filter() in python
filter() function is used to extract elements from an iterable (like a list, tuple or set) that satisfy a given condition. It works by applying a function to each element and keeping only those for which function returns True.
Basic Example
This simple example shows how to keep only the words starting with the letter 'a' from a list of fruits.
def starts_a(w):
return w.startswith("a")
li = ["apple", "banana", "avocado", "cherry", "apricot"]
res = filter(starts_a, li)
print(list(res))
Output
['apple', 'avocado', 'apricot']
Explanation: function starts_a checks if a word begins with 'a' and filter() applies this function to each fruit and returns only matching ones.
Syntax
filter(function, iterable)
Parameters:
- function: tests each element and if return, True - Keep the element, if False - Discard the element
- iterable: Any iterable (list, tuple, set, etc.).
Return Value: A filter object (an iterator), which can be converted into a list, tuple, set, etc.
Examples of filter function
Let's explore some examples of filter() function and see how it is used.
Example 1: Using filter() with a Named Function
This code defines a regular function to check if a number is even and then uses filter() to extract all even numbers from a list.
def even(n):
return n % 2 == 0
a = [1, 2, 3, 4, 5, 6]
b = filter(even, a)
print(list(b)) # Convert filter object to a list
Output
[2, 4, 6]
Explanation:
- even function checks if a number is divisible by 2.
- filter() applies this function to each item in a.
- Only even numbers are included in output.
Example 2: Using filter() with a Lambda Function
Instead of creating a separate named function, use a lambda function for concise code. Below code uses a lambda function with filter() to select even numbers from a list.
a = [1, 2, 3, 4, 5, 6]
b = filter(lambda x: x % 2 == 0, a)
print(list(b))
Output
[2, 4, 6]
Explanation: filter(lambda x: x % 2 == 0, a) keeps only numbers divisible by 2 (even numbers).
Example 3: Filtering and Transforming Data
In this Example lambda functions is used with filter() and map() to first get even numbers from a list and then double them.
a = [1, 2, 3, 4, 5, 6]
b = filter(lambda x: x % 2 == 0, a)
c = map(lambda x: x * 2, b)
print(list(c))
Output
[4, 8, 12]
Explanation:
- filter(lambda x: x % 2 == 0, a): Selects only even numbers from the list ([2, 4, 6]).
- map(lambda x: x * 2, b): Doubles each of the filtered even numbers ([4, 8, 12]).
Example 4: Filtering Strings
Here, lambda function is used with filter() to keep only words that have more than 5 letters from a list of fruits.
a = ["apple", "banana", "cherry", "kiwi", "grape"]
b = filter(lambda w: len(w) > 5, a)
print(list(b))
Output
['banana', 'cherry']
Example 5: Filtering with None (Truthiness Check)
This code uses filter() with None as the function to remove all falsy values (like empty strings, None and 0) from a list.
L = ["apple", "", None, "banana", 0, "cherry"]
A = filter(None, L)
print(list(A))
Output
['apple', 'banana', 'cherry']
Explanation: filter(None, L) removes all falsy values (empty string, None and 0) and keeps only truthy ones.
Filter() Function in Python
