![Hash Table Overview
S Adding Jane
S int index = GetIndex(Jane.Name);
S _array[index] = Jane;
S What does GetIndex() do? It hashes the string](https://image.slidesharecdn.com/hashtables-130111134936-phpapp01/85/Hash-tables-8-320.jpg)
Hash tables
- 1. Hash Table Code Review 1/20/11 S
- 2. Outline S Hash Table Overview S Hashing Overview S Add Items S Remove Items S Search For Items S Enumerate Items
- 3. Hash Table Overview S Associative Array S Storage of Key / Value Pairs S Like an array, but the index can be any comparable type S Each Key is Unique, though Keys can point to the same value S The Key Type is mapped to an Index
- 4. Hashed??? S Hashing derives a fixed size result from an input S every hash returns same size and type S Stable S The same input generates the same output ALWAYS S Uniform S The hash value use should be uniformly distributed through available space (though impossible to have perfect uniformity) S Efficient S The cost of generating a hash must be balanced with application needs S Secure S The cost of finding data that produces a given hash is prohibitive
- 5. Hashing A String S Naïve implementation S Summing the ASCII value for each character F O O 102 111 111 324 S Pros S Stable S Efficient S Cons S Not Uniform S AdditiveHash(“foo”) = AdditiveHash(“oof ”) S Not Secure
- 6. Hashing A String S Somewhat better S “Folds” bytes of every four characters into an integer (32bit) Lore m ip sum dolo r 170199844 1885937773 54404403 1869377380 114 -707031079 -162986676 1706390704 1706390818 S Pros S Stable, Efficient and better uniformity S Cons S Not secure (can be treated essentially as additive)
- 7. Hashing Functions S There are lots of good hashing algorithms, you don’t have to write your own. S Pick the right hash for the job at hand (all these are available in the .net framework) Name Stable Uniform Efficient Secure Additive ✔ ✔ Folding ✔ ✔ ✔ CRC32 ✔ ✔ ✔ MD5 ✔ ✔ SHA-2 ✔ ✔ ✔
- 8. Hash Table Overview S Adding Jane S int index = GetIndex(Jane.Name); S _array[index] = Jane; S What does GetIndex() do? It hashes the string
- 10. Handling Collisions S Two distinct items have the same hash value S Items are assigned to the same index in the hash table S Two common strategies S Open Addressing S Moving to next index in the table S Chaining S Storing items into a linked list S Frequency of Collisions S # of slots in the array S # of filled slots in the array
- 11. Finding Items S Items are found by key S Person p = HashTable.Find(“Jane”) S Open Addressing S Get the index of the key S If the value != null S If keys match, return value S If keys don’t match, check next index S Chaining S Get index of the key S Find index in the list
- 12. Removing Items S Items are removed by key S HashTable.Remove(“Jane”) S Open Addressing S Get index of the key S If value != null S If keys match, remove S If keys don’t match, check next index S Chaining S Get index of the key S Remove item from the linked list
- 13. Enumerating Keys & Values S Open Addressing S Foreach(item in array) {if(item!=null) return item;} S Chaining S Foreach(list in array) {if(list !=null) {foreach(item in list){return item;}} }