Josh's Blog; Data.Code.Cloud

Python Lists, Part 1

Python Lists, Part 1

Josh Campos's photo
Josh Campos
·

3 min read

Table of contents

This two part series will provide an introduction to using lists and how to work with them in python, from traversing a for loop to using map() and list comprehension strategies.

Lists

A list in python is a collection of various elements. A list can be a collection of integers, characters, dictionaries, or all of the above, not to be confused with an array. An array requires all elements to be of the same type. 

# Empty List
empty_list = []

# List of integers
number_list = [1, 2, 3, 4, 5]

# List of strings
alpha_list = ["alpha", "beta", "charlie", "delta", "echo"]

# List of dictionaries
dict_list = [{"id": 1, "role": "admin"}, {"id": 2, "role": "manager"}] 

# List of mixed value types
mixed_list = [1, "two", 3, False]

# check the type
type(number_list)
# OUTPUT: <class 'list'>

Index & Length

# Access list items by a positive index
alpha_list[1]
# OUTPUT: 'beta'

# Negative index can also be used to access list elements
number_list[-2]
# OUTPUT: 4

# Length of the list
len(alpha_list)
# OUTPUT: 5

Slicing

Using indexes, access a subset of a list by using two indexes seperated by a colon [x:y], x element is inclusive, y element is not. Omitting the first index, the slice starts at index 0. Omitting the second index, the slice continues until the end of the string.

number_list[1:3]
# OUTPUT: [2, 3]

number_list[:4]
# OUTPUT: [1, 2, 3, 4]

number_list[2:]
# OUTPUT: [3, 4, 5]

# Negative indexes with slicing can also be used
number_list[-3:]
# OUTPUT: [3, 4, 5]

number_list[:-3]
# OUTPUT: [1, 2]

Join & Split

String variables can be split into a list. Default delimiter is a space but a character delimeter can be specified. 

"Hello world".split()
# OUTPUT: ['Hello', 'world']

"Hello world".split('o')
# OUTPUT: ['Hell', ' w', 'rld']

# The Join method does the opposite and combines a list into a string
" ".join(['Hello', 'world'])
# OUTPUT:  'Hello world'

"-".join(['Hello', 'world'])
# OUTPUT:  'Hello-world'

Mutable

Lists are mutable. 

# Add elements by appending to the end of the list
alpha_list = ["alpha", "beta"]
alpha_list.append("charlie")

alpha_list
# OUTPUT: ['alpha', 'beta', 'charlie']

#Insert elements at a specific index
alpha_list.insert(0, "delta")
alpha_list
#OUTPUT; ['delta', 'alpha', 'beta', 'charlie']

Remove/ Delete

Elements can also be removed/deleted from the list.

# del will remove the element at a specific index
del alpha_list[3]
alpha_list
#OUTPUT; ['delta', 'alpha', 'beta']

# Remove will remove the first match in the list
# If multiple matches exist in the list, not all elements are removed
alpha_list.remove('alpha')
alpha_list
#OUTPUT; ['delta', 'beta']

Other Helper Methods

# Reverse the order of the list
alpha_list = ["alpha", "beta"]
alpha_list.reverse()
alpha_list
# OUTPUT: ['beta', 'alpha']


# Sort the list
alpha_list.sort()
alpha_list
# OUTPUT; ['delta', 'beta']

For Loop

# Create a List of grades
grades = [90, 80, 95, 100, 75]

# Iterate over the list and print each individual grade
for i in grades:
        print(i)

# OUTPUT:
# 90
# 80
# 95
# 100
# 75

# Show indexes for elements in list using the enumerate function in a for loop
alpha_list = ["alpha", "beta", "charlie"]
for index, alpha  in enumerate(alpha_list):
        print(f"The word at index: {index} is {alpha} ")

# OUTPUT: 
# The word at index: 0 is alpha
# The word at index: 1 is beta
# The word at index: 2 is charlie

Part 2 of this series will include the more functional programming concepts such as map() and list comprehension examples.

Programming BlogsPython