Python | Filter even values from a list

Given a list of numbers, the task is to make a new list containing only even values. 

Examples: 

Input: list = [1, 2, 3, 4, 5]
Output: [2, 4]

Input: list = [12, 14, 95, 3]
Output: [12, 14]

Method #1 : Using For loop Iterate each element in the list using for loop and check if num % 2 == 0. If the condition satisfies, then append it in the output list. 

Python3

# Python code to filter even values from a list

# Initialisation of list
lis = [1, 2, 3, 4, 5]

# Output list initialisation
out = []

for num in lis:

    # checking condition
    if num % 2 == 0:
        out.append(num)


# printing output
print(out)

Output:

[2, 4]

Time complexity: O(n) – It will iterate through each element of the list once.
Auxiliary space: O(m) – It will create a new list ‘out’ to store the even numbers, which could potentially be as large as the original list. Therefore, the space complexity is linear with respect to the size of the input list.

Method #2: Using While loop 

Python3

# Python code to filter even values from a list

# Initialisation of list
lis = [1, 2, 3, 4, 5]
num = 0

# Output list initialisation
out = []
while(num & lt
      len(lis)):

    # checking condition
    if lis[num] % 2 == 0:
        out.append(lis[num])
    # increment num
    num += 1


# printing output
print(out)

Output:

[2, 4]

Time complexity: O(n) – It will iterate through each element of the list once.
Auxiliary space: O(m) – It will create a new list ‘out’ to store the even numbers, which could potentially be as large as the original list. Therefore, the space complexity is linear with respect to the size of the input list.

Method #3: Using list Comprehension 

Python3

# Python code to filter even values from a list

# Initialisation of list
lis = [1, 2, 3, 4, 5]

lis2 = [i for i in lis if i % 2 == 0]

# Printing output
print(lis2)

Output:

[2, 4]

Time Complexity: O(n), where n is length of lis list.
Auxiliary Space: O(n), where n is length of lis2 list.

Method #4 : Using filter() 

Python3

# Python code to filter even values from a list

# Initialisation of list
lis1 = [1, 2, 3, 4, 5]


def is_even(x): return x % 2 == 0


# using filter
lis2 = list(filter(is_even, lis1))

# Printing output
print(lis2)

Output:

[2, 4]

Method#5: Using numpy.array()

Python3

# Python code to filter even values from a list
# Using numpy
import numpy as np
# Initialisation of list
lis1 = np.array([1, 2, 3, 4, 5])

# Using numpy method
is_even = lis1[lis1 % 2 == 0]


# Printing output
print(is_even)

Output:

[2 4]

Method#6: Using a generator function 

Approach is using a function that returns a generator. Generators are a type of iterable, like lists or tuples, but instead of creating the entire sequence in memory at once, they generate the values one at a time, on the fly. This can be useful when working with large lists or when the values in the list are expensive to compute.

Here is an example of how you could use a generator function to filter even values from a list:

Python3

def even_values(lst):
    for value in lst:
        if value % 2 == 0:
            yield value

evens = list(even_values([1, 2, 3, 4, 5]))
print(evens)  # [2, 4]

#This code is contributed by Edula Vinay Kumar Reddy

Output

[2, 4]

The time complexity of the generator function even_values is O(n), since it needs to iterate over all n elements of the list. The space complexity is also O(n), since it stores all of the even values in the list evens.