Question

So I am creating a Hash Table that uses an Array of Linked Lists of Arrays. Let me take a second to explain why this is.

So I have previously implemented Hash Tables by creating an Array, and each element of the array is a Linked List. This way I could quickly look up a LL of 450,000 elements by searching for the hash value first in the array, and searching the elements of this LL. I should add that this is a project for school and I cannot just use the Hash Tables that comes with java.

Now I want to do something similar… but I massive have a LL of Arrays that I need to search. Here each element of the LL is line of a text file, which represented by a 4 element array, where each of the 4 elements is a different string that was tab delimited in the input file. I need to be able to quickly access the 2nd, 3rd, and 4th string that was located in each line, and that is now an element of this array.

So What I want is to be able to create an Array of LL of Arrays… first I will find the sum of the ascii values of the second element of an array. Then I will hash the entire array using this value into by Hash Table. Then when I later need to find this element, I will go to the corresponding element of the array, where I have a list of arrays. I will the search for the 2nd value of each array in the list. If i find the one I want, then I return that array, and use the 3rd and 4th element of this array.

As I said, I have this working fine for an Array of LL, but adding the extra dimension of Arrays inside has thrown me off completely. I think it is mostly just figuring out syntax, since I have successfully initialized a Array of LL of Arrays (public static LinkedList[] RdHashLL) so it appears that Java is okay with this in principal. However, I have no idea how to put elements into the Hash Table, and how to read them out.

Below is my code for a ARRAY OF LINKED LISTS that works FINE. I just need help getting it to work for an ARRAY OF LL OF ARRAYS!

public class TableOfHash{

public static LinkedList<String>[] HashLL;

//HASH FUNCTION - Finds sum of ascii values for string
public static int charSum(String s){
    int hashVal = 0;
    int size = 1019; //Prime Number around size of 8 char of 'z', (8 chars is amoung largest consistantly in dictionary)

    for(int i = 0; i < s.length(); i++){
        hashVal += s.charAt(i);
    }   
    return hashVal % size;
}

//CREATE EMPTY HASH TABLE - Creates an array of LL
public static void makeHash(){
    HashLL = new LinkedList[1019];
    for(int i=0; i<HashLL.length; i++){
        HashLL[i] = new LinkedList<String>();
    }
}

//HASH VALUES INTO TABLE!
public static void dictionary2Hash(LinkedList<String> Dict){
    for(String s : Dict){
        HashLL[charSum(s)].add(s);
        //Finds sum of char vales of dictionary element i,
        //and then word at i to the HashLL at point defined
        //by the char sum.
    }   
    //Print out part of Hash Table (for testing! for SCIENCE!)
    //System.out.println("HASH TABLE::");
    //printHashTab();
}

//SEARCH HashTable for input word, return true if found
public boolean isWord(String s){

    if(HashLL[charSum(s)].contains(s)){
        wordsfound++;
        return true;
    }
    return false;
}

}

I have made some attempts to change this, but for things like if(HashLL[charSum(s)].contains(s)) which searches the LL at the element returned by charsum(s)… I have no idea how to get it to work when it is a LL of Arrays and not of Strings. I have tired HashLL[charSum(s)].[1].contains(s)), and HashLL[charSum(s)][1].contains(s)), and various other things.

The fact that a Google search for “Array of Linked Lists of Arrays” (with quotes) turns up empty has not helped.

Last bit. I realize there might be another data structure that would do what I want, but unless you believe that a Array of LL of Arrays is a totally hopeless cause, I’d like to get it to work as is.

Answer 1

if you have

LinkedList<String[]>[] hashLL;

you can read a specific String like this (one of many ways)

String str = hashLL[outerArrayIndex].get(listIndex)[innerArrayIndex];

To write into the fields, this is possible (assuming everything is initialized correctly).

String[] arr = hashLL[outerArrayIndex].get(listIndex);
arr[index] = "value";