Data Structures- Hashmaps, Sets, Hash Tables, Hashing and Collisions
Observing hashmaps with python dictionaries
- What is a Hashtable/Hashmap?
- What is Hashing and Collision?
- What is a Set?
- Dictionary Example
- Hacks
- My Own Album (m.A.A.d city)
What is a Hashtable/Hashmap?
A hashtable is a data structure 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?
# In the output, we notice that the duplicates are removed and the resulting set contains only unique elements
# Why do you think Sets are in the same tech talk as Hashmaps/Hashtables?
# Sets are in the same tech talk as Hashmaps/Hashtables because they both use hash tables as their underlying data structure. A hash table is a data structure that allows for efficient insertion, deletion, and lookup of elements based on a key.
# Sets use a hash table where the elements themselves serve as the key. This allows for fast membership testing and ensures that all elements in a set are unique. Similarly, hashmaps/hashtables use a hash table to store key-value pairs, where the keys serve as unique identifiers for the values. The use of hash tables is what makes sets and hashmaps/hashtables efficient data structures for certain operations, such as membership testing and fast lookups.
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?
# Key-value pairs for general information about the "Lover" album, such as the title, artist, year, and genre.
# A nested dictionary for the track list, where each track number is a key and each track title is the corresponding value.
# 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?
# we can change the producer key from a list to a set. A set only contains unique elements, so any duplicate producers will automatically be removed.
# 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"))
lover_album["tracks"].update({19: "All Of The Girls You Loved Before"})
# How would add an additional genre to the dictionary, like electropop?
# To add an additional genre like electropop to the "genre" key in the "lover_album" dictionary, we can use the append() method to add the new genre to the existing list.
# 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
# See below code block
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}")
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?
#
#
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
Done
- Create a diagram or comparison illustration (Canva).
- What are the pro and cons of using this data structure?
- Dictionary vs List
Done
- Expand upon the code given to you, possible improvements in comments
Done
- Build your own album showing features of a python dictionary
Done
- For Mr. Yeung's class:Justify your favorite Taylor Swift song, answer may effect seed
Mixtape = {
"title": "m.A.A.d city",
"artist": "Kendrick Lamar",
"year": 2012,
"genre": ["Rap"],
"tracks": {
1: "Compton",
2: "Money Trees",
3: "Good Kid",
4: "Real",
5: "The Art of peer Pressure",
6: "Backseat Freestyle",
7: "Swimming Pools (drank)",
8: "Sing About Me, I'm Dying of Thirst",
9: "m.A.A.d city",
10: "sherane a.k.a master splinter’s daughter",
11: "Don't Kill My Vibe",
12: "Poetic Justice",
13: "Humble"
}
}
# Printing the dictionary
print(Mixtape)
# get + index
print(Mixtape.get('tracks')[4])
# double indexing
print(Mixtape['tracks'][4])
Mixtape["producer"] = set(['Lil Xeem', 'Sorrowbringer', 'zkup'])
# Printing the dictionary
print(Mixtape)
# Printing the producer set
print(Mixtape["producer"])
Mixtape["genre"].append("Pop")
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}")
# 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()