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Python | Perform append at beginning of list

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The usual append operation of a Python list adds the new element at the end of the list. But in certain situations, we need to append each element to the front of the list. If we perform brute force techniques, we need to perform unnecessary shifts of elements and hence, having shorthand for it is useful. Here, we will see how to add to a list in Python.

Example

Input: [1, 3, 5, 7, 9]
Output: [100, 1, 3, 5, 7, 9]
Explanation: In the above list we have added the '100' at index=0

Add Element to Front of List in Python

Let us see a few different methods to see how to add to a list in Python and append a value at the beginning of a Python list.

  • Using Insert() Method
  • Using [ ] and + Operator
  • Using List Slicing
  • Using collections.deque.appendleft()
  • using extend() method
  • using list() and itertools.chain() Functions
  • Using numpy.concatenate() Function
  • using reduce() Function

Insert Element to Front of List Using insert() Method

The insert() method in Python generally inserts the element at any position in the list and also performs the necessary shifts required internally and hence can also be used to perform this very task. 

Python3

# initializing list
test_list = [1, 3, 4, 5, 7]
 
# printing initial list
print("Original list: " + str(test_list))
 
# using insert() to append
# at beginning. append 6
test_list.insert(0, 6)
          
# printing resultant list
print("Resultant list is: " + str(test_list))

                    

Output
Original list: [1, 3, 4, 5, 7]
Resultant list is: [6, 1, 3, 4, 5, 7]

Time Complexity: O(n)
Auxiliary Space: O(1)

Insert Element to Front of List Using [ ] and + Operator

The square brackets [] and the + Arithmetic operator can be combined to perform this task. We convert the element to a list in Python and python add to list and then perform the list concatenation operation. 

Python3

# initializing list
test_list = [1, 3, 4, 5, 7]
 
# printing initial list
print("Original list: " + str(test_list))
 
# using [] and + to append
# at beginning append 6
test_list = [6] + test_list
          
# printing resultant list
print("Resultant list is: " + str(test_list))

                    

Output
Original list: [1, 3, 4, 5, 7]
Resultant list is: [6, 1, 3, 4, 5, 7]

Time complexity: O(n)
Auxiliary space: O(n) where n is the length of the list.

Append integer to the beginning of the list in Python using List Slicing

List Slicing is also another method to perform this particular task. We just assign the 0th sliced list to the list converted from the element. This does the trick and is quite elegant. 

Python3

# initializing list
test_list = [1, 3, 4, 5, 7]
 
# printing initial list
print("Original list: " + str(test_list))
 
# using slicing to append
# at beginning. append 6
test_list[:0] = [6]
          
# printing resultant list
print("Resultant list is: " + str(test_list))

                    

Output
Original list: [1, 3, 4, 5, 7]
Resultant list is: [6, 1, 3, 4, 5, 7]

Time Complexity: O(n), where n is the number of elements in the list.
Auxiliary Space: O(1), as no additional space is used to store the elements.

Append Integer to the Beginning of list using Collections.deque.appendleft()

The Python list can be converted to deque using the dequeue() function of the collections module and then the appendleft() function can be used to perform the push-like operation from the front of the double-ended queue. 

Python3

from collections import deque
 
# initializing list
test_list = [1, 3, 4, 5, 7]
 
# printing initial list
print("Original list: " + str(test_list))
 
# using collections.deque.pushleft()
# to append at beginning
# append 6
test_list = deque(test_list)
test_list.appendleft(6)
test_list = list(test_list)
 
# printing resultant list
print("Resultant list is: " + str(test_list))

                    

Output
Original list: [1, 3, 4, 5, 7]
Resultant list is: [6, 1, 3, 4, 5, 7]

Time complexity: O(1)
Auxiliary space: O(1)

Append Integer to Beginning of List Using Extend() Method

In this method, we will convert the element to be appended to a list in Python and then extend this list with the list to which we want to add the element using the extend() function.

Python3

# initializing list
test_list = [1, 3, 4, 5, 7]
 
# printing initial list
print("Original list: " + str(test_list))
x = [6]
x.extend(test_list)
 
# printing resultant list
print("Resultant list is: " + str(x))

                    

Output
Original list: [1, 3, 4, 5, 7]
Resultant list is: [6, 1, 3, 4, 5, 7]

Time Complexity: O(n), where n is the length of the original list.
Auxiliary Space: O(n)

Append Integer to Beginning of List Using List() and itertools.chain() Functions

Here we will use the chain() function of the itertools module to combine the element and the list and then convert it to a list using the list() method.

Python3

import itertools
 
# initializing list
test_list = [1, 3, 4, 5, 7]
 
# printing initial list
print("Original list: " + str(test_list))
 
# using itertools to append
# at beginning. append 6
test_list = list(itertools.chain([6], test_list))
 
# printing resultant list
print("Resultant list is: " + str(test_list))

                    

Output
Original list: [1, 3, 4, 5, 7]
Resultant list is: [6, 1, 3, 4, 5, 7]

Time Complexity: O(n)
Auxiliary Space: O(n+1)

Append Integer to Beginning of List Using Numpy.concatenate() Function

We can also add an element to the front of the list by converting it to a Python numpy array using the numpy.array() function. Then we will use numpy.concatenate() method and pass the element and the list to it as parameters. Finally, convert it back to the list using the tolist() function in Python.

Python3

import numpy as np
 
# initializing list
test_list = [1, 3, 4, 5, 7]
 
# printing initial list
print("Original list: " + str(test_list))
 
# element to be added
element = 6
 
# convert the input list to a numpy array
arr = np.array(test_list)
 
# create a numpy array of the element to be added
arr_elem = np.array([element])
 
# concatenate both arrays along the first dimension
concatenated_arr = np.concatenate((arr_elem, arr), axis=0)
 
# convert the concatenated array back to a list
result_list = concatenated_arr.tolist()
 
# printing resultant list
print("Resultant list is: " + str(result_list))

                    

Output

Original list : [1, 3, 4, 5, 7]
Resultant list is : [6, 1, 3, 4, 5, 7]

Time Complexity: O(n)
Auxiliary Space: O(n), as we are creating a numpy array to store the element to be added.

Append Integer to Beginning of List Using Reduce() Function

In this method, we will use the reduce() function of the functools module which takes two parameters. The first parameter is a lambda function which takes two lists as input and returns their concatenation. The second parameter is a list containing an empty list and the input list. The lambda function concatenates the two lists, placing the first list (containing the element to be added) at the beginning of the result.

Python3

from functools import reduce
 
# Initializing list
test_list = [1, 3, 4, 5, 7]
 
# Printing initial list
print("Original list: ", test_list)
 
# Using reduce() function to insert an element at the beginning of the list
test_list = reduce(lambda x, y: [6] + x + y, [[], test_list])
 
# Printing the resultant list
print("Resultant list: ", test_list)

                    

Output
Original list:  [1, 3, 4, 5, 7]
Resultant list:  [6, 1, 3, 4, 5, 7]

Time Complexity: O(n).
Auxiliary Space: O(n)



Last Updated : 13 Dec, 2023
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