Data Structures- Hashmaps, Sets, Hash Tables, Hashing and Collisions
Observing hashmaps with python dictionaries
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(n).
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?
# I notice that the output only has 3 values, with no repetition.
# In the original list, there are repeats for a total of 5 values.
# Why do you think Sets are in the same tech talk as Hashmaps/Hashtables?
# I think sets are in the same tech talk as Hashmaps/Hashtables since they both don't allow duplicates.
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?
# A list (genre)
# A dictionary (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"] = 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 the list into a set, since sets don't allow for duplicates (like above)
# you can also just take out Taylor Swift
# 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?
lover_album["genre"].append("electropop")
# You could add a new genre by updating the list (not .update)
# Printing the dictionary
print(lover_album["genre"])
for k,v in lover_album.items(): # iterate using a for loop for key and value
print(str(k) + ": " + str(v))
print("\n")
# printing code so tracks are in a prettier format:
print("title: ", lover_album["title"], "\nartist: ", lover_album["artist"], "\nyear: ", lover_album["year"], "\ngenre: ", lover_album["genre"], "\nproducer: ", lover_album["producer"])
print("tracks: ")
for k, v in lover_album["tracks"].items():
print(str(k) + ": " + str(v))
# Write your own code to print tracks in readable format
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?
# I can improve upon this code by making the code actually retrieve a random song (track) like the comment says.
# I could do this by importing random and generating a random song by using the random.randint function
# it would have the parameters as 1 to 19, which selects a random key and returns the corresponding value (which is the random song.)
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
- My favs: Enchanted, Our Song, Red
TakeMeHome_album = {
"title": "Take Me Home (Expanded Edition)",
"artist": "One Direction",
"year": 2012,
"genre": ["Pop", "Pop-teen", "Pop-rock"],
"tracks": {
1: "Live While We're Young",
2: "Kiss You",
3: "Little Things",
4: "C'mon, C'mon",
5: "Last First Kiss",
6: "Heart Attack",
7: "Rock Me",
8: "Change My Mind",
9: "I Would",
10: "Over Again",
11: "Back For You",
12: "They Don't Know About Us",
13: "Summer Love",
14: "She's Not Afraid",
15: "Loved You First",
16: "Nobody Compares",
17: "Still The One",
18: "Truly Madly Deeply",
19: "Magic",
20: "Irresistible"
}
}
# printing album
print("title: ", TakeMeHome_album["title"], "\nartist: ", TakeMeHome_album["artist"], "\nyear: ", TakeMeHome_album["year"], "\ngenre: ", TakeMeHome_album["genre"])
print("tracks: ")
for k, v in TakeMeHome_album["tracks"].items():
print(str(k) + ": " + str(v))