The filter function helps create filtered views of data. By the end of this lesson, you'll know how to use the filter function effectively, both with lambda functions and predefined functions.

The filter function is built into Python to create an iterator from elements of an iterable that satisfy a function. It’s useful for extracting specific elements from a list, tuple, or any iterable based on a condition.

Here’s the basic syntax:

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1filter(function, iterable)

• function: A function that tests elements in the iterable.
• iterable: Any iterable (e.g., list, tuple).

Let's see a basic example. Suppose you have a list of numbers and want to keep only the even numbers:

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1def is_even(x):
2    return x % 2 == 0
3
4if __name__ == "__main__":
5    # List of numbers
6    numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
7
8    # Filter even numbers
9    evens = list(filter(is_even, numbers))
10    print("Even numbers:", evens)  # Even numbers: [2, 4, 6, 8, 10]

The is_even function checks if a number is even. The filter function keeps only elements for which is_even returns True.

As before, we can utilize lambdas to make the code more clean. Here’s how to use a lambda function to filter even numbers:

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1if __name__ == "__main__":
2    # List of numbers
3    numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
4
5    # Filter even numbers using lambda
6    evens = list(filter(lambda x: x % 2 == 0, numbers))
7    print("Even numbers:", evens)  # Even numbers: [2, 4, 6, 8, 10]

The lambda function lambda x: x % 2 == 0 does the same job as is_even, but more concisely.

Predefined functions are helpful when you have a complex condition. Let’s filter out prime numbers from a list. First, define a function to check if a number is prime:

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1def is_prime(x):
2    if x < 2:
3        return False
4    for i in range(2, int(x ** 0.5) + 1):
5        if x % i == 0:
6            return False
7    return True

Now, use the is_prime function with filter:

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1def is_prime(x):
2    if x < 2:
3        return False
4    for i in range(2, int(x ** 0.5) + 1):
5        if x % i == 0:
6            return False
7    return True
8
9if __name__ == "__main__":
10    # List of numbers
11    numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
12
13    # Filter prime numbers
14    primes = list(filter(is_prime, numbers))
15    print("Prime numbers:", primes)  # Prime numbers: [2, 3, 5, 7]

The is_prime function checks if a number is prime. The filter function uses it to keep only prime numbers from numbers.

We covered:

• The filter function’s purpose and usage.
• Using filter with lambda functions for simple filters.
• Using filter with predefined functions for complex conditions.

Now, move to the practice section to apply your knowledge. You will filter different datasets using lambda and predefined functions to solidify your understanding of the filter function in Python.