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 0(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?
I noticed that the output is printing only one of each value in the list my_set. It removes the duplicates in a list.
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 because both result in outputing only unique values in order to be hashable and they remove all the 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"
}
}
# Printing the dictionary
print(lover_album)
What data structures do you see?
I see that a list is used to set up the genres and that a dictionary is used for the names of the songs.
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']
# Printing the dictionary
print(lover_album)
What can you change to make sure there are no duplicate producers?
I can change the code to make sure there are no duplicate producers by putting the word "set" in front of the list of names. Look at the code segment below.
lover_album["producer"] = set(['Taylor Swift', 'Jack Antonoff', 'Joel Little', 'Taylor Swift', 'Louis Bell', 'Frank Dukes'])
# Printing the dictionary with no duplicate producers
print(lover_album)
lover_album["tracks"].update({19: "All Of The Girls You Loved Before"})
# Printing the dictionary
print(lover_album)
How would you add an additional genre to the dictionary like electropop?
I could add an additional genre to the dictionary like electropop by using .append. Look at the code segment 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
#
Write your own code to print tracks in readable format:
Look at the code segment below.
print("title: " + str(lover_album["title"]))
print("artist: " + str(lover_album["artist"]))
print("year: " + str(lover_album["year"]))
print("genre: " + str(lover_album["genre"]))
print("tracks:")
for k,v in lover_album["tracks"].items():
print(f"{str(k)}: {str(v)}")
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?
An addition that would make the program better to use would be to make the program run continuously until the user told it to stop by typing in the word "exit". This would allow the user to search up all the categories of the album without needing to restart the program each time. Another addition that could be useful is printing out the categories for the users to see. This would make it easier for the user to know what they can search for. I also could make it so that the answer is easier to distinguish from the category names and other answers.
Look at the code segment below for these improvements.
for k,v in lover_album.items():
print(str(k))
print("exit(end the program)")
def search():
endcode = 0
while endcode == 0:
search = input("What would you like to know about the album?")
if search.lower() == "exit":
endcode = 1
print("exited program")
else:
if lover_album.get(search.lower()) == None:
print("Invalid Search")
else:
print()
print(search.lower() + ":")
print(lover_album.get(search.lower()))
search()
commiting_album = {
"title": "Commiting",
"artist": "Lucas Moore",
"year": 2023,
"genre": ["coding","binary","python"],
"producers": ['Lucas Moore', 'Lance Moore'],
"tracks": {
1: "code, code, code",
2: "merge conflict",
3: "1s and 0s",
4: "if only else",
5: "My Favorite type of error",
6: "pushing and pulling"
}
}
for k,v in commiting_album.items():
print(str(k))
print("exit(end the program)")
def search():
endcode = 0
while endcode == 0:
search = input("What would you like to know about the album?")
if search.lower() == "exit":
endcode = 1
print("exited program")
else:
if commiting_album.get(search.lower()) == None:
endcode = 1
print("exited program")
else:
print()
print(search.lower() + ":")
print(commiting_album.get(search.lower()))
search()