Python | Get first element of each sublist

Given a list of lists, write a Python program to extract first element of each sublist in the given list of lists. Examples:

Input : [[1, 2], [3, 4, 5], [6, 7, 8, 9]]
Output : [1, 3, 6]

Input : [['x', 'y', 'z'], ['m'], ['a', 'b'], ['u', 'v']]
Output : ['x', 'm', 'a', 'u']

  Approach #1 : List comprehension 

# Python3 program to extract first and last 
# element of each sublist in a list of lists

def Extract(lst):
    return [item[0] for item in lst]
    
# Driver code
lst = [[1, 2], [3, 4, 5], [6, 7, 8, 9]]
print(Extract(lst))

[1, 3, 6]

Time Complexity: O(n)

Space Complexity: O(n) (length of the list given as argument)

  Approach #2 : Using zip and unpacking(*) operator This method uses zip with * or unpacking operator which passes all the items inside the 'lst' as arguments to zip function. Thus, all the first element will become the first tuple of the zipped list. Returning the 0^th element will thus, solve the purpose. 

# Python3 program to extract first and last 
# element of each sublist in a list of lists

def Extract(lst):
    return list(list(zip(*lst))[0])
    
# Driver code
lst = [[1, 2], [3, 4, 5], [6, 7, 8, 9]]
print(Extract(lst))

[1, 3, 6]

Time Complexity: O(n) (zip function has complexity of O(1) and conversion to list has O(n))

Space Complexity: O(n) (length of the list given as argument)

Another method of using zip is given below:- 

def Extract(lst):
    return list(next(zip(*lst)))

  Approach #3 : Using itemgetter() 

# Python3 program to extract first and last 
# element of each sublist in a list of lists
from operator import itemgetter

def Extract(lst):
    return list( map(itemgetter(0), lst ))
    
# Driver code
lst = [[1, 2], [3, 4, 5], [6, 7, 8, 9]]
print(Extract(lst))

[1, 3, 6]

Time Complexity: O(n) (itemgetter has O(1) and conversion to list O(n))

Space Complexity: O(n)

 Approach #4 : Using reduce

You can use the reduce function from the functools library to iteratively apply a lambda function to the list of lists and build a new list containing the first elements of each sublist.

from functools import reduce
# Python3 program to extract first and last
# element of each sublist in a list of lists
def extract_first(lst):
    return reduce(lambda acc, x: acc + [x[0]] if x else acc, lst, [])
# Driver code
lst = [[1, 2], [3, 4, 5], [6, 7, 8, 9]]
print(extract_first(lst))

[1, 3, 6]

Time Complexity: O(n)

Space Complexity: O(n) 

Method#5: using for loop

Approach

this approach uses a for loop to iterate through each sublist and append the first element of each sublist to a new list. The resulting list is returned. 

Algorithm

1. Initialize an empty list to hold the first elements.
2. Iterate through each sublist.
3. For each sublist, append the first element to the list of first elements.
4. Return the list of first elements.

--

def get_first_element(list_of_lists):
    first_elements = []
    for sublist in list_of_lists:
        first_elements.append(sublist[0])
    return first_elements


list_of_lists = [[1, 2], [3, 4, 5], [6, 7, 8, 9]]
print(get_first_element(list_of_lists))

[1, 3, 6]

Time complexity: O(n), where n is the number of sublists in the input list. We iterate once over each sublist.

Space complexity: O(n), where n is the number of sublists in the input list. We create a new list to hold the first elements.