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# Hashing - PowerPoint PPT Presentation

Hashing. What is hashing. Simply, generating a numeric key using an algorithm (hash function) Definition: A function that maps keys to integers, usually to get an even distribution on a smaller set of values. The very simplest hash function is to use the modulus operator %

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## PowerPoint Slideshow about 'Hashing' - merton

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Presentation Transcript

### Hashing

• Simply, generating a numeric key using an algorithm (hash function)

• Definition: A function that maps keys to integers, usually to get an even distribution on a smaller set of values.

• The very simplest hash function is to use the modulus operator %

• Input range % key range

• Example. We want to store 7 digit telephone numbers so that they can be quickly retrieved.

• Number of expected entries = 100

• Range of telephone numbers = 0 – 9999999

Simple hashing algorithm

hash = inputNumber % 100

What’s the effect?

• File management – working out where to store records

• Comparing complex values

• Cryptography – creating digital signatures – eg: md5

• Where the hash value returned for two keys is the same.

• What to do?

• Open hashing

• Closed hashing

• Deleting

• The 2/3rds rule

• “An algorithm produced by Professor Daniel J. Bernstein and shown first to the world on the usenet newsgroup comp.lang.c. It is one of the most efficient hash functions ever published. “

def DJBHash(key):

hash = 5381

for i in range(len(key)):

hash = ((hash << 5) + hash) + ord(key[i])

returnhash