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Hash tables

Posted on 2000-03-27
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Last Modified: 2010-04-02
Hi. I'm trying to implement the following hash functions.
1. h(x)= 1 + (x/k) * M
2. h(x)= 1 + x mod M
3. h(x)=1 + @x mod M

Where M is the table size. They should change keys into integers in the range from 1 to M. I'm using this to generate the numbers and send them to the hash function:

#include<iostream.h>
#include<stdlib.h>
main()
{
 int tablesize=32;
 int a[200];
 for(int i=0;i<200;i++)
 {
      i=rand()%5001;
        h(x)=hash(tablesize,i)
   }
 return 0;
}

1. How do I make the hash function?
2. Can hashing be used with text? Can you provide some examples?
Thanx
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Question by:q_low
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Expert Comment

by:RONSLOW
ID: 2662927
something like this would probably do...

int hash1(int M, int x) { return 1 + (x / k) * M; }
int hash2(int M, int x) { return 1 + x % m; }
int hash3(int M, int x) { return 1 + ???x % m; }
// whatever @x means
// don't know what you're talking about there
// you'll have to code that yourself

static int method = 1;

int hash(int M, int x) {
  switch (method) {
  case 1: return hash1(M,x);
  case 2: return hash1(M,x);
  case 3: return hash1(M,x);
  }
  return 0; // not a valid hash method
}

You can then set method to be 1, 2 or 3 as required.

And I think you want you code to say
  int x;
  for(int i=0;i<200;i++) {
    x=rand()%5001;
    a[x]=hash(tablesize,x);
  }
And then print out (or do something with) the hash values rather than just end the program.

To use with text, you usually loop for each each character, hash it and combine with the overall running hash value (eg add, or shift and add or whatever).  There are lots of examples around of hashing.

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Author Comment

by:q_low
ID: 2663948
in the first equation k will be the key.
what does the key means if i have a table of size 32?
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Author Comment

by:q_low
ID: 2667660
....
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Accepted Solution

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RONSLOW earned 50 total points
ID: 2672700
the way you have shown the code, x is the key.  k being the key doesn't make sense .. because then what is x and where does k come from.

if your table size is 32, then .. well, your table size is 32.  Don't think it means anything in particular.

I think I answred you question satisfactorily.

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Author Comment

by:q_low
ID: 2692064
thanx
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