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 O(1).

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?
# The output is given in a set and does not print out any duplicates.

# Why do you think Sets are in the same tech talk as Hashmaps/Hashtables?
# Sets are in the same tech talk as Hasmaps/Hashtables because sets are able to remove duplicates
{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?
# lists (genre) and dictionary (tracks)

# 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'}}
print(lover_album.get('tracks'))
# or
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'}
print(lover_album.get('tracks')[4])
# or
print(lover_album['tracks'][4])
The Man
The Man
lover_album["producer"] = set(['Taylor Swift', 'Jack Antonoff', 'Joel Little', 'Taylor Swift', 'Louis Bell', 'Frank Dukes'])

# What can you change to make sure there are no duplicate producers?
# you can make it into a set
#

# 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'}, 'producer': {'Taylor Swift', 'Frank Dukes', 'Jack Antonoff', 'Joel Little', 'Louis Bell'}}
lover_album["tracks"].update({19: "All Of The Girls You Loved Before"})

# How would add an additional genre to the dictionary, like electropop? 
# .update

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': {'Taylor Swift', 'Frank Dukes', 'Jack Antonoff', 'Joel Little', 'Louis Bell'}}
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
tracks_dict = lover_album["tracks"]
track = 1
for song in tracks_dict.values():
    print("Track " + str(track) + ": " + str(song))
    track += 1
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: {'Taylor Swift', 'Frank Dukes', 'Jack Antonoff', 'Joel Little', 'Louis Bell'}
Track 1: I Forgot That You Existed
Track 2: Cruel Summer
Track 3: Lover
Track 4: The Man
Track 5: The Archer
Track 6: I Think He Knows
Track 7: Miss Americana & The Heartbreak Prince
Track 8: Paper Rings
Track 9: Cornelia Street
Track 10: Death By A Thousand Cuts
Track 11: London Boy
Track 12: Soon You'll Get Better (feat. Dixie Chicks)
Track 13: False God
Track 14: You Need To Calm Down
Track 15: Afterglow
Track 16: Me! (feat. Brendon Urie of Panic! At The Disco)
Track 17: It's Nice To Have A Friend
Track 18: Daylight
Track 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?
# implementing crud
# you could add a spot for error check. if the input is invalid guide them to do something else
Taylor Swift

Hacks

  • Answer ALL questions in the code segments
  • Create a diagram or comparison illustration (Canva).
    • What are the pro and cons of using this data structure?
    • Dictionary vs List

image

  • Expand upon the code given to you, possible improvements in comments
  • Build your own album showing features of a python dictionary
folklore_album = {
    "title": "folklore",
    "artist": "Taylor Swift",
    "year": 2020,
    "genre": ["Alternative rock", "Indie folk", "Chamber pop", "Folktronica", "Indietronica"],
    "tracks": {
        1: "the 1",
        2: "cardigan",
        3: "the last great american dynasty",
        4: "exile (feat. Bon Iver)",
        5: "my tears ricochet",
        6: "mirrorball",
        7: "seven",
        8: "august",
        9: "this is me trying",
        10: "illicit affairs",
        11: "invisible string",
        12: "mad woman",
        13: "epiphany",
        14: "betty",
        15: "peace",
        16: "hoax",
        17: "the lakes - bonus track",
    }
}

print(folklore_album)
{'title': 'folklore', 'artist': 'Taylor Swift', 'year': 2020, 'genre': ['Alternative rock', 'Indie folk', 'Chamber pop', 'Folktronica', 'Indietronica'], 'tracks': {1: 'the 1', 2: 'cardigan', 3: 'the last great american dynasty', 4: 'exile (feat. Bon Iver)', 5: 'my tears ricochet', 6: 'mirrorball', 7: 'seven', 8: 'august', 9: 'this is me trying', 10: 'illicit affairs', 11: 'invisible string', 12: 'mad woman', 13: 'epiphany', 14: 'betty', 15: 'peace', 16: 'hoax', 17: 'the lakes - bonus track'}}
print(folklore_album.get('tracks'))
# or
print(folklore_album['tracks'])
{1: 'the 1', 2: 'cardigan', 3: 'the last great american dynasty', 4: 'exile (feat. Bon Iver)', 5: 'my tears ricochet', 6: 'mirrorball', 7: 'seven', 8: 'august', 9: 'this is me trying', 10: 'illicit affairs', 11: 'invisible string', 12: 'mad woman', 13: 'epiphany', 14: 'betty', 15: 'peace', 16: 'hoax', 17: 'the lakes - bonus track'}
{1: 'the 1', 2: 'cardigan', 3: 'the last great american dynasty', 4: 'exile (feat. Bon Iver)', 5: 'my tears ricochet', 6: 'mirrorball', 7: 'seven', 8: 'august', 9: 'this is me trying', 10: 'illicit affairs', 11: 'invisible string', 12: 'mad woman', 13: 'epiphany', 14: 'betty', 15: 'peace', 16: 'hoax', 17: 'the lakes - bonus track'}
for k,v in folklore_album.items():
    print(str(k) + ": " + str(v))
title: folklore
artist: Taylor Swift
year: 2020
genre: ['Alternative rock', 'Indie folk', 'Chamber pop', 'Folktronica', 'Indietronica']
tracks: {1: 'the 1', 2: 'cardigan', 3: 'the last great american dynasty', 4: 'exile (feat. Bon Iver)', 5: 'my tears ricochet', 6: 'mirrorball', 7: 'seven', 8: 'august', 9: 'this is me trying', 10: 'illicit affairs', 11: 'invisible string', 12: 'mad woman', 13: 'epiphany', 14: 'betty', 15: 'peace', 16: 'hoax', 17: 'the lakes - bonus track'}
tracks_dict = folklore_album["tracks"]
track = 1
for song in tracks_dict.values():
    print("Track " + str(track) + ": " + str(song))
    track += 1
Track 1: the 1
Track 2: cardigan
Track 3: the last great american dynasty
Track 4: exile (feat. Bon Iver)
Track 5: my tears ricochet
Track 6: mirrorball
Track 7: seven
Track 8: august
Track 9: this is me trying
Track 10: illicit affairs
Track 11: invisible string
Track 12: mad woman
Track 13: epiphany
Track 14: betty
Track 15: peace
Track 16: hoax
Track 17: the lakes - bonus track