Lambda and filter in Python Examples

Before diving into examples, let’s quickly understand the function of lambda and filter() in Python:

• lambda function: A small, anonymous function defined using the lambda keyword. It can take any number of arguments but has only one expression.
• filter() function: A built-in function that filters elements from an iterable based on a condition (function) that returns True or False.

For example, suppose we want to filter even numbers from a list, here's how we can do it using filter() and lambda:

a = [1, 2, 3, 4, 5, 6]

e = list(filter(lambda x: x % 2 == 0, a))

print(e)

[2, 4, 6]

Explanation: lambda x: x % 2 == 0 returns True for even numbers, and filter() keeps only those elements.

To learn about lambda and filter in detail, refer to: Lambda in Python, filter() in Python

Syntax of filter() with Lambda

filter(function, iterable)

Parameters:

• function: A function (most often a lambda function) that returns True or False for each element.
• iterable: The sequence (list, tuple, etc.) to be filtered.

Return Type: it returns a filter object, which is an iterator (you have to convert it to list, tuple, etc).

Examples of lambda with filter()

Example 1: Find Numbers Divisible by 13

Given a list of numbers, we want to extract only those numbers divisible by 13.

a = [12, 65, 54, 39, 102, 339, 221, 50, 70]

# Using filter() with lambda to get numbers divisible by 13
res = list(filter(lambda x: (x % 13 == 0), a))

# Printing the result
print(res)

[65, 39, 221]

Explanation:

• lambda function lambda x: (x % 13 == 0) checks if each number is divisible by 13.
• filter() function extracts only the numbers that satisfy this condition.

Example 2: Finding Palindromes in a List

A palindrome is a string that reads the same forward and backward. Let's filter all palindromes from a given list of words.

s = ["geeks", "geeg", "keek", "practice", "aa"]

# Using filter() with lambda to find palindromes
res = list(filter(lambda x: (x == "".join(reversed(x))), s))

print(res)

['geeg', 'keek', 'aa']

Explanation:

• lambda function lambda x: (x == "".join(reversed(x))) checks if the word is equal to its reversed version.
• filter() function keeps only those words that are palindromes.

Example 3: Finding Anagrams from a List

An anagram is a word formed by rearranging the letters of another word. Let's find all anagrams of a given string in a list.

from collections import Counter  

s = ["geeks", "geeg", "keegs", "practice", "aa"]
ts = "eegsk"

# Using filter() with lambda to find anagrams of target_str
res = list(filter(lambda x: (Counter(ts) == Counter(x)), my_list))


print(res)

['geeks', 'keegs']

Explanation:

• Counter(x) creates a frequency dictionary of characters in x.
• lambda function checks if Counter(target_str) == Counter(x), meaning they have the same letter frequencies.
• filter() function extracts words that are anagrams of "eegsk".