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
- Hashmaps pros and cons
- Dictionary vs List
- Build your own album
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 printed the set without duplicate values
# Why do you think Sets are in the same tech talk as Hashmaps/Hashtables?
# Both sets and hashmaps/hashtables don't allow there to be duplicate values
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?
# The lover_album is a dictionary that contains the keys: "title", "artist", "year", "genre", and "tracks". Tracks is a dictionary within the lover_album dictionary, therefore it is a nested 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 function in order 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 an append function to add an electropop genre shown below
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
#
print("-----")
print("Tracks:")
for track_number, track_name in lover_album["tracks"].items():
print(f"{track_number}: {track_name}")
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?
# Formatting output so that it is easier to read, like tracks
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
When I Fall in Love 3:03 Slipping Away 3:56 No More Looking for Love 3:16 Whenever You Need Somebody 3:57 The Love Has Gone 4:20 It Would Take a Strong Strong Man 3:45 You Move Me 3:46 Together Forever 3:27 Dont't Say Goodbye 4:12 Never Gonna Give You Up
album2 = {
"title": "Whenever You Need Somebody",
"artist": " Rick Astley",
"year": 1987,
"genre": ["Pop", "Electronic dance"],
"rickroll" : "Never Gonna Give You Up",
"tracks": {
1: "Never Gonna Give You Up",
2: "Dont't Say Goodbye",
3: "Together Forever",
4: "You Move Me",
5: "It Would Take a Strong Strong Man",
6: "The Love Has Gone",
7: "Whenever You Need Somebody",
8: "No More Looking for Love",
9: "Slipping Away",
10: "When I Fall in Love"
}
}
print(album2)
album2["genre"].append("Disco")
def search():
search = input("What would you like to know about the album?")
if album2.get(search.lower()) == None:
print("Invalid Search")
else:
if type(album2.get(search.lower())) is dict:
for k, v in album2.get(search.lower()).items():
print(v)
pass
elif type(album2.get(search.lower())) is list:
for i in album2.get(search.lower()):
print(i)
pass
elif type(album2.get(search.lower())) is int or str:
return(album2.get(search.lower()))
search()
print("----")
search()