Methods for Working with Collections in Python

Python provides a series of built-in functions and methods that facilitate the manipulation and processing of lists, tuples, and other iterables.

Functions on Iterables

map() Function

The map() function in Python is used to apply a function to each element of a sequence and return an iterable with the results.

# Basic usage of map():
def square(x):
    return x ** 2

numbers = [1, 2, 3, 4, 5]
squares = map(square, numbers)

print(list(squares)) # [1, 4, 9, 16, 25]

In this example, map(square, numbers) applies the square() function to each element of the numbers list, returning a map object that contains the squares of the numbers.

Often, we will use the map() function with a lambda function to make the code more concise:

# Using map() with a lambda function:
numbers = [1, 2, 3, 4, 5]
squares = map(lambda x: x ** 2, numbers)

print(list(squares)) # [1, 4, 9, 16, 25]

filter() Function

The filter() function in Python is used to filter elements from a sequence based on a condition defined by a function.

# Basic usage of filter():
def is_even(x):
    return x % 2 == 0

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
evens = filter(is_even, numbers)

In this example, filter(is_even, numbers) filters the numbers from the numbers list and returns a filter object that contains only the even numbers.

Just like with map(), a lambda function can also be used with filter():

# Basic usage of filter():
def is_even(x):
    return x % 2 == 0

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
evens = filter(is_even, numbers)

print(list(evens))  # Output: [2, 4, 6, 8, 10]

any() and all() Functions

The any() and all() functions in Python are boolean functions that return True based on the values of an iterable:

• any() returns True if at least one element is True
• all() returns True if all elements are True

# Using any() and all():
values = [True, False, True, False, True]
is_any_true = any(values)  # True
are_all_true = all(values)  # False

These functions are useful for checking conditions in iterables like lists or generators.

They are often used together with map to apply a function to the values of an iterable. For example, like this:

values = [1, 2, 3, 4, 5]
is_any_greater_than_ten = any(map(lambda x: x > 10, values))  # False

sorted() Function

The sorted() function in Python returns a new sorted list from the elements of the specified iterable.

# Basic usage of sorted():
numbers = [3, 1, 4, 1, 5, 9, 2, 6, 5, 3]
sorted_numbers = sorted(numbers)

print(sorted_numbers) # Output: [1, 1, 2, 3, 3, 4, 5, 5, 6, 9]

In this example, sorted(numbers) will sort the numbers list in ascending order and assign the result to sorted_numbers.

Optionally, we can specify:

• A key function to customize the sorting
• A reverse argument to reverse the order

# Using sorted() with key and reverse:
words = ["python", "java", "c++", "ruby", "javascript"]
sorted_words = sorted(words, key=len, reverse=True)

print(sorted_words) # ['javascript', 'python', 'java', 'c++', 'ruby']

Here, sorted(words, key=len, reverse=True) sorts the words list by the length of the strings in descending order.

sum() Function

The sum() function in Python returns the sum of the elements of the specified iterable, optionally starting from a specified initial value (default is 0).

# Basic usage of sum():
numbers = [1, 2, 3, 4, 5]

total_sum = sum(numbers)  # 15

This example sums all the numbers in the numbers list.

enumerate() Function

The enumerate() function in Python returns an enumerated object that produces tuples with a counter and an element from the specified iterable, optionally starting from a specified index (default is 0).

# Basic usage of enumerate():
teams = ["Real Madrid", "Barcelona", "Atlético de Madrid"]
enumerated_teams = list(enumerate(teams, start=1))

print(enumerated_teams) # [(1, 'Real Madrid'), (2, 'Barcelona'), (3, 'Atlético de Madrid')]

In this example, enumerate(teams, start=1) enumerates the football teams starting from 1 and produces a list of tuples (index, team).

reduce() Function

The reduce() function in Python is used to reduce a sequence of elements to a single value by repeatedly applying a two-argument function to the elements of the sequence.

# Basic usage of reduce():
from functools import reduce

def add(x, y):
    return x + y

numbers = [1, 2, 3, 4, 5]
total_sum = reduce(add, numbers)

print(total_sum) # Output: 15

In this example, reduce(add, numbers) applies the add() function cumulatively to the elements of the numbers list, resulting in the total sum of all the numbers.

zip() Function

The zip() function in Python combines elements from multiple iterables into tuples until the shortest iterable is exhausted.

# Basic usage of zip():
names = ["Luis", "María", "Pedro"]
ages = [30, 25, 35, 40]

print(persons) # [("Luis", 30), ("María", 25), ("Pedro", 35)]

In this example, zip(names, ages) combines the names and ages lists into a list of tuples (name, age).

In this case, it returns a collection of 3 tuples because names only has 3 values. Each tuple combines a value from the names iterable and from the ages iterable.