Week 10 Workshop 7 Linear Data Structures – Part 3 Application - Hashing

what is hashing?

Hashing involves using an array for the efficient storage and retrieval of information

what is the idea effeicency for hashing for storage and retrival?

O(1)

how are collisions resolved?

Collision is resolved by finding a free location. Next location (1) is still free - use this location. Key 10 maps to position 2

what defines a " good hash function"

A good hash function must: Quick and easy to compute Minimise collisions Achieve an even distribution of keys Aim: Efficiency O(1) for storage and retrieval of data

what is truncation?

Ignores parts of the key and uses remaining parts as the hash value. Fast but can fail to distribute keys uniformly.

what is folding?

Split key into several parts and then combine the parts to obtain hash value. Often gives better spread than truncation.

what is Modula Arithmetic?

Divide key by table size - use remainder as the hash value For good spread - table size must be prime.

what is Collision Resolution with Chaining

Uses singly linked lists to resolve the collisions.

what is Collision Resolution with Open Addressing using Linear Rehash or Linear Probing

A linear search of the table from the hashed position is used to find an empty slot.

what is Collision Resolution with Open Addressing using Quadratic Rehash or Quadratic Probing

The following slots are tried to find an empty slot: h + i2 (mod table size) for i = 1,2,3... ie: slots tried are: h, h+1, h+4, h+9, h+16 ...... (mod table size) Works well if the table size is prime.

what is Collision Resolution with Open Addressing using Add the hash Rehash

The hash value is added to the hashed value repetitively to find an empty slot. ie: slots tried are: h, 2h, 3h, 4h, ... (mod table size) Works well if the table size is prime.

what is Collision Resolution with Open Addressing using random Rehash

a pseudo‑random number generator is used to obtain the increments Uses a seed to initiate the sequence of random numbers. Excellent in avoiding collisions but slower