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Python – Append Element to the Left in Deque

In this tutorial, we will look at how to append an element to the left of a deque in Python with the help of some examples.

deque in Python

The collections module in Python comes with a deque class that you can use to implement a stack or queue data structure in Python. Deques support thread-safe, memory efficient appends and pops from either side of the deque with approximately the same O(1) performance in either direction.

In simple terms, you can think of a deque in Python as a list-like data structure but with efficient insertion and removal of items from either end (left and right). This makes them very useful when implementing a stack or queue functionality in Python.

How to append an element to the left a deque in Python?

append element to the left of deque in python

The deque class comes with a number of built-in functions to help perform common operations (like adding and removing elements).

To append an element to the left of a deque in Python, use the appendleft() function. The following is the syntax –

# append element "x" to the left of deque "queue"
queue.appendleft(x)

It adds the element x to the left of the deque, queue.

Examples

Let’s now look at some examples of using the above syntax –

Example 1 – Append element to the left of a non-empty deque

First, let’s create a deque with some elements.

from collections import deque

# create a deque
queue = deque([1, 2, 3])
# print the deque
print(queue)

Output:

deque([1, 2, 3])

Here, we imported the deque class from the collections module and created a deque object, queue with three elements – 1, 2, and 3.

Now, let’s add a new element to this deque object on its left side.

# add element to the left of deque
queue.appendleft(4)
# print the deque
print(queue)

Output:

deque([4, 1, 2, 3])

Here, we used the appendleft() function to add the element 4 to the left side of the deque queue.

Example 2 – Append element to the left of an empty deque

Let’s now create an empty deque object (a deque with no elements).

# create an empty deque
queue = deque()
# print the deque
print(queue)

Output:

deque([])

Now, let’s add an element to the left of the deque using appendleft().

# add element to the left of deque
queue.appendleft(4)
# print the deque
print(queue)

Output:

deque([4])

Note that, you would get the same result if you add the element to the right of the deque since you’re appending to an empty deque (the left and the right ends are the same in an empty deque).

Difference between appendleft() and append()

Both the appendleft() and the append() functions are used to add an element to a deque in Python. The main difference between them is that the append() function is used to add the element to the end (the right side) of the deque whereas the appendleft() function is used to add the element to the start (the left side).

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Author

  • Piyush

    Piyush is a data scientist passionate about using data to understand things better and make informed decisions. In the past, he's worked as a Data Scientist for ZS and holds an engineering degree from IIT Roorkee. His hobbies include watching cricket, reading, and working on side projects.