c++dictionaryhashdouble-hashing

Double hashing function returning wrong value


I am creating a double hashing map, but the remove function does not work after inserting. I follow the same format for increasing the index, but it just does not hit the correct index.

class RHHM {
    unsigned int hash2( int key ) {

        return key % (M-1) + 1;

    }

    //Private variables

    hashNode ** map;        //Backing array
    unsigned int M;   //Capacity of array

    //If index that key hashes to is empty, insert. Else, replace value at hashed index.
    int insert( int key, char value ) {

        int f = hash( key );
        int f2 = hash2 ( key );
        int p = 0;
        int h = f + (p * f2) % M;

        while( map[h] != NULL ) {

            if( p == M )
                return -1 * p;

            if( map[h]->key == key ) {
                map[h]->value = value;
                return p;
            }
            else {
                ++p;
                h = f + (p * f2) % M;
            }
        }

        map[h] = new hashNode( key, value );
        return p;
    }

int remove( int key, char &value) {

        int f = hash( key );
        int f2 = hash2 ( key );
        int p = 0;                         //Keeps track of how many indexes have been checked
        int h = f + (p * f2) % M;

        while( map[h] != NULL ) {

            if( p  == M )              //If item is not found in table, return false
                return -1 * p;

            if( key == map[h]->key )        //If key is found, break out of loop
                break;
            else {
                ++p;
                h = f + (p * f2) % M;  //Wrap around array if necessary
            }

        }

        if( map[h] == NULL )                //If end of cluster reached, return false
            return -1 * p;

        value = map[h]->value;              //Stores the value of the item to be deleted
        delete map[h];                      //Delete the item the user specified
        map[h] = NULL;
        ++p;
        h = f + (p * f2) % M;
        for( ; map[h] != NULL; h = f + (p * f2) % M) {     //While still in the cluster, remove and     reinsert all items
            int tempKey = map[h]->key;
            char tempValue = map[h]->value;
            delete map[h];
            map[h] = NULL;
            insert(tempKey, tempValue);
            ++p;
        }
        return p;

    }

}

And here is my main:

RHHM bh(10);
bh.insert(0, 'A');
bh.insert(10, 'B');
bh.insert(20, 'C');
bh.print(std::cout);

Output:

<[ 0:A, - , 10:B, 20:C, - , - , - , - , - , - ]>

As you can see, the first index hashes to 0. Since the 10 key collides with 0, double hash ( 10 ) should hash to 1, but its hashing to 2. Why is it returning the wrong value?


Solution

  • This is because the hash2 function returns a value 2 for the key 10.For hash2(10), hash2 can be shown as.

    return 10 % (10-1) + 1
    

    this in turn by operator precedence evaluated to the value 2 as.

    return (10 %(10-1))+1
    

    and in the insert function when there is a collision you add the hash2 value to the hash value to get the new index which evaluates as.

    h= f + ( P * f2 ) % M
    h= 0 + (1 * 2 ) % 10 \\ this evaluates to 2.
    

    thats why you get the new index as 2.

    Edit: The code has quiet a few issues with operator precedence. First h = f + (p * f2) % M; //Wrap around array if necessary

    This doesn't wrap around the array. since % has more precedence than +. f has value in the range[0-M-1] and the (p *f2) % M has values in the range [0-M-1]. So the above expression may evaluates to a value in the range [0-2*M-2]. This holds true for other such expressions in the code. This is a reason some of the key may be missing in hash table.