# Lists Python Lists are ordered collections of items, where each item can be of any data type, and they are enclosed within square brackets `[]`. Lists support various operations such as appending, indexing, slicing, and iterating. They can contain heterogeneous data types, allowing flexibility in storing different kinds of elements like integers, strings, floats, or even other lists (nested lists). In addition, they are mutable, meaning their elements can be changed after creation, making them versatile for dynamic data storage and manipulation. Furthermore, they support indexing and slicing to access elements or sublists; and have their own methods, which we will see below. Dataset: ```python basketball_champions= [ 'Los Angeles Lakers', 17, 'Boston Celtics', 17, 'Golden State Warriors', 7, 'Chicago Bulls', 6, 'San Antonio Spurs', 5, 'Philadelphia 76ers', 3, 'Detroit Pistons', 3, 'Miami Heat', 3, 'New York Knicks', 2, 'Houston Rockets', 2, 'Milwaukee Bucks', 2, 'Cleveland Cavaliers', 1, 'Atlanta Hawks', 1, 'Washington Wizards', 1, 'Oklahoma City Thunder', 1, 'Portland Trail Blazers', 1, 'Dallas Mavericks', 1, 'Sacramento Kings', 1, 'Toronto Raptors', 1, 'Denver Nuggets', 1 ] ``` ## 1. Subsetting Lists ### 1.1. Single Element ```python # number of elements/items in a list num = len(basketball_champions) print(f"Number of elements: {num}") # 40 # last element try: print(basketball_champions[40]) # error, indexing start at 0 except IndexError: print("Oops! Out of Index!") # last element (again) print(basketball_champions[39]) #or print(basketball_champions[num-1]) #or (preferrable) print(basketball_champions[-1]) # first element print(basketball_champions[0]) # OUTPUT """ Number of elements: 40 Oops! Out of Index! 1 1 1 Los Angeles Lakers """ ``` ### 1.2. Multiple Elements (Slicing) ```python # print first two teams with their championships print(basketball_champions[0:4]) # print last two teams with their championships print(basketball_champions[-4:]) # print only teams print(basketball_champions[::2]) # OUTPUT """ ['Los Angeles Lakers', 17, 'Boston Celtics', 17] ['Toronto Raptors', 1, 'Denver Nuggets', 1] ['Los Angeles Lakers', 'Boston Celtics', 'Golden State Warriors', 'Chicago Bulls', 'San Antonio Spurs'] """ ``` ## 2. Updating Lists ### 2.1. Changing list elements ```python # change championship for Lakers to 25 basketball_champions[1] = 25 print(basketball_champions[:2]) # increase championship for Denver Nuggets basketball_champions[-1] = 2 print(basketball_champions[-2:]) # OUTPUT """ ['Los Angeles Lakers', 25] ['Denver Nuggets', 2] """ ``` ### 2.2. Adding and removing elements ```python # adding Phoenix Suns with 0 championships to our list basketball_champions += ['Phoenix Suns',0] print(basketball_champions[-6:]) # 1.Deleting by Index # a. using del # deleting 'Phoenix Suns' and its championships value from the list del basketball_champions[-2:] print(basketball_champions[-6:]) # b. using .pop() # deleting the last element will be deleted basketball_champions.pop() # if no index given, then the last element will be deleted # or the same as basketball_champions.pop(-1) print(basketball_champions[-6:]) # Deleting by Value # removing a specific item (first occurence of that item!) basketball_champions.remove('Denver Nuggets') print(basketball_champions[-6:]) # OUTPUT """ ['Toronto Raptors', 1, 'Denver Nuggets', 2, 'Phoenix Suns', 0] ['Sacramento Kings', 1, 'Toronto Raptors', 1, 'Denver Nuggets', 2] [1, 'Sacramento Kings', 1, 'Toronto Raptors', 1, 'Denver Nuggets'] ['Dallas Mavericks', 1, 'Sacramento Kings', 1, 'Toronto Raptors', 1] """ ``` {% hint style="info" %} In python **del** is a keyword and **remove(), pop()** are in-built methods (see section #3 for more methods). The difference between **del** and **pop()** is that even though they both delete the values at a particular index, **pop()** returns the deleted value: ```python del basketball_champions[0] # No OUTPUT basketball_champions.pop(0) # OUTPUT: 'Los Angeles Lakers' ``` {% endhint %} ## 3. List Methods
MethodDescription
append()Adds an element at the end of the list
clear()Removes all the elements from the list
copy()Returns a copy of the list
count()Returns the number of elements with the specified value
extend()Add the elements of a list (or any iterable), to the end of the current list
index()Returns the index of the first element with the specified value
insert()Adds an element at the specified position
pop()Removes the element at the specified position
remove()Removes the first item with the specified value
reverse()Reverses the order of the list
sort()Sorts the list
Click on the methods practice them on W3schools ```python # insert Chicago Bulls and 99 to the list basketball_champions.insert(0,'Chicago Bulls') basketball_champions.insert(1,99) print(basketball_champions[:6]) # retrieve Chicago Bulls index print(basketball_champions.index('Chicago Bulls')) # retrieve Chicago Bulls' original index print(basketball_champions.index('Chicago Bulls',1)) # count the frequency of a specific value print(f'Number of "Chicago Bulls" entries: {basketball_champions.count("Chicago Bulls")}') print(f"Number of teams with single championships: {basketball_champions.count(1)}") # OUTPUT """ ['Chicago Bulls', 99, 'Los Angeles Lakers', 17, 'Boston Celtics', 17] 0 8 Number of "Chicago Bulls" entries: 2 Number of teams with single championships: 9 """ # delete Chicago Bulls with 99 championships basketball_champions.pop(0) basketball_champions.pop(0) ``` Lists are great data types to store data; however, they fall short when it comes to arithmetic: ```python # extract championships only to do arithmetic champs = basketball_champions[1::2] print(champs) # multiply each element with itself champs * champs # OUTPUT """ [17, 17, 7, 6, 5, 3, 3, 3, 2, 2, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1] --------------------------------------------------------------------------- TypeError Traceback (most recent call last) Cell In[76], line 4 1 champs = basketball_champions[1::2] 2 print(champs) ----> 4 champs * champs TypeError: can't multiply sequence by non-int of type 'list' """ ``` To do such calculation we need the help of a `for` loop: ```python # elementwise multiplication ch_ch = [i*i for i in champs] print(ch_ch) # OUTPUT """ [289, 289, 49, 36, 25, 9, 9, 9, 4, 4, 4, 1, 1, 1, 1, 1, 1, 1, 1, 1] """ ``` A better approach would be to use the [Numpy](https://numpy.org/) package! We first convert our list to a numpy array and then can do the multiplication without having use of a `for` loop. ```python import numpy as np arr_champs = np.array(champs) print(arr_champs * arr_champs) # OUTPUT """ [289 289 49 36 25 9 9 9 4 4 4 1 1 1 1 1 1 1 1 1] """ ``` ## Conclusion Python lists are versatile data structures that can hold a collection of elements of different types. They are mutable, meaning we can modify their contents after creation. Lists support indexing and slicing to access elements or sublists and have built-in methods to operate on them more easily. They are dynamic in size and can grow or shrink as needed, making them a fundamental building block for many Python programs. All being said, because of the nature of the dataset we used (i.e. having only pairs of 'team' and 'championships') it is better if we use another Python's data structure called [Dictionary](https://dshub.gitbook.io/ds-hub/python/basics/variables-and-datatypes/dictionaries). --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://dshub.gitbook.io/ds-hub/python/basics/variables-and-datatypes/lists.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.