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How to use the "filter" function in Python?

In Python, the filter() function is a built-in function used to create an iterator that yields only the elements from an iterable that satisfy a specified condition. It filters out the elements that don't meet the condition and returns an iterator containing the filtered elements.

The general syntax of the filter() function is as follows:

filter(function, iterable)
  • function: The function (predicate) that determines the condition to be applied to each element of the iterable. It should return a boolean value (True or False).
  • iterable: The iterable (e.g., list, tuple, string) containing the elements to be filtered.

Here's an example to demonstrate the usage of the filter() function:

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

my_list = [1, 2, 3, 4, 5]
filtered_iterator = filter(is_even, my_list)

# Using a for loop to iterate over the filtered_iterator
for result in filtered_iterator:
    print(result)

# Output:
# 2
# 4

In this example, the filter() function applies the is_even() function to each element in my_list. The resulting iterator, filtered_iterator, yields only the elements that satisfy the condition of being even when iterated over.

The filter() function can also be used with lambda functions, which are anonymous functions defined inline. Here's an example using a lambda function:

my_list = [1, 2, 3, 4, 5]
filtered_iterator = filter(lambda x: x % 2 == 0, my_list)

# Using a list comprehension to collect the results of the filtered_iterator
even_numbers = [result for result in filtered_iterator]
print(even_numbers)

# Output: [2, 4]

In this example, the filter() function applies the lambda function lambda x: x % 2 == 0 to each element in my_list. The list comprehension [result for result in filtered_iterator] collects the filtered elements from the filtered_iterator and stores them in the even_numbers list.

The filter() function is useful when you want to select or filter out elements from an iterable based on a specific condition. It provides a convenient and concise way to extract the elements that meet a particular criterion without the need for explicit loops.

It's important to note that the filter() function returns an iterator, which means it only produces results on-the-fly as you iterate over it. If you need to store the filtered elements in a list or another data structure, you can use a list comprehension or the list() function to convert the iterator into a list.

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