Hash tables in C

Outline Data storage problems Hash Table Design Hash function terminology “Good” hash functions Example in C Acknowledgements

Data Storage Limitations with data structures we have learned so far: Suppose we have two entities e.g. word and definition {we are probably making a dictionary} Limitations with data structures we have learned so far: Arrays: Have to form two ‘parallel’ arrays-> search one containing the words-> retrieve same key from second array Linked list: Go through list elements one by one ->find cell containing word -> retrieve definition

Problems: Ar[..] Arrays : Have to follow two structures Double operations Linked Lists: Too slow, dangling pointers

Solution: Combine fast retrieval power of arrays and linear characteristic of linked lists. Symtab[NHASH] NULL Name 1 Value 1 NULL Name 2 Value 2 NULL Name 3 Value 3

Design Analyze data Storing: Come up with “good” hash function Hash key to find “bucket” Store Retrieving: Hash key Lookup table Get data Other possible facilitators: Deletion Replacement

Terminology Using our dictionary program application example: Key => word Hash function (convert key to hash) Hash => encoded word Hash table => ‘table’ of hashed values Bucket => ‘cell’ in a hash table Value => word definitions Chaining => technique of linking definitions

Overcoming Same Hash Problems Two main ways: Overwriting Chaining Picked according to programming challenge being tackled

Chaining! Symtab[NHASH] NULL Name 1 Value 1 Name 3 Value 3

Good hash functions Good function: Perfect function: One that randomly spreads out all keys into buckets perfectly randomly Symtab[NHASH] NULL -+=-+=-+=-+=-+=-+= Symtab[NHASH] -+=-+=-+=-+=-+=-+= Good function: Very close to perfect if not perfect

Bad hash functions Multiple chains Doesn’t consider data e.g. common multiples Different hashes for the same key Simply: bad algorithm Symtab[NHASH] NULL -+=-+=-+=-+=-+=-+=

C example C-ing is believing

for words in our dictionary Hash Function for words in our dictionary

Bad hash function example: Bad modulation Not accounting for multiples Not accounting for constants in names e.g. java using constants in names .class In urls (^http://*, *.htm$, *.html$) Using a random number generator to spread out data

Uses of hash tables in real world IP and DNS tables Memory allocation Almost any two-part data More parts with custom data types

? ? Questions

Acknowledgements