Question

I am implementing a Hashtable class in C++.
The collision resolution method that I am using is linear probing with lazy deletion.
I have seen implementations of this but had a question with regards to the insert method.
Each cell of the hashtable has a state (active, deleted, empty). For some reason in the implementation I have seen when inserting a new element, they hash the key and then probe the table until an EMPTY cell is found (or until a cell already containing the same key is found).

Example Code:

int findPos(const string &key){
     int currentPos=hash(key);
     while(data[currentPos].state!=EMPTY && data[currentPos].key!=key){
         currentPos++;
         if (currentPos>=data.size())
            currentPos-=data.size()
         }
      return currentPos;
}

bool insert(const string &key){
     int currentPos=findPos(key);
     if (isActive(currentPos))
          return false; //already exists
     data[currentPos]=hashEntry(key,ACTIVE);
     if (++currentSize>data.size()/2)
          rehash();
     return true;   //element inserted
}

My question is: Is there a reason not to stop and insert into a cell that has been tagged as deleted? In other words, in the findPos method, why not change the while statement to while(data[currentPos].state==ACTIVE && data[currentPos].key!=key) so that the loop ends either when we find the cell with the key or the first deleted/empty cell. Then in the insert, test the state of the cell. If active the entry already exists, so return false; else insert the element.

Answer 1

The key could have been inserted further on, and later one of the intervening cells could have been marked as deleted. Then you would insert a duplicate instance of the same key.