Understanding Python's Filter Function

The filter() function evaluates each item in an iterable with a specified function that returns a boolean value. This can be particularly useful for processing lists.

Syntax

filter(function, iterable) --> filter object

The filter() function takes two arguments: a function and a sequence (e.g., a list). Each item in the sequence is processed by the function, which returns True or False. Only those items that return True are included in a filter object, which can be converted into a list, tuple, etc.

Prime Number Example

Consider the following isPrime() function. This function returns True if the input is a prime number, and False otherwise. It is then used within filter() to identify prime numbers in a range.

def isPrime(x):
    for n in range(2, x):
        if x % n == 0:
            return False
    return x > 1

fltrObj = filter(isPrime, range(10))
print('Prime numbers between 1-10:', list(fltrObj))

Output:

Prime numbers between 1-10: [2, 3, 5, 7]

Using Lambda with Filter

A lambda function can also be utilized with filter(). Below is an example that filters a list to include only even numbers.

fltrObj = filter(lambda x: x % 2 == 0, range(10))
print(list(fltrObj))

Output:

[0, 2, 4, 6, 8]

By using filter(), we can efficiently process and extract data from sequences based on custom criteria.