Collision Resolution Techniques-

 

Before you go through this article, make sure that you have gone through the previous article on Collision Resolution Techniques.

 

We have discussed-

• Hashing is a well-known searching technique.
• Collision occurs when hash value of the new key maps to an occupied bucket of the hash table.
• Collision resolution techniques are classified as-

 

 

In this article, we will discuss about Open Addressing.

 

Open Addressing-

 

In open addressing,

• Unlike separate chaining, all the keys are stored inside the hash table.
• No key is stored outside the hash table.

 

Techniques used for open addressing are-

• Linear Probing
• Quadratic Probing
• Double Hashing

 

Operations in Open Addressing-

 

Let us discuss how operations are performed in open addressing-

 

Insert Operation-

 

• Hash function is used to compute the hash value for a key to be inserted.
• Hash value is then used as an index to store the key in the hash table.

 

In case of collision,

• Probing is performed until an empty bucket is found.
• Once an empty bucket is found, the key is inserted.
• Probing is performed in accordance with the technique used for open addressing.

 

Search Operation-

 

To search any particular key,

• Its hash value is obtained using the hash function used.
• Using the hash value, that bucket of the hash table is checked.
• If the required key is found, the key is searched.
• Otherwise, the subsequent buckets are checked until the required key or an empty bucket is found.
• The empty bucket indicates that the key is not present in the hash table.

 

Delete Operation-

 

• The key is first searched and then deleted.
• After deleting the key, that particular bucket is marked as “deleted”.

 

NOTE-

 

• During insertion, the buckets marked as “deleted” are treated like any other empty bucket.
• During searching, the search is not terminated on encountering the bucket marked as “deleted”.
• The search terminates only after the required key or an empty bucket is found.

 

Open Addressing Techniques-

 

Techniques used for open addressing are-

 

1. Linear Probing-

 

In linear probing,

• When collision occurs, we linearly probe for the next bucket.
• We keep probing until an empty bucket is found.

 

Advantage-

 

• It is easy to compute.

 

Disadvantage-

 

• The main problem with linear probing is clustering.
• Many consecutive elements form groups.
• Then, it takes time to search an element or to find an empty bucket.

 

Time Complexity-

 

Worst time to search an element in linear probing is O (table size).

 

This is because-

• Even if there is only one element present and all other elements are deleted.
• Then, “deleted” markers present in the hash table makes search the entire table.

 

2. Quadratic Probing-

 

In quadratic probing,

• When collision occurs, we probe for i²‘th bucket in i^th iteration.
• We keep probing until an empty bucket is found.

 

3. Double Hashing-

 

In double hashing,

• We use another hash function hash2(x) and look for i * hash2(x) bucket in i^th iteration.
• It requires more computation time as two hash functions need to be computed.

 

Comparison of Open Addressing Techniques-

 

	Linear Probing	Quadratic Probing	Double Hashing
Primary Clustering	Yes	No	No
Secondary Clustering	Yes	Yes	No
Number of Probe Sequence (m = size of table)	m	m	m2
Cache performance	Best	Lies between the two	Poor

 

Conclusions-

 

• Linear Probing has the best cache performance but suffers from clustering.
• Quadratic probing lies between the two in terms of cache performance and clustering.
• Double caching has poor cache performance but no clustering.

 

Load Factor (α)-

 

Load factor (α) is defined as-

 

 

In open addressing, the value of load factor always lie between 0 and 1.

 

This is because-

• In open addressing, all the keys are stored inside the hash table.
• So, size of the table is always greater or at least equal to the number of keys stored in the table.

 

PRACTICE PROBLEM BASED ON OPEN ADDRESSING-

 

Problem-

 

Using the hash function ‘key mod 7’, insert the following sequence of keys in the hash table-

50, 700, 76, 85, 92, 73 and 101

 

Use linear probing technique for collision resolution.

 

Solution-

 

The given sequence of keys will be inserted in the hash table as-

 

Step-01:

 

• Draw an empty hash table.
• For the given hash function, the possible range of hash values is [0, 6].
• So, draw an empty hash table consisting of 7 buckets as-

 

 

Step-02:

 

• Insert the given keys in the hash table one by one.
• The first key to be inserted in the hash table = 50.
• Bucket of the hash table to which key 50 maps = 50 mod 7 = 1.
• So, key 50 will be inserted in bucket-1 of the hash table as-

 

 

Step-03:

 

• The next key to be inserted in the hash table = 700.
• Bucket of the hash table to which key 700 maps = 700 mod 7 = 0.
• So, key 700 will be inserted in bucket-0 of the hash table as-

 

 

Step-04:

 

• The next key to be inserted in the hash table = 76.
• Bucket of the hash table to which key 76 maps = 76 mod 7 = 6.
• So, key 76 will be inserted in bucket-6 of the hash table as-

 

 

Step-05:

 

• The next key to be inserted in the hash table = 85.
• Bucket of the hash table to which key 85 maps = 85 mod 7 = 1.
• Since bucket-1 is already occupied, so collision occurs.
• To handle the collision, linear probing technique keeps probing linearly until an empty bucket is found.
• The first empty bucket is bucket-2.
• So, key 85 will be inserted in bucket-2 of the hash table as-

 

 

Step-06:

 

• The next key to be inserted in the hash table = 92.
• Bucket of the hash table to which key 92 maps = 92 mod 7 = 1.
• Since bucket-1 is already occupied, so collision occurs.
• To handle the collision, linear probing technique keeps probing linearly until an empty bucket is found.
• The first empty bucket is bucket-3.
• So, key 92 will be inserted in bucket-3 of the hash table as-

 

 

Step-07:

 

• The next key to be inserted in the hash table = 73.
• Bucket of the hash table to which key 73 maps = 73 mod 7 = 3.
• Since bucket-3 is already occupied, so collision occurs.
• To handle the collision, linear probing technique keeps probing linearly until an empty bucket is found.
• The first empty bucket is bucket-4.
• So, key 73 will be inserted in bucket-4 of the hash table as-

 

 

Step-08:

 

• The next key to be inserted in the hash table = 101.
• Bucket of the hash table to which key 101 maps = 101 mod 7 = 3.
• Since bucket-3 is already occupied, so collision occurs.
• To handle the collision, linear probing technique keeps probing linearly until an empty bucket is found.
• The first empty bucket is bucket-5.
• So, key 101 will be inserted in bucket-5 of the hash table as-

 

 

To gain better understanding about Open Addressing,

Watch this Video Lecture

 

Next Article- Separate Chaining Vs Open Addressing

 

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