1. Hash tables in C

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

3. 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

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

5. 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

6. 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

7. 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

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

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

10. 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

11. 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
-+=-+=-+=-+=-+=-+=

12. C example
C-ing is believing

15. for words in our dictionary
Hash Function
for words in our dictionary

16. 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 (^ *.htm$, *.html$)
Using a random number generator to spread out data

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

18. ? ?
Questions

19. Acknowledgements