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?
# 
# The output only displays 1, 2, 3 and not the repeated numbers, they are in order from least to greatest
#

# Why do you think Sets are in the same tech talk as Hashmaps/Hashtables?
#
# Sets are in the same lesson because this shows how to organize the data and only shows one set of the data
#

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?
#
# list for genre and dictionary for the tracks
#

# Printing the dictionary
print(lover_album)
print(lover_album.get('tracks'))
# or
print(lover_album['tracks'])
print(lover_album.get('tracks')[4])
# or
print(lover_album['tracks'][4])
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?
#
# can make it into a set and set function, make lover_album["producer"] = ..set(['Taylor Swift, ect.'])...
#

# Printing the dictionary
print(lover_album)
lover_album["tracks"].update({19: "All Of The Girls You Loved Before"})

# How would add an additional genre to the dictionary, like electropop? 
# 
# cannot use .update and genre is updating a list so have to figure out another way >>>
# lover_album["genre"].append("electropop")
#

# Printing the dictionary
print(lover_album)
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
#
# iterate so use k for key and v for value
#

Printing Tracks to a readable format

#printing the separate genres so they can be organized
#printing the title
print(f"TITLE: {lover_album['title']}")
#print the artist aka ily TAYLOR :)
print(f"ARTIST: {lover_album['artist']}")
#print the year
print(f"YEAR: {lover_album['year']}")
#print the separate genres
print("GENRE:")
for genre in lover_album['genre']:
    print(f"{genre}")
#print all of the tracks and add the numbers and titles
print("TRACKS:")
for track_number, track_title in lover_album['tracks'].items():
    print(f"{track_number}.{track_title}")
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?
#
#also make it so that the search is better and more focused on options
#this is an easy search option and not much error checking and doesn't tell or let to do something else, display property option and format output in loop menu
#
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()

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
  • Expand upon the code given to you, possible improvements in comments
  • Build your own album showing features of a python dictionary

  • For Mr. Yeung's class: Justify your favorite Taylor Swift song, answer may effect seed

Own Album

noway_album = {
    "title": "No Way!!!",
# lists for the genre and the artists []
    "artist": ["EW", "KV", "PBC"],
    "year": 2022,
    "genre": ["Rap", "Hip-Hop"],
# dictionary for the tracks {}
    "tracks": {
        1: "Cash Sh*t",
        2: "Doja",
        3: "The Grinch",
        4: "#miles",
        5: "Took Her To The O",
        6: "Top",
        7: "Disaster",
        8: "North Memphis",
        9: "Runnin",
        10: "Hardy Boys",
        11: "STATS",
        12: "Drunk And Nasty",
        13: "Watch This - ARIZONATEARS",
        14: "Fast",
        15: "Time To Live",
        16: "Twin Nem",
        17: "R.I.P.",
        18: "16",
        20: "ILoveUIHateU",
        21: "vent",
        22: "TURN IT UP",
        23: "booman",
        24: "D*MN HOMIE (feat. Lil Yachty)",
        25: "BENZO",
        26: "Maytag (Tax Free)",
        27: "HONEST",
        28: "Tonka 2",
        29: "Dreaming",
        30: "Talk",
    }
}
# Printing the dictionary
print(noway_album)
{'title': 'No Way!!!', 'artist': ['EW', 'KV', 'PBC'], 'year': 2022, 'genre': ['Rap', 'Hip-Hop'], 'tracks': {1: 'Cash Sh*t', 2: 'Doja', 3: 'The Grinch', 4: '#miles', 5: 'Took Her To The O', 6: 'Top', 7: 'Disaster', 8: 'North Memphis', 9: 'Runnin', 10: 'Hardy Boys', 11: 'STATS', 12: 'Drunk And Nasty', 13: 'Watch This - ARIZONATEARS', 14: 'Fast', 15: 'Time To Live', 16: 'Twin Nem', 17: 'R.I.P.', 18: '16', 20: 'ILoveUIHateU', 21: 'vent', 22: 'TURN IT UP', 23: 'booman', 24: 'D*MN HOMIE (feat. Lil Yachty)', 25: 'BENZO', 26: 'Maytag (Tax Free)', 27: 'HONEST', 28: 'Tonka 2', 29: 'Dreaming', 30: 'Talk'}}
def search():
    search = input("What would you like to know about the album?")
    if noway_album.get(search.lower()) == None:
        print("Property not found. Please try again. \n")
    else:
        print(noway_album.get(search.lower()))

# Define the options as a list
options = []

for k,v in noway_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: {noway_album[options[0]]} \n')
        
    elif selected_option == "artist":
        print("\n Artist(s): ")
        for a in noway_album['artist']:
            print(f"- {a}")
        
    elif selected_option == "year":
        print(f'\n Year: {noway_album[options[2]]} \n')
        
    elif selected_option == "genre":
        print("\n Genre(s):")
        for g in noway_album['genre']:
            print(f"- {g}")
            
    elif selected_option == "tracks":
        print("\n Tracks:")
        for track_number, track_title in noway_album['tracks'].items():
            print(f"{track_number}) {track_title}")
            
    elif selected_option == "producer":
        print("\n Producer(s):")
        for p in noway_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

 Title: No Way!!! 


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

 Artist(s): 
- EW
- KV
- PBC

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

 Year: 2022 


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

 Genre(s):
- Rap
- Hip-Hop

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