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# Data Structure MCQ – Hash Tables

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

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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.

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

a) Diffusion

b) Replication

c) Collision

d) Duplication

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Answer: c

Explanation: In a hash table, if sevaral elements are computing for the same bucket then there will be a clash among elements. This condition is called Collision. The Collision is reduced by adding elements to a linked list and head address of linked list is placed in hash table.

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) Same array position for all keys

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Answer: a

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

What is the search complexity in direct addressing?

a) O(n)

b) O(logn)

c) O(nlogn)

d) O(1)

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Answer: d

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

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) A function that computes the location of the values in the array

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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.

Which of the following is not a technique to avoid a collision?

a) Make the hash function appear random

b) Use the chaining method

c) Use uniform hashing

d) Increasing hash table size

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Answer: d

Explanation: On increasing hash table size, space complexity will increase as we need to reallocate the memory size of hash table for every collision. It is not the best technique to avoid a collision. We can avoid collision by making hash function random, chaining method and uniform hashing.

What is the load factor?

a) Average array size

b) Average key size

c) Average chain length

d) Average hash table length

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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.

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) Elements has Random probability of hashing into array slots

d) Elements are hashed based on priority

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Answer: a

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

In simple uniform hashing, what is the search complexity?

a) O(n)

b) O(logn)

c) O(nlogn)

d) O(1)

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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.

In simple chaining, what data structure is appropriate?

a) Singly linked list

b) Doubly linked list

c) Circular linked list

d) Binary trees

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Answer: b

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