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
- working on hacks
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 of the function is {1,2,3}.
# There are no duplicate values
# probably b/c we can't have duplciates in hashes.
# Why do you think Sets are in the same tech talk as Hashmaps/Hashtables?
# Sets are a good example of how collisions in hashing works and why we shouldn't have duplicate
# Sets are a tool we can use for hashing if we don't want to use dictionaries.
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?
# Lists
# all of this is compiled into a 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?
# Just remove the duplicate manually lol
# or use sets (that worked)
settest = set(lover_album["producer"])
# Printing the dictionary
print(lover_album["producer"])
print(settest)
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"] =["Pop", "Synth-pop", "electropop"] # will work, but maybe there is another way
#
lover_album["genre"].append("new genre") #this method uses list methods but we now have duplicates if we run multiple times
# 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)) dont need this....
pass
# Write your own code to print tracks in readable format
#
#
exe = 0
trackS = lover_album["tracks"]
num = 0
for k,v in trackS.items(): # iterate using a for loop for key and value
exe = exe + 1
num = num + 1
print("song", exe, ":", trackS[num])
def search():
x = 1
while x == 1: # use while loop so we can continue searching
search = input("What would you like to know about the album? (type quit to exit)")
if search == "quit":
x = 0
elif 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?
# use while loop so that we can continue searching
#
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_album = { # initialize the album need to run each time
"title": "I know how to use dictionaries",
"artist": "artist",
"year": 1984,
"genre": [],
"tracks": {
1: "U.N Owen Was her?",
}
}
def search_personal():
x = 1
while x == 1: # use while loop so we can continue searching
search = input("What would you like to know about the album? (type quit to leave)")
if search == "quit":
print("you have quit lol")
x = 0
elif my_album.get(search.lower()) == None:
print("Invalid Search")
else:
print(my_album.get(search.lower()))
#search_personal()
my_tracks = my_album["tracks"]
def addtracks():
x = 1
while x == 1: # use while loop so we can continue searching
newTrack = input("add a new track (type tracks), change artist (type artist), show album (type search), add genre (type genre) or type quit")
if newTrack == "quit":
x = 0
elif newTrack == "artist":
artists = input("type in artist name")
my_album["artist"] = artists
elif newTrack == "tracks":
newTrackz = input("type in new track name")
newTrack2 = input("add position of new track:")
my_tracks[newTrack2] = newTrackz
elif newTrack == "search":
search_personal()
elif newTrack == "genre":
genres = input("add a genre to the program")
my_album["genre"].append(genres)
newGenre = []
[newGenre.append(x) for x in my_album["genre"] if x not in newGenre] # use list comprehension to remove duplicates
my_album["genre"] = newGenre # add genre
print(my_album["genre"])
else:
print("unrecognized command!")
addtracks()
#search_personal()
print(my_album["tracks"])