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?
#No duplicates exist, and each category of numbers is only displayed once. The numbers are presented in ascending order, beginning with 1, then 2, and so on.
# Why do you think Sets are in the same tech talk as Hashmaps/Hashtables?
# Sets and Hashmaps/Hashtables are discussed in the same technical presentation because they both prohibit duplicates in data structures.
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?
# strings, integers, list, dictionary
#
# 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?
# use the "set" syntax to remove duplicates
#
# 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?
# use 'append'
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
title = lover_album["title"]
artist = lover_album["artist"]
year = lover_album["year"]
genres = ", ".join(lover_album["genre"])
tracks = "\n".join([" {}: {}".format(k, v) for k, v in lover_album["tracks"].items()])
print("Title: {}".format(title))
print("Artist: {}".format(artist))
print("Year: {}".format(year))
print("Genre: {}".format(genres))
print("Tracks:\n{}".format(tracks))
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?
# Allow user to make specific searches within a dictionary.
#
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
lover_album = {
"title": "Certified Lover Boy",
"artist": "Drake",
"year": 2021,
"genre": ["Hip-hop", "R&B"],
"tracks": {
1: "Champagne Poetry",
2: "Papi's Home",
3: "Girls Want Girls (feat. Lil Baby)",
4: "In The Bible (feat. Lil Durk & Giveon)",
5: "Love All (feat. Jay-Z)",
6: "Fair Trade (feat. Travis Scott)",
7: "Way 2 Sexy (feat. Future & Young Thug)",
8: "TSU",
9: "N 2 Deep (feat. Future)",
10: "Pipe Down",
11: "Yebba's Heartbreak",
12: "No Friends In The Industry",
13: "Knife Talk (feat. 21 Savage & Project Pat)",
14: "7am On Bridle Path",
15: "Race My Mind",
16: "Fountains (feat. Tems)",
17: "Get Along Better (feat. Ty Dolla $ign)",
18: "You Only Live Twice (feat. Rick Ross & Lil Wayne)",
19: "IMY2 (feat. Kid Cudi)",
20: "Fucking Fans"
}
}
# What data structures do you see?
# Dictionary (lover_album) with string, integer and list values as keys and values
# Nested dictionary (tracks) as a value inside the lover_album dictionary
# 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(['Drake', '40', 'Noel Cadastre', 'Oliver El-Khatib', 'Metro Boomin'])
# Printing the dictionary
print(lover_album)
lover_album["tracks"].update({21: "The Remorse"})
# How would you add an additional genre to the dictionary, like soul?
# We can simply append it to the existing genre list:
lover_album["genre"].append("Soul")
# Printing the dictionary
print(lover_album)
# To print the tracks in a readable format, we can iterate over the nested tracks dictionary and print each track number and title:
for track_number, title in lover_album["tracks"].items():
print(f"{track_number}. {title}")