Hash table mod#

This effectively means that each location in the hash table is not limited to store one value. As new collisions occur, the linked list grows to accommodate those collisions forming a chain. In chaining, each location in a hash table stores a pointer to a linked list that contains all the key values that were hashed to that location. Hash collision resolved by separate chaining There are two popular collision resolution techniques: Open Addressing The method used to solve the problem of collisions is called the collision resolution technique. Two records cannot be stored in the same location of a hash table normally. CollisionsĬollisions occur when the hash function maps two different keys to the same location. The algorithm works well because most or all digits of the key-value contribute to the resulting hash.

The middle r digits of the result are extracted.This can be represented as: h(k) = fractional_part Mid-Square Method The result of Step 3 is multiplied by the size of the hash table ( m).The fractional part of kA is extracted.A constant is chosen which is between 0 and 1, say A.The Multiplication method has the following steps:

Hash table mod#

This function could be represented as: h(k) = k mod M Multiplication Method Generally, M is chosen to be a prime number because a prime number increases the likelihood that the keys are mapped with uniformity in the output range of values. Any integer, for example, x is divided by a number M and the remainder obtained is used as the hash. This is the most simple method of hashing.

Search Engines: As the number of pages to be crawled is huge, a hash function can be used to determine if the page is unique or it had already been crawled before, without comparing the contents of the whole webpage.

This can then be used in combination with other variables to uniquely identify a person.

Identification Databases: A hash function can make a unique signature from never changing data like our Date of Birth.

The cost of executing a hash function must be small so that using the hashing technique becomes preferable over other traditional approaches. DeterministicĪ hash function must always generate the same hash value for a given input value. This means that the probability of generating every hash value in the output range should roughly be the same. Properties of a Good Hash Function UniformityĪ good hash function must map the keys as evenly as possible. It produces a unique set of integers within some suitable range in order to reduce the number of collisions. The main aim of a hash function is that elements should be uniformly distributed. Hash FunctionsĪ hash function is a mathematical formula which, when applied to a key, produces a value which can be used as an index for the key in the hash table. Hash tables are particularly efficient when the maximum number of entries can be predicted in advance. The access time of an element is on average O(1), therefore lookup could be performed very fast. The main advantage of hash tables over other data structures is speed.

The process of mapping the keys to appropriate locations (or indices) in a hash table is called hashing. The value stored in a hash table can be searched in O(1) time, by using the same hash function which generates an address from the key.