Difference between revisions of "Hashtable"
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* One basic approach. Calculate a hash for each key and store the hash value in an array index of the same value. Store the key, value pair in the same index positions in other arrays. | * One basic approach. Calculate a hash for each key and store the hash value in an array index of the same value. Store the key, value pair in the same index positions in other arrays. | ||
* However, this approach suffers from wasted space in the array - for those array indexes for which corresponding hash values are never generated. | * However, this approach suffers from wasted space in the array - for those array indexes for which corresponding hash values are never generated. | ||
| + | * Another problem is collisions - a single hash value can represent many keys. | ||
| + | * To solve wasted space use : hashcode modulo arraysize. | ||
| + | * To solve collisions use linkedlists to store key->value pairs for each hash value. | ||
Revision as of 01:36, 22 October 2012
- Hashing is designed to solve the problem of needing to efficiently find or store an item in a collection. It is a technique to narrow down the search space.
- Hashing means using some function or algorithm to map object data to some representative integer value.
- One basic approach. Calculate a hash for each key and store the hash value in an array index of the same value. Store the key, value pair in the same index positions in other arrays.
- However, this approach suffers from wasted space in the array - for those array indexes for which corresponding hash values are never generated.
- Another problem is collisions - a single hash value can represent many keys.
- To solve wasted space use : hashcode modulo arraysize.
- To solve collisions use linkedlists to store key->value pairs for each hash value.
