Python Dictionaries

Question 1

What is a python dictionary?

Answer 1

A collection which is unordered, changeable and indexed

Question 2

What is the time complexity to access a key in a dictionary?

Answer 2

O(1) time complexity

Question 3

How are dictionaries recorded in memory?

Answer 3

A dictionary is recorded in memory as a hash table.

Question 4

What is a hash table?

Answer 4

A hash table is a way of doing key-value lookups. You store the values In an array, and then use a hash function to find the index of the array cell that corresponds to your key-value pair.

Question 5

What are collisions in a hash table?

Answer 5

Different keys have the same hash value.

Question 6

Ways to add key value pairs to a dictionary and update

Answer 6

• myDict[‘age’] = 17

side note: o(1) time complexity

Question 7

Set a new key value if it doesnt exist in Dictionary

Answer 7

myDict.setdefault(key, value)

if the key value already exists it does not set the value.

Question 8

Different ways to pull values from a dictionary

Answer 8

• myDict[key]: this will pull the value. if the key does not exist then it will return an error.
• myDict.get(key, value): This will pull the value and return None if it does not exist. if value is set then this will be the default value.

Question 9

simple way to traverse over keys in dict

Answer 9

for key in myDict:
print(key, dict[key])

O(n) time complexity

Question 10

How to search for a value in a dictionary

Answer 10

def searchDict(dict, value):
for key in dict:
if dict[key] == value:
return key, value
return None

O(n)
O(1) for space complexity.

Question 11

What are the 4 ways to delete a value or values from a Dictionary

Answer 11

1. myDict.pop(key):
   removes and returns the keys value
2. myDict.popitem():
   removes a random key
3. myDict.clear():
   removes all the keys in the dictionary
4. del myDict[key]:
   removes a specific key from the dictionary

Question 12

method used to create a copy of a dictionary

Answer 12

the .copy() creates a copy of the dictionaries. The original one will not change when using this

Question 13

How do you create a dictionary using a list of keys

Answer 13

{}.fromkeys(sequence[], value])
- value is optional
- values will be None if the value is not set

Question 14

how to get a dictionary as a list of tuples in python. Good for iteration.

Answer 14

.items() method

result example: [(key1, value1), (key2, value2)]

Question 15

How to get all the keys in a dictionary

Answer 15

dict.keys() will return a list of keys

Question 16

getting a list of just the values in a dictionary

Answer 16

dict.values()

Question 17

How to combine a dictionary to another dictionary

Answer 17

dict1.update(dict2)

• updates the dict from another element
• adds elements to the dictionary if the key does not already exist
• if the key exists then the value is updated to the new value
• it can also take a tuple of key value pairs

Question 18

How to determine if a key exists in the dictionary

Answer 18

key in dict

• the in operator in a dictionary is super efficient because it is a hash table
• O(1) time complexity

Question 19

determine if a value is in a dictionary

Answer 19

value in dict.values()

Question 20

returns only true when every value in the dictionary is true

Answer 20

all(dict)

• if dict is empty then this will return true.

Question 21

if any element of a collection is true in a dict it will return true.

Answer 21

any(dict)

• if it is an empty collection it will return false

Question 22

count the number of keys in a dictionary
count the number of values in a dictionary

Answer 22

: len(dict1) returns number of keys
: len(dict.values()) returns number of keys

Question 23

how to order a dictionary keys

Answer 23

sorted(myDict , reverse=False, key=None)
- reverse will reverse the order of the sort
- key will change how the sort function is sorting the keys

Question 24

What are the key differences between a list and a dictionary

Answer 24

• Dictionary, unordered. list: ordered
• Dictionary: Collection of key value pairs List: Collection of elements
• Dictionary cannot have duplicate while lists can