Introducing The Python Filter Function

We've previously introduced map() as a built-in higher order function in Python.

Remember, a higher order function is a function that either accepts one or more functions as arguments, or returns a function as its result.

Now, let's talk about another built-in higher order function in Python: the filter() function.

The filter() function allows you to filter out elements from an iterable (like a list or a tuple) based on a certain condition.

The filter() function takes in two arguments: a function and an iterable. It applies the function to all elements of the iterable, and returns only those elements for which the function returns True.

filtered_result = filter(function, iterable)

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Here's a simple example of how to use the filter() function:

numbers = [1, 2, 3, 4, 5, 6]
def is_greater_than_three(n):
    return n > 3
 
filtered_numbers = filter(is_greater_than_three, numbers)
print(list(filtered_numbers))

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Output

In this example, we're using the filter() function to filter out all numbers that are not greater than 3.

Just like with the map() function, the filter() function returns a filter object, which is an iterator. To get a list of results, we need to convert the filter object to a list using the list() function.

result = filter(function, iterable)
# Convert the filter object to a list
list(result)

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What will be the output?

numbers = [1, 2, 3, 4, 5, 6]
def is_even(n):
    return n % 2 == 0
 
filtered_numbers = filter(is_even, numbers)
print(list(filtered_numbers))

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What will be the output?

numbers = [1, 2, 3, 4, 5, 6]
def is_greater_than_two(n):
    return n > 2
 
filtered_numbers = filter(is_greater_than_two, numbers)
print(filtered_numbers)

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Just like with the map() function, we can use lambda functions with the filter() function. This can make our code more concise.

Here's how we can rewrite our previous example using a lambda function:

numbers = [1, 2, 3, 4, 5, 6]
filtered_numbers = filter(lambda n: n > 3, numbers)
print(list(filtered_numbers))

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Output

What will be the output?

words = ['apple', 'banana', 'cherry', 'date']
filtered_words = filter(lambda word: len(word) > 5, words)
print(list(filtered_words))

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What will be the output?

strings = ['hello', 'world', 'python', 'is', 'great']
filtered_strings = filter(lambda s: 'o' in s, strings)
print(list(filtered_strings))

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