Hash Files

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Hash Files

• CIS 402 File Management

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How It Works
rrn
Key Value
Mapping Function
N-1
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Collisions - Synonyms

• Avoid Collisions
• Reduce the Number of Collisions
• Spread out records
• Use extra memory
• Put more than one record at an address

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Hashing Function

• Converting a String to a number
• Prime Number division remainder
• Digit extraction
• Folding
• Radix Conversion
• Mid-square

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Handling Collisions

• Open Addressing
• Linear Probing
• Quadratic Probing
• Double Hashing
• Chaining
• Restructuring the hash file - buckets

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Bucket Algorithms - Insert
// Creates an entry and places the entry on the
page //If it fits and returns 0. //If it does
not fit, the function returns the addr of the
next //bucket int Insert(string key, int
address) if it doesnt fit return
Next_Bucket else add to next slot in
bucket add to NumEntries return 0 //
success indicator
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Bucket Algorithms - Find
// Find looks in the bucket for the key. If it
finds it, it assigns //the associated address to
addr and returns 0. If it doesnt find //it, it
returns the address of the next bucket. int
Find(string key, int addr) while iltNumEntries
entry not the one at i increment i if (i
gt NumEntries) // not in bucket return
NextBucket else assign address at position i
to addr return 0
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Hash File Insert
//Hash the key and call the recursive
insert //Return 1 if success and 0 if failed int
Insert(string key, int address) bucketNo
hash(key) return _Insert(key, address,
bucketNo)
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Hash File Algorithms - Insert
int _Insert(string key, int address, int
bucketNo) read the bucket at bucket
number Insert the key/address in the bucket if
( Insert succeeded) Write the bucket return
1 if (Insert gave back 1 ) //didnt fit, no
next bucket yet get a new bucket insert the
key/address into the new bucket Write the new
bucket link the new bucket to old
bucket Write the old bucket return 1 return
_Insert(key, address, bucketNo back from insert
to bucket call)
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Hash File Search
//Hash the key and call the recursive
search //Return the address associated with key
if success //and -1 if failed int Search(string
key) bucketNo hash(key) return _Search(key,
bucketNo)
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Hash File Algorithms - Search
//Searches at the given bucketNo for key // It
returns associated address if found and 1
otherwise. int _Search(string key, int
bucketNo) read the bucket at address
bucketNo find the value in the bucket if find
succeeded return 1 if find returned
1 return 1 return _Search(key, the return
from find)