Hash map vs hash set in Data Structures Theory - Practice Questions

Challenge - 5 Problems

🧠 Conceptual

intermediate

Difference in data storage between hash map and hash set

Which statement correctly describes the difference between a hash map and a hash set?

A A hash map stores key-value pairs, while a hash set stores only unique keys without associated values.

B Both hash map and hash set store key-value pairs but differ in sorting order.

C A hash map stores only unique keys, while a hash set stores key-value pairs.

D A hash map stores values only, while a hash set stores keys only.

💡 Hint

Think about whether each structure associates values with keys or just stores keys.

A hash map stores pairs of keys and their associated values, allowing lookup by key. A hash set stores only unique keys without any associated values.

📋 Factual

intermediate

Typical use case for a hash set

Which of the following is a common use case for a hash set?

A Checking if an item has already been seen to avoid duplicates.

B Storing ordered pairs of data.

C Mapping user IDs to user profiles.

D Counting the frequency of words in a text.

💡 Hint

Consider when you only need to know if something exists or not.

Hash sets are used to store unique items and quickly check if an item is already present, which helps avoid duplicates.

🔍 Analysis

advanced

Performance difference in lookup operations

Given a hash map and a hash set both implemented with good hashing functions, which statement about their lookup performance is true?

A Lookup in a hash map is generally slower than in a hash set because it retrieves values, not just keys.

B Lookup time complexity is generally the same (average O(1)) for both hash map and hash set.

C Hash sets have slower lookup because they store only keys without values.

D Hash maps have O(n) lookup time while hash sets have O(1).

💡 Hint

Think about how hashing works for both structures.

Both hash maps and hash sets use hashing to achieve average constant time (O(1)) lookup for keys. The presence of values in hash maps does not affect the lookup time complexity.

❓ Comparison

advanced

Memory usage comparison between hash map and hash set

Which statement best describes the memory usage difference between a hash map and a hash set storing the same number of keys?

A A hash set uses more memory because it stores keys twice for faster lookup.

B A hash map uses less memory because it compresses values.

C Both use the same amount of memory since they store the same keys.

D A hash map uses more memory because it stores both keys and values, while a hash set stores only keys.

💡 Hint

Consider what extra data each structure holds besides keys.

Hash maps store key-value pairs, so they require memory for both keys and values. Hash sets store only keys, so they use less memory for the same number of keys.

❓ Reasoning

expert

Choosing between hash map and hash set for a problem

You need to track unique user IDs that have logged into a system and also store the last login timestamp for each user. Which data structure is the best choice?

A Use a hash set to store user IDs and a separate list to store timestamps.

B Use a hash set only, storing timestamps as keys.

C Use a hash map with user IDs as keys and timestamps as values.

D Use a list of tuples containing user IDs and timestamps.

💡 Hint

Think about how to associate each user ID with its timestamp efficiently.

A hash map allows storing each user ID as a key with its last login timestamp as the associated value, enabling quick lookup and update. A hash set cannot store associated values.