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filter() in python

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  • Difficulty Level : Easy
  • Last Updated : 22 Feb, 2023
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The filter() method filters the given sequence with the help of a function that tests each element in the sequence to be true or not. 


filter(function, sequence)
function: function that tests if each element of a 
sequence true or not.
sequence: sequence which needs to be filtered, it can 
be sets, lists, tuples, or containers of any iterators.
returns an iterator that is already filtered.


# function that filters vowels
def fun(variable):
    letters = ['a', 'e', 'i', 'o', 'u']
    if (variable in letters):
        return True
        return False
# sequence
sequence = ['g', 'e', 'e', 'j', 'k', 's', 'p', 'r']
# using filter function
filtered = filter(fun, sequence)
print('The filtered letters are:')
for s in filtered:


The filtered letters are:

Application: It is normally used with Lambda functions to separate list, tuple, or sets. 


# a list contains both even and odd numbers.
seq = [0, 1, 2, 3, 5, 8, 13]
# result contains odd numbers of the list
result = filter(lambda x: x % 2 != 0, seq)
# result contains even numbers of the list
result = filter(lambda x: x % 2 == 0, seq)


[1, 3, 5, 13]
[0, 2, 8]

Please refer Python Lambda functions for more details. 

python filter() functions:

In this  program, the is_multiple_of_3() function checks if a number is a multiple of 3. The filter() function is used to apply this function to each element of the numbers list, and a for statement is used within the lambda function to iterate over each element of the list before applying the condition. This way, we can perform additional operations on each element before applying the condition.


#Define a function to check if a number is a multiple of 3
def is_multiple_of_3(num):
    return num % 3 == 0
# Create a list of numbers to filter
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
# Use filter and a lambda function to filter the list of numbers
# and only keep the ones that are multiples of 3
result = list(filter(lambda x: is_multiple_of_3(x), numbers))
# Print the result
print(result)  # [3, 6, 9]


[3, 6, 9]

Time complexity analysis:
1.The filter function is used to filter the list of numbers, and it applies the lambda function to each element of the list. The time complexity of the filter function is O(n), where n is the number of elements in the list.
2.The time complexity of the lambda function is constant, O(1), since it only performs a single arithmetic operation. Therefore, the overall time complexity of the program is O(n).

Auxiliary Space analysis:
The program uses a list to store the filtered numbers, so the space complexity  is proportional to the number of filtered numbers. In the worst case, if all numbers are multiples of 3, the filtered list will have n/3 elements. Therefore, the space complexity is O(n/3), which simplifies to O(n) in big O notation.

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