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 ___.
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?
# Only outputs unique numbers
# Curly braces indicative of dictionaries
# Why do you think Sets are in the same tech talk as Hashmaps/Hashtables?
# No 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?
# Lists
# Dictionaries
# Printing the dictionary
print(lover_album)
print(lover_album.get('tracks'))
# or
print(lover_album['tracks'])
print(lover_album.get('tracks').get(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?
# Make a set
#
# 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?
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
for i in lover_album:
itemType = type(lover_album[i])
print(str(i.upper()) + ":")
if itemType == list or itemType == set:
for n in lover_album[i]:
print(" " + str(n))
# not using "\t" because the indent looks to big for me
elif itemType == dict:
for n in lover_album[i]:
print(" " + str(n) + ": " + str(lover_album[i][n]))
else:
print(" " + str(lover_album[i]))
print("")
# 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?
# Better formatting
# loop for invalid searches
def albumSearch():
search = input("What would you like to know about the album?").lower()
if search == "":
print("search cancelled")
return
elif lover_album.get(search) == None:
print("Invalid Search")
albumSearch()
else:
if search == "tracks":
print(search.upper())
for i in lover_album["tracks"]:
print(" " + str(i) + ": " + str(lover_album["tracks"][i]))
elif search == "producer":
print(str(search.upper()) + ":")
for i in lover_album["producer"]:
print(" " + str(i))
else:
print(search.upper() + ":\n " + lover_album[search])
albumSearch()
Hacks
- Answer ALL questions in the code segments
- Create a venn diagram or other compare and contrast tool related to hashmaps.
- 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
thenownow = {
"name":"The Now Now",
"artist":"Gorillaz",
"year":2018,
"genres":False,
"tracks":[
"funk",
"synth-pop",
"psychadelic pop"
]
}
print(thenownow)
Oh no! I've accidentally forgotten all of the tracks, set genres equal to False, and set the tracks to the genres!
Also the formatting sucks!!!!!
tracks = [
"Humility",
"Tranz",
"Hollywood",
"Kansas",
"Sorcererz",
"Idaho",
"Lake Zurich",
"Magic City",
"Fire Flies",
"One Percent",
"Souk Eye"
]
# Set the genres equal to what tracks currently is, which is the actual genres
thenownow["genres"] = thenownow["tracks"]
# Set the tracks equal to the list of actual tracks
thenownow["tracks"] = tracks
print(thenownow)
Hooray! The information is correct now! But the formatting still stuck!!!!!!!
def albumSearch():
# Keep exact search to return to user in text
exactSearch = input("Enter search or press Escape to exit (Name, artist, year, genres, or tracks)")
# Lower actual search to show results even if capitalization is different
search = exactSearch.lower()
print('You searched for: "' + exactSearch + '"\n')
# Back out if Escape is pressed or nothing is entered
if search == "":
return
# Loop function if no result is found
elif thenownow.get(search) == None:
print("Results not found, please try again\n")
albumSearch()
# If results are found, state category of result before items in result
else:
print("Results found\n\n" + search.upper() + ":")
# If result is a list, iterate through and print each item
if type(thenownow[search]) == list:
for item in thenownow[search]:
print(" " + item)
# If result is not a list, print item
else:
print(" " + thenownow[search])
albumSearch()