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(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 is a dictionary. The set converts this list into a dictionary.
# Additionally, there are no duplicate values. The extra 2 and 1 are removed from the dictionary which is outputted.
# 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 and Hashtables because they allow for data to be cleaned and have only one unique value in a dictionary.
# The set function may be used as a function which can prepare a dictionary which is a hashmaps.
#
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?
#
# I see a list and dictionary. There is also a tracks dictionary inside of 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(['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 put the list of producers into the set function.
# Printing the dictionary
print(lover_album["producer"])
lover_album["tracks"].update({19: "All Of The Girls You Loved Before"})
# How would add an additional genre to the dictionary, like electropop?
#
# You could use the append function to add an additional genre to the genre list.
# Printing the dictionary
print(lover_album)
for k,v in [(k,v) for k, v in lover_album.items() if k != "tracks"]: # 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("Tracks:")
for l, s in lover_album["tracks"].items():
print(str(l) + ": " + str(s))
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?
options = ['title', 'artist', 'year', 'genre', 'tracks']
def album_search():
print("Options:")
for option in options:
print(option)
usrsearch = input("What would you like to know about the album? ")
if usrsearch.lower() not in options:
print("Invalid search")
elif usrsearch.lower() == 'tracks':
tracksearch = input("All --> for all tracks; 1-19 --> for a specific track")
if tracksearch.lower() == 'all':
for track_num, track_name in lover_album["tracks"].items():
print(str(track_num) + ": " + track_name)
else:
trackno = int(tracksearch)
print('Track ' + str(trackno) + ': ' + lover_album["tracks"].get(trackno, "Invalid track number"))
else:
print(lover_album.get(usrsearch.lower()))
album_search()
This code improves the options for selection by showcasing multiple options and lets the user select one specific track if they want to.
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
from IPython.display import Image
comp_data = Image(filename='../images/compare_data.png')
display(comp_data)
chiefs = {
"Name": "Kansas City Chiefs",
"HC": "Andy Reid",
"OC": "Eric Bienemy",
"DC": "Steve Spagnuolo",
"Stars": ["Travis Kelce", "Patrick Mahomes", "Chris Jones", "Orlando Brown Jr."],
"Super Bowl Wins": {
1: "IV",
2: "LIV"
}
}
print(chiefs.items())
# add and remove players from a list
print(chiefs.get("Stars"))
chiefs["Stars"].remove("Orlando Brown Jr.")
chiefs["Stars"].append("Harrison Butker")
print(chiefs["Stars"])
# add to Super Bowl Wins
print(chiefs.get("Super Bowl Wins"))
chiefs["Super Bowl Wins"].update({3: "LVII"})
# add another list to the dictionary
chiefs.update({"Retired Numbers": [3, 16, 18, 28, 33, 36, 58, 63, 78, 86]})
print(chiefs["Retired Numbers"])
# printing new dictionary
print(chiefs)