Question

I was wondering if i could get some help to prevent a duplicate entry into my hashtable:

bool hashmap::put(const stock& s, int& usedIndex, int& hashIndex, int& symbolHash)
{
    hashIndex = this->hashStr( s.m_symbol ); // Get remainder, Insert at that index.
    symbolHash = (int&)s.m_symbol;
    usedIndex = hashIndex;

    while ( hashTable[hashIndex].m_symbol != NULL ) // collision found
    {
        ++usedIndex %= maxSize; // if necessary wrap index around
        if ( hashTable[usedIndex].m_symbol == NULL )
        {
            hashTable[usedIndex] = s;
            return true;
        }
    }
    hashTable[hashIndex] = s; // insert if no collision 
    return true;

}

The paramater hashIndex uses a java algorithm to generate an appropriate index to insert into the table. The usedIndex paramater is where I insert into the table when a collision is found; symbolHash just gets the address of the name passed in. stock is a class object.

My question is just, how do i prevent a duplicate entry?

Answer 1

Just look at the objects you’re storing in that index. If you have to move the index because of a collision check those also. If they match what you’re trying to store, throw an exception or something.

I’m guessing at your code so…

bool hashmap::put(const stock& s, int& usedIndex, int& hashIndex, int& symbolHash)
{
    hashIndex = this->hashStr( s.m_symbol ); // Get remainder, Insert at that index.
    symbolHash = (int&)s.m_symbol;
    usedIndex = hashIndex;

    if(hastTable[usedIndex].m_symbol == s.m_symbol){
        return false;
    }
    while ( hashTable[hashIndex].m_symbol != NULL ) // collision found
    {
            ++usedIndex %= maxSize; // if necessary wrap index around
            if ( hashTable[usedIndex].m_symbol == NULL )
            {
                    hashTable[usedIndex] = s;
                    return true;
            }else if(hastTable[usedIndex].m_symbol == s.m_symbol){
                 return false;
            }
    }
    hashTable[hashIndex] = s; // insert if no collision 
    return true;

}