Maps / Dictionaries

Question 1

What is a map in data structures?

Answer 1

A collection that stores key-value pairs.

Question 2

What is a dictionary in Python?

Answer 2

A built-in data structure that functions as a map.

Question 3

What is the primary purpose of a map?

Answer 3

To associate a unique key with a value.

Question 4

Can map keys be duplicated?

Answer 4

No, all keys in a map must be unique.

Question 5

Can map values be duplicated?

Answer 5

Yes, values can be repeated.

Question 6

What is the time complexity of inserting into a hash map?

Answer 6

O(1) average case.

Question 7

What is the time complexity of searching in a hash map?

Answer 7

O(1) average case.

Question 8

What is the time complexity of deleting from a hash map?

Answer 8

O(1) average case.

Question 9

What is a hash map?

Answer 9

A map implemented using a hash table.

Question 10

What is a tree map?

Answer 10

A map implemented using a balanced binary search tree.

Question 11

What is the time complexity of operations in a tree map?

Answer 11

O(log n).

Question 12

What is the role of a hash function in a map?

Answer 12

To convert a key into an index for storage.

Question 13

What happens when two keys hash to the same index?

Answer 13

A collision occurs, which must be resolved.

Question 14

What are the common ways to resolve hash collisions?

Answer 14

Chaining and open addressing.

Question 15

How do you access a value by key in a map?

Answer 15

Use the key as an index (e.g., map[key]).

Question 16

What happens if you access a nonexistent key in a map?

Answer 16

It raises an error or returns null/None, depending on the language.

Question 17

How do you safely access a value in Python dictionaries?

Answer 17

Use the get() method.

Question 18

How do you insert or update a value in a map?

Answer 18

Assign a value to the key (e.g., map[key] = value).

Question 19

What is the space complexity of a map?

Answer 19

O(n), where n is the number of key-value pairs.

Question 20

What is an ordered map?

Answer 20

A map that maintains insertion order (e.g., Python 3.7+, LinkedHashMap in Java).

Question 21

What is a default dictionary?

Answer 21

A dictionary that returns a default value when a key is missing.

Question 22

What is a nested dictionary?

Answer 22

A dictionary where values are also dictionaries.

Question 23

How do you iterate through a map?

Answer 23

Use a for-loop over keys, values, or key-value pairs.

Question 24

How do you delete a key-value pair from a map?

Answer 24

Use the del statement or remove method.

Question 25

What is the keys() method in a dictionary?

Answer 25

Returns a view of all keys.

Question 26

What is the values() method in a dictionary?

Answer 26

Returns a view of all values.

Question 27

What is the items() method in a dictionary?

Answer 27

Returns a view of all key-value pairs.

Question 28

What happens if you insert a key that already exists?

Answer 28

Its value is updated.

Question 29

Can keys in a map be mutable?

Answer 29

No, keys must be hashable and immutable.

Question 30

What is the difference between a list and a dictionary?

Answer 30

Lists use integer indices; dictionaries use keys.

Question 31

What is a common use case for maps?

Answer 31

Counting frequency of elements.

Question 32

What is a multi-map?

Answer 32

A map that allows multiple values for a single key.

Question 33

How do you simulate a multi-map in Python?

Answer 33

Use a dictionary with lists or sets as values.

Question 34

What is the difference between map and set?

Answer 34

Maps store key-value pairs; sets store only keys.

Question 35

Can a map have null or None as a key?

Answer 35

Yes, depending on the language.

Question 36

What is the load factor in a hash map?

Answer 36

The ratio of the number of elements to the number of buckets.

Question 37

What happens when the load factor is too high?

Answer 37

The map may resize or performance may degrade.

Question 38

What is rehashing in a map?

Answer 38

Resizing the map and re-inserting all elements.

Question 39

How is a map implemented in C++?

Answer 39

Using std::map (tree) and std::unordered_map (hash).

Question 40

How is a map implemented in Java?

Answer 40

Using HashMap, TreeMap, or LinkedHashMap.

Question 41

How do you check if a key exists in a map?

Answer 41

Use the in keyword or containsKey method.

Question 42

How do you get the number of elements in a map?

Answer 42

Use the len() function or size() method.

Question 43

What is the time complexity of checking key existence in a hash map?

Answer 43

O(1) on average.

Question 44

What is a symbol table?

Answer 44

Another name for a map, often used in compilers.

Question 45

What is a sparse map?

Answer 45

A map used to store keys with widely spaced or rare values.

Question 46

What is a frequency map?

Answer 46

A map where keys are elements and values are their counts.

Question 47

What is the key difference between HashMap and TreeMap in Java?

Answer 47

HashMap is unordered and faster; TreeMap is sorted.

Question 48

Can a map be used to sort data?

Answer 48

Yes, if the map maintains order, like a TreeMap.

Question 49

Can values in a map be complex objects?

Answer 49

Yes, values can be any data type including lists or other maps.