is : deque([9, 8, 7, 1, 2, 3, 4, 5, 6]) The deque after rotating deque is : deque([1, 2, 3, 4, 5, 6, 9, 8, 7]) The deque after reversing deque is : deque([7, 8, 9, 6, 5, 4, 3, 2, 1])
Refer end for complexity analysis.
Complexity Analysis:
Methods | Time Complexity | Auxiliary Space |
|---|---|---|
append() | O(1) | O(1) |
appendleft() | O(1) | O(1) |
pop() | O(1) | O(1) |
popleft() | O(1) | O(1) |
index(ele, beg, end) | O(N) | O(1) |
insert(i, a) | O(N) | O(1) |
remove() | O(N) | O(1) |
count() | O(N) | O(1) |
len(dequeue) | O(1) | O(1) |
Deque[0] | O(1) | O(1) |
Deque[-1] | O(1) | O(1) |
extend(iterable) | O(K) | O(1) |
extendleft(iterable) | O(K) | O(1) |
reverse() | O(N) | O(1) |
rotate() | O(K) | O(1) |
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