What is a Hashtable/Hashmap?

A hashtable is a data structure that with a collection of key-value pairs, where each key maps to a value, and the keys must be unique and hashable.

  • In Python there is a built in hashtable known as a dictionary.

The primary purpose of a hashtable is to provide efficient lookup, insertion, and deletion operations. When an element is to be inserted into the hashtable, a hash function is used to map the key to a specific index in the underlying array that is used to store the key-value pairs. The value is then stored at that index. When searching for a value, the hash function is used again to find the index where the value is stored.

The key advantage of a hashtable over other data structures like arrays and linked lists is its average-case time complexity for lookup, insertion, and deletion operations.

  • The typical time complexity of a hashtable is constant.

What is Hashing and Collision?

Hashing is the process of mapping a given key to a value in a hash table or hashmap, using a hash function. The hash function takes the key as input and produces a hash value or hash code, which is then used to determine the index in the underlying array where the value is stored. The purpose of hashing is to provide a quick and efficient way to access data, by eliminating the need to search through an entire data structure to find a value.

However, it is possible for two different keys to map to the same hash value, resulting in a collision. When a collision occurs, there are different ways to resolve it, depending on the collision resolution strategy used.

Python's dictionary implementation is optimized to handle collisions efficiently, and the performance of the dictionary is generally very good, even in the presence of collisions. However, if the number of collisions is very high, the performance of the dictionary can degrade, so it is important to choose a good hash function that minimizes collisions when designing a Python dictionary.

What is a Set?

my_set = set([1, 2, 3, 2, 1])
print(my_set)  

# What do you notice in the output?
# 
# It doesn't output the repeated numbers (Only outputs one of each)


# Why do you think Sets are in the same tech talk as Hashmaps/Hashtables?
# 
# Both will always contain unique values (It disregards the duplicate items)
{1, 2, 3}

Dictionary Example

Below are just some basic features of a dictionary. As always, documentation is always the main source for all the full capablilties.

lover_album = {
    "title": "Lover",
    "artist": "Taylor Swift",
    "year": 2019,
    "genre": ["Pop", "Synth-pop"],
    "tracks": {
        1: "I Forgot That You Existed",
        2: "Cruel Summer",
        3: "Lover",
        4: "The Man",
        5: "The Archer",
        6: "I Think He Knows",
        7: "Miss Americana & The Heartbreak Prince",
        8: "Paper Rings",
        9: "Cornelia Street",
        10: "Death By A Thousand Cuts",
        11: "London Boy",
        12: "Soon You'll Get Better (feat. Dixie Chicks)",
        13: "False God",
        14: "You Need To Calm Down",
        15: "Afterglow",
        16: "Me! (feat. Brendon Urie of Panic! At The Disco)",
        17: "It's Nice To Have A Friend",
        18: "Daylight"
    }
}

# What data structures do you see?
# 
# Dictionary, list

# Printing the dictionary
print(lover_album)
{'title': 'Lover', 'artist': 'Taylor Swift', 'year': 2019, 'genre': ['Pop', 'Synth-pop'], 'tracks': {1: 'I Forgot That You Existed', 2: 'Cruel Summer', 3: 'Lover', 4: 'The Man', 5: 'The Archer', 6: 'I Think He Knows', 7: 'Miss Americana & The Heartbreak Prince', 8: 'Paper Rings', 9: 'Cornelia Street', 10: 'Death By A Thousand Cuts', 11: 'London Boy', 12: "Soon You'll Get Better (feat. Dixie Chicks)", 13: 'False God', 14: 'You Need To Calm Down', 15: 'Afterglow', 16: 'Me! (feat. Brendon Urie of Panic! At The Disco)', 17: "It's Nice To Have A Friend", 18: 'Daylight'}}
# get method
print(lover_album.get('tracks'))
# or

# index method
print(lover_album['tracks'])
{1: 'I Forgot That You Existed', 2: 'Cruel Summer', 3: 'Lover', 4: 'The Man', 5: 'The Archer', 6: 'I Think He Knows', 7: 'Miss Americana & The Heartbreak Prince', 8: 'Paper Rings', 9: 'Cornelia Street', 10: 'Death By A Thousand Cuts', 11: 'London Boy', 12: "Soon You'll Get Better (feat. Dixie Chicks)", 13: 'False God', 14: 'You Need To Calm Down', 15: 'Afterglow', 16: 'Me! (feat. Brendon Urie of Panic! At The Disco)', 17: "It's Nice To Have A Friend", 18: 'Daylight'}
{1: 'I Forgot That You Existed', 2: 'Cruel Summer', 3: 'Lover', 4: 'The Man', 5: 'The Archer', 6: 'I Think He Knows', 7: 'Miss Americana & The Heartbreak Prince', 8: 'Paper Rings', 9: 'Cornelia Street', 10: 'Death By A Thousand Cuts', 11: 'London Boy', 12: "Soon You'll Get Better (feat. Dixie Chicks)", 13: 'False God', 14: 'You Need To Calm Down', 15: 'Afterglow', 16: 'Me! (feat. Brendon Urie of Panic! At The Disco)', 17: "It's Nice To Have A Friend", 18: 'Daylight'}
# get + index
print(lover_album.get('tracks')[4])
# or

# double indexing
print(lover_album['tracks'][4])
The Man
The Man
lover_album["producer"] = ['Taylor Swift', 'Jack Antonoff', 'Joel Little', 'Taylor Swift', 'Louis Bell', 'Frank Dukes']

# What can you change to make sure there are no duplicate producers?
#
# make lover_album["producer"] = set(['Taylor Swift', 'Jack Antonoff', 'Joel Little', 'Taylor Swift', 'Louis Bell', 'Frank Dukes'])

lover_album["producer"] = set(['Taylor Swift', 'Jack Antonoff', 'Joel Little', 'Taylor Swift', 'Louis Bell', 'Frank Dukes'])

# Printing the dictionary
print(lover_album)
# Printing the producer set (notice no duplicates even though there are duplicates above)
print(lover_album.get("producer"))
{'title': 'Lover', 'artist': 'Taylor Swift', 'year': 2019, 'genre': ['Pop', 'Synth-pop'], 'tracks': {1: 'I Forgot That You Existed', 2: 'Cruel Summer', 3: 'Lover', 4: 'The Man', 5: 'The Archer', 6: 'I Think He Knows', 7: 'Miss Americana & The Heartbreak Prince', 8: 'Paper Rings', 9: 'Cornelia Street', 10: 'Death By A Thousand Cuts', 11: 'London Boy', 12: "Soon You'll Get Better (feat. Dixie Chicks)", 13: 'False God', 14: 'You Need To Calm Down', 15: 'Afterglow', 16: 'Me! (feat. Brendon Urie of Panic! At The Disco)', 17: "It's Nice To Have A Friend", 18: 'Daylight'}, 'producer': {'Frank Dukes', 'Taylor Swift', 'Louis Bell', 'Joel Little', 'Jack Antonoff'}}
{'Frank Dukes', 'Taylor Swift', 'Louis Bell', 'Joel Little', 'Jack Antonoff'}
lover_album["tracks"].update({19: "All Of The Girls You Loved Before"})

# How would add an additional genre to the dictionary, like electropop? 
# 
lover_album["genre"].append("electropop")

# Printing the dictionary
print(lover_album)
{'title': 'Lover', 'artist': 'Taylor Swift', 'year': 2019, 'genre': ['Pop', 'Synth-pop', 'electropop'], 'tracks': {1: 'I Forgot That You Existed', 2: 'Cruel Summer', 3: 'Lover', 4: 'The Man', 5: 'The Archer', 6: 'I Think He Knows', 7: 'Miss Americana & The Heartbreak Prince', 8: 'Paper Rings', 9: 'Cornelia Street', 10: 'Death By A Thousand Cuts', 11: 'London Boy', 12: "Soon You'll Get Better (feat. Dixie Chicks)", 13: 'False God', 14: 'You Need To Calm Down', 15: 'Afterglow', 16: 'Me! (feat. Brendon Urie of Panic! At The Disco)', 17: "It's Nice To Have A Friend", 18: 'Daylight', 19: 'All Of The Girls You Loved Before'}, 'producer': {'Frank Dukes', 'Taylor Swift', 'Louis Bell', 'Joel Little', 'Jack Antonoff'}}
for k,v in lover_album.items(): # iterate using a for loop for key and value
    print(str(k) + ": " + str(v))

# Write your own code to print tracks in readable format
#
# See code segment below this cell
title: Lover
artist: Taylor Swift
year: 2019
genre: ['Pop', 'Synth-pop', 'electropop']
tracks: {1: 'I Forgot That You Existed', 2: 'Cruel Summer', 3: 'Lover', 4: 'The Man', 5: 'The Archer', 6: 'I Think He Knows', 7: 'Miss Americana & The Heartbreak Prince', 8: 'Paper Rings', 9: 'Cornelia Street', 10: 'Death By A Thousand Cuts', 11: 'London Boy', 12: "Soon You'll Get Better (feat. Dixie Chicks)", 13: 'False God', 14: 'You Need To Calm Down', 15: 'Afterglow', 16: 'Me! (feat. Brendon Urie of Panic! At The Disco)', 17: "It's Nice To Have A Friend", 18: 'Daylight', 19: 'All Of The Girls You Loved Before'}
producer: {'Frank Dukes', 'Taylor Swift', 'Louis Bell', 'Joel Little', 'Jack Antonoff'}
print(f"Title: {lover_album['title']}")
print(f"Artist: {lover_album['artist']}")
print(f"Year: {lover_album['year']}")
print("Genres:")
for genre in lover_album['genre']:
    print(f"- {genre}")
print("Tracks:")
for track_number, track_title in lover_album['tracks'].items():
    print(f"{track_number}. {track_title}")
Title: Lover
Artist: Taylor Swift
Year: 2019
Genres:
- Pop
- Synth-pop
- electropop
Tracks:
1. I Forgot That You Existed
2. Cruel Summer
3. Lover
4. The Man
5. The Archer
6. I Think He Knows
7. Miss Americana & The Heartbreak Prince
8. Paper Rings
9. Cornelia Street
10. Death By A Thousand Cuts
11. London Boy
12. Soon You'll Get Better (feat. Dixie Chicks)
13. False God
14. You Need To Calm Down
15. Afterglow
16. Me! (feat. Brendon Urie of Panic! At The Disco)
17. It's Nice To Have A Friend
18. Daylight
19. All Of The Girls You Loved Before
def search():
    search = input("What would you like to know about the album?")
    if lover_album.get(search.lower()) == None:
        print("Invalid Search")
    else:
        print(lover_album.get(search.lower()))

search()

# This is a very basic code segment, how can you improve upon this code?
#
# We can display the property options and format the outputs in a looping menu
['Pop', 'Synth-pop', 'electropop']
def search():
    search = input("What would you like to know about the album?")
    if lover_album.get(search.lower()) == None:
        print("Property not found. Please try again. \n")
    else:
        print(lover_album.get(search.lower()))

# Define the options as a list
options = []

for k,v in lover_album.items(): # iterate using a for loop for key and value
    options.append(str(k.lower()))

# Define the menu function
def menu():
    # Display the menu options
    print('\n Select an option from the list below:')
    for option in options:
        print(option)

    # Get the user's choice
    search = input("What would you like to know about the album from the list of properties shown? Type the full word or press enter to exit.").lower()

    # Find the selected option in the options list
    selected_option = None
    for option in options:
        if search == str(option):
            selected_option = str(option)
            break
    

    if selected_option == "title":
        print(f'\n Title: {lover_album[options[0]]} \n')
    elif selected_option == "artist":
        print(f'\n Artist: {lover_album[options[1]]} \n')
    elif selected_option == "year":
        print(f'\n Year: {lover_album[options[2]]} \n')
    elif selected_option == "genre":
        print("\n Genre(s):")
        for g in lover_album['genre']:
            print(f"- {g}")
    elif selected_option == "tracks":
        print("\n Tracks:")
        for track_number, track_title in lover_album['tracks'].items():
            print(f"{track_number}) {track_title}")
    elif selected_option == "producer":
        print("\n Producer(s):")
        for p in lover_album['producer']:
            print(f"- {p}")
    elif search == "":
        return
    else:
        print("\n Property not found. Please try again. \n")

    # Repeat the menu
    menu()


menu()
 Select an option from the list below:
title
artist
year
genre
tracks
producer

 Title: Lover 


 Select an option from the list below:
title
artist
year
genre
tracks
producer

 Artist: Taylor Swift 


 Select an option from the list below:
title
artist
year
genre
tracks
producer

 Year: 2019 


 Select an option from the list below:
title
artist
year
genre
tracks
producer

 Genre(s):
- Pop
- Synth-pop
- electropop

 Select an option from the list below:
title
artist
year
genre
tracks
producer

 Tracks:
1) I Forgot That You Existed
2) Cruel Summer
3) Lover
4) The Man
5) The Archer
6) I Think He Knows
7) Miss Americana & The Heartbreak Prince
8) Paper Rings
9) Cornelia Street
10) Death By A Thousand Cuts
11) London Boy
12) Soon You'll Get Better (feat. Dixie Chicks)
13) False God
14) You Need To Calm Down
15) Afterglow
16) Me! (feat. Brendon Urie of Panic! At The Disco)
17) It's Nice To Have A Friend
18) Daylight
19) All Of The Girls You Loved Before

 Select an option from the list below:
title
artist
year
genre
tracks
producer

 Producer(s):
- Frank Dukes
- Taylor Swift
- Louis Bell
- Joel Little
- Jack Antonoff

 Select an option from the list below:
title
artist
year
genre
tracks
producer

Hacks

  • Answer ALL questions in the code segments

    Done

  • Create a diagram or comparison illustration (Canva).

    • What are the pro and cons of using this data structure?
    • Dictionary vs List

      See graphic below

Comparing Dictionary and List

  • Expand upon the code given to you, possible improvements in comments

    Done

  • Build your own album showing features of a python dictionary

    Done (see below)

  • For Mr. Yeung's class:Justify your favorite Taylor Swift song, answer may effect seed> No comment

My Own Album (Yes... made by me)

# Creating a dictionary with information about the album "2021: The Mixtape"
Mixtape = {
    "title": "2021: The Mixtape",
    "artist": "Lil Xeem",
    "year": 2021,
    "genre": ["Rap"],
    "tracks": {
        1: "love is dead",
        2: "Stress",
        3: "Stay (Remake)",
        4: "Secrets",
        5: "Run Away",
        6: "Broken Heart",
        7: "Interlude",
        8: "No Way",
        9: "Beast Mode",
        10: "No Limits",
        11: "Vroom",
        12: "Skrt Skrt (feat. A.G.E. The Gawd)",
        13: "Talk My Sh*t"
    }
}

# Printing the dictionary
print(Mixtape)
{'title': '2021: The Mixtape', 'artist': 'Lil Xeem', 'year': 2021, 'genre': ['Rap'], 'tracks': {1: 'love is dead', 2: 'Stress', 3: 'Stay (Remake)', 4: 'Secrets', 5: 'Run Away', 6: 'Broken Heart', 7: 'Interlude', 8: 'No Way', 9: 'Beast Mode', 10: 'No Limits', 11: 'Vroom', 12: 'Skrt Skrt (feat. A.G.E. The Gawd)', 13: 'Talk My Sh*t'}}
# Retrieve value from a dictionary inside a dictionary

# get + index
print(Mixtape.get('tracks')[4])

# double indexing
print(Mixtape['tracks'][4])
Secrets
Secrets
# Adding a producer set

Mixtape["producer"] = set(['Lil Xeem', 'Sorrowbringer', 'zkup'])

# Printing the dictionary
print(Mixtape)

# Printing the producer set
print(Mixtape["producer"])
{'title': '2021: The Mixtape', 'artist': 'Lil Xeem', 'year': 2021, 'genre': ['Rap'], 'tracks': {1: 'love is dead', 2: 'Stress', 3: 'Stay (Remake)', 4: 'Secrets', 5: 'Run Away', 6: 'Broken Heart', 7: 'Interlude', 8: 'No Way', 9: 'Beast Mode', 10: 'No Limits', 11: 'Vroom', 12: 'Skrt Skrt (feat. A.G.E. The Gawd)', 13: 'Talk My Sh*t'}, 'producer': {'Sorrowbringer', 'zkup', 'Lil Xeem'}}
{'Sorrowbringer', 'zkup', 'Lil Xeem'}
# Updating the genre with Pop

Mixtape["genre"].append("Pop")
# Pretty printing

print(f"Title: {Mixtape['title']}")
print(f"Artist: {Mixtape['artist']}")
print('Producer(s):')
for prod in Mixtape['producer']:
    print(f"- {prod}")
print(f"Year: {Mixtape['year']}")
print("Genres:")
for genre in Mixtape['genre']:
    print(f"- {genre}")
print("Tracks:")
for track_number, track_title in Mixtape['tracks'].items():
    print(f"{track_number}. {track_title}")
Title: 2021: The Mixtape
Artist: Lil Xeem
Producer(s):
- Sorrowbringer
- zkup
- Lil Xeem
Year: 2021
Genres:
- Rap
- Pop
Tracks:
1. love is dead
2. Stress
3. Stay (Remake)
4. Secrets
5. Run Away
6. Broken Heart
7. Interlude
8. No Way
9. Beast Mode
10. No Limits
11. Vroom
12. Skrt Skrt (feat. A.G.E. The Gawd)
13. Talk My Sh*t
# Menu to find info about the album

# Define the options as a list
options = []

for k,v in Mixtape.items(): # iterate using a for loop for key and value
    options.append(str(k.lower()))

# Define the menu function
def menu():
    # Display the menu options
    print('\n Select an option from the list below:')
    for option in options:
        print(option)

    # Get the user's choice
    search = input("What would you like to know about the album from the list of properties shown? Type the full word or press enter to exit.").lower()

    # Find the selected option in the options list
    selected_option = None
    for option in options:
        if search == str(option):
            selected_option = str(option)
            break
    

    if selected_option == "title":
        print(f'\n Title: {Mixtape[options[0]]} \n')
    elif selected_option == "artist":
        print(f'\n Artist: {Mixtape[options[1]]} \n')
    elif selected_option == "year":
        print(f'\n Year: {Mixtape[options[2]]} \n')
    elif selected_option == "genre":
        print("\n Genre(s):")
        for g in Mixtape['genre']:
            print(f"- {g}")
    elif selected_option == "tracks":
        print("\n Tracks:")
        for track_number, track_title in Mixtape['tracks'].items():
            print(f"{track_number}) {track_title}")
    elif selected_option == "producer":
        print("\n Producer(s):")
        for p in Mixtape['producer']:
            print(f"- {p}")
    elif search == "":
        return
    else:
        print("\n Property not found. Please try again. \n")

    # Repeat the menu
    menu()

menu()
 Select an option from the list below:
title
artist
year
genre
tracks
producer

 Title: 2021: The Mixtape 


 Select an option from the list below:
title
artist
year
genre
tracks
producer

 Artist: Lil Xeem 


 Select an option from the list below:
title
artist
year
genre
tracks
producer

 Year: 2021 


 Select an option from the list below:
title
artist
year
genre
tracks
producer

 Genre(s):
- Rap
- Pop

 Select an option from the list below:
title
artist
year
genre
tracks
producer

 Tracks:
1) love is dead
2) Stress
3) Stay (Remake)
4) Secrets
5) Run Away
6) Broken Heart
7) Interlude
8) No Way
9) Beast Mode
10) No Limits
11) Vroom
12) Skrt Skrt (feat. A.G.E. The Gawd)
13) Talk My Sh*t

 Select an option from the list below:
title
artist
year
genre
tracks
producer

 Producer(s):
- Sorrowbringer
- zkup
- Lil Xeem

 Select an option from the list below:
title
artist
year
genre
tracks
producer