collision resolution techniques

Tag: collision resolution techniques

Separate Chaining Vs Open Addressing

Collision Resolution Techniques-

In Hashing, collision resolution techniques are classified as-

In this article, we will compare separate chaining and open addressing.

Separate Chaining Vs Open Addressing-

Separate Chaining	Open Addressing
Keys are stored inside the hash table as well as outside the hash table.	All the keys are stored only inside the hash table. No key is present outside the hash table.
The number of keys to be stored in the hash table can even exceed the size of the hash table.	The number of keys to be stored in the hash table can never exceed the size of the hash table.
Deletion is easier.	Deletion is difficult.
Extra space is required for the pointers to store the keys outside the hash table.	No extra space is required.
Cache performance is poor. This is because of linked lists which store the keys outside the hash table.	Cache performance is better. This is because here no linked lists are used.
Some buckets of the hash table are never used which leads to wastage of space.	Buckets may be used even if no key maps to those particular buckets.

Which is the Preferred Technique?

The performance of both the techniques depend on the kind of operations that are required to be performed on the keys stored in the hash table-

Separate Chaining-

Separate Chaining is advantageous when it is required to perform all the following operations on the keys stored in the hash table-

Insertion Operation
Deletion Operation
Searching Operation

NOTE-

Deletion is easier in separate chaining.
This is because deleting a key from the hash table does not affect the other keys stored in the hash table.

Open Addressing-

Open addressing is advantageous when it is required to perform only the following operations on the keys stored in the hash table-

Insertion Operation
Searching Operation

NOTE-

Deletion is difficult in open addressing.
This is because deleting a key from the hash table requires some extra efforts.
After deleting a key, certain keys have to be rearranged.

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Open Addressing | Linear Probing | Collision

Data Structures

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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Separate Chaining | Collision Resolution Techniques

Data Structures

Hashing in Data Structure-

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

We have discussed-

Hashing is a well-known searching technique.
It minimizes the number of comparisons while performing the search.
It completes the search with constant time complexity O(1).

In this article, we will discuss about Collisions in Hashing.

Collision in Hashing-

In hashing,

Hash function is used to compute the hash value for a key.
Hash value is then used as an index to store the key in the hash table.
Hash function may return the same hash value for two or more keys.

When the hash value of a key maps to an already occupied bucket of the hash table,

it is called as a Collision.

Collision Resolution Techniques-

Collision Resolution Techniques are the techniques used for resolving or handling the collision.

Collision resolution techniques are classified as-

Separate Chaining
Open Addressing

In this article, we will discuss about separate chaining.

Separate Chaining-

To handle the collision,

This technique creates a linked list to the slot for which collision occurs.
The new key is then inserted in the linked list.
These linked lists to the slots appear like chains.
That is why, this technique is called as separate chaining.

Time Complexity-

For Searching-

In worst case, all the keys might map to the same bucket of the hash table.
In such a case, all the keys will be present in a single linked list.
Sequential search will have to be performed on the linked list to perform the search.
So, time taken for searching in worst case is O(n).

For Deletion-

In worst case, the key might have to be searched first and then deleted.
In worst case, time taken for searching is O(n).
So, time taken for deletion in worst case is O(n).

Load Factor (α)-

Load factor (α) is defined as-

If Load factor (α) = constant, then time complexity of Insert, Search, Delete = Θ(1)

PRACTICE PROBLEM BASED ON SEPARATE CHAINING-

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 separate chaining technique for collision resolution.