How do map, reduce and filter functions work in Python?

Python's map(), filter(), and reduce() functions bring functional programming concepts to Python. These functions accept a function and an iterable, applying the function to elements in different ways to produce results.

map() Function

The map() function applies a given function to each item in an iterable and returns a map object (iterator) with the results. Unlike reduce(), it operates independently on each item rather than producing a single cumulative result.

Syntax

map(function, iterable)

Parameters

• function ? The function to apply to each element

• iterable ? The sequence to iterate over (list, tuple, etc.)

Example

Here's how to multiply each number in a list by 3 using map() ?

# creating a function that returns multiplication result
def multiply_by_three(number):
    # returning number after multiplying with 3
    return number * 3

# input list
numbers = [1, 3, 5, 2, 6, 10]

# map() function applies the multiply_by_three function to each element
result = map(multiply_by_three, numbers)

# Converting map object to list and printing results
print("Multiplying list elements with 3:")
print(list(result))

Multiplying list elements with 3:
[3, 9, 15, 6, 18, 30]

filter() Function

The filter() function creates an iterator from elements of an iterable for which the function returns True. It's used to filter out elements based on a condition.

Syntax

filter(function, iterable)

Parameters

• function ? A function that returns True/False for filtering

• iterable ? The sequence to filter

Example

Filter ages to find who is eligible for voting (18 and above) ?

# creating a function that checks voting eligibility
def is_voting_age(age):
    # checking whether the age is greater than or equal to 18
    return age >= 18

# input list of ages
ages = [3, 20, 18, 6, 14, 25, 19]

# Getting only values that are greater than or equal to 18
eligible_ages = filter(is_voting_age, ages)

# converting filter object to list
print("Eligible ages for voting:", list(eligible_ages))

Eligible ages for voting: [20, 18, 25, 19]

reduce() Function

The reduce() function applies a function cumulatively to items in an iterable, reducing it to a single value. It's available in the functools module.

Syntax

reduce(function, iterable)

Parameters

• function ? A function that takes two arguments

• iterable ? The sequence to reduce

Example

Calculate the sum of all numbers in a list using reduce() ?

# importing reduce() function from functools module
from functools import reduce

# function that returns the sum of two numbers
def add_numbers(x, y):
    return x + y

# input list
numbers = [12, 4, 10, 15, 6, 5]

# Calculate sum using reduce() function
total = reduce(add_numbers, numbers)
print("The sum of all list items:", total)

The sum of all list items: 52

Comparison

Conclusion

Use map() to transform each element, filter() to select elements based on conditions, and reduce() to combine all elements into a single value. These functions provide elegant alternatives to traditional loops for functional programming patterns.