Data Structures Multiple choice Questions and Answers on Hash Tables

1. What is a hash table?

a) A structure that maps values to keys
b) A structure that maps keys to values
c) A structure used for storage
d) A structure used to implement stack and queue

Answer: b

Explanation: A hash table is used to implement associative arrays which has a key-value pair, so the has table maps keys to values.

2. If several elements are competing for the same bucket in the hash table, what is it called?

a) Diffusion
b) Replication
c) Collision
d) None of the mentioned

Answer: c

Explanation: By definition

3. What is direct addressing?

a) Distinct array position for every possible key
b) Fewer array positions than keys
c) Fewer keys than array positions
d) None of the mentioned

Answer: a

Explanation: Direct addressing is possible only when we can afford to allocate an array that has one position for every possible key.

4. What is the search complexity in direct addressing?

a) O(n)
b) O(logn)
c) O(nlogn)
d) O(1)

Answer: d

Explanation: Since every key has a unique array position, searching takes a constant time

5. What is a hash function?

a) A function has allocated memory to keys
b) A function that computes the location of the key in the array
c) A function that creates an array
d) None of the mentioned

Answer: b

Explanation: In a hash table, there are fewer array positions than the keys, so the position of the key in the array has to be computed, this is done using the hash function.

6. What can be the techniques to avoid collision?

a) Make the hash function appear random
b) Use the chaining method
c) Use uniform hashing
d) All of the mentioned

Answer: d

Explanation: Making the hash function random is not really a good choice, although it is considered one of the techniques to avoid collisions along with chaining and simple uniform hashing.

7. What is the load factor?

a) Average array size
b) Average key size
c) Average chain length
d) None of the mentioned

Answer: c

Explanation: In simple chaining, load factor is the average number of elements stored in a chain, and is given by the ratio of number of elements stored to the number of slots in the array.

8. What is simple uniform hashing?

a) Every element has equal probability of hashing into any of the slots
b) A weighted probabilistic method is used to hash elements into the slots
c) All of the mentioned
d) None of the mentioned

Answer: a

Explanation: In simple uniform hashing, any given element is equally likely to hash into any of the slots available in the array.

9. In simple uniform hashing, what is the search complexity?

a) O(n)
b) O(logn)
c) O(nlogn)
d) O(1)

Answer: d

Explanation: There are two cases, once when the search is successful and when it is unsuccessful, but in both the cases, the complexity is O(1+alpha) where 1 is to compute the hash function and alpha is the load factor.

10. In simple chaining, what data structure is appropriate?

a) Singly linked list
b) Doubly linked list
c) Circular linked list
d) Binary trees

Answer: b

Explanation: Deletion becomes easier with doubly linked list, hence it is appropriate.