CODEWITHSUNDEEP
javaarrayshashtable
Slaike
Slaike

Reputation: 23

Resizing array when elements are more than 1/2 of his size

I'm trying to resizing my array when N, the number of elements, is greater than m/2, m is the initial size of the array, but it doesn't work and I don't understand why. This array should work like an hashtable, so I have an hashing function before every insert, and after the resizing I want to insert again every element with a new hashing (m value changed). This is the error: 

Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
    at JumpHashing.resize(JumpHashing.java:55)
    at JumpHashing.put(JumpHashing.java:50)
    at JumpHashing.hashing(JumpHashing.java:40)
    at JumpHashing.resize(JumpHashing.java:61)
    at JumpHashing.put(JumpHashing.java:50)
    at JumpHashing.hashing(JumpHashing.java:40)
    at JumpHashing.resize(JumpHashing.java:61)
    at JumpHashing.put(JumpHashing.java:50)
    at JumpHashing.hashing(JumpHashing.java:40)
    at JumpHashing.resize(JumpHashing.java:61)
    at JumpHashing.put(JumpHashing.java:50)
    at JumpHashing.hashing(JumpHashing.java:40)
    at JumpHashing.resize(JumpHashing.java:61)
    at JumpHashing.put(JumpHashing.java:50)
    at JumpHashing.hashing(JumpHashing.java:40)
    at JumpHashing.resize(JumpHashing.java:61)
    at JumpHashing.put(JumpHashing.java:50)
    at JumpHashing.hashing(JumpHashing.java:40)
    at JumpHashing.resize(JumpHashing.java:61)
    at JumpHashing.put(JumpHashing.java:50)
    at JumpHashing.hashing(JumpHashing.java:40)
    at JumpHashing.resize(JumpHashing.java:61)
    at JumpHashing.put(JumpHashing.java:50)
    at JumpHashing.hashing(JumpHashing.java:40)
    at JumpHashing.resize(JumpHashing.java:61)
    at JumpHashing.put(JumpHashing.java:50)
    at JumpHashing.hashing(JumpHashing.java:40)
    at JumpHashing.resize(JumpHashing.java:61)
    at JumpHashing.put(JumpHashing.java:50)
    at JumpHashing.hashing(JumpHashing.java:40)
    at JumpHashing.resize(JumpHashing.java:61)
    at JumpHashing.put(JumpHashing.java:50)

The problem is clearly the resizing, without it (with less than 23 elements) it works.

Inititial size of m is 23, this is the actual code (Class "In" for reading file from algs4): 

public class JumpHashing{
    private int m;
    private int[] hashTable; 
    private static int id;
    private int N;

    public JumpHashing(){
        m = 23;
        hashTable = new int[m];
        N = 0;
    }

    public void hashing(int value) {
            int key = (value*11)%m;
            put(key, value);
    }

    public void put(int key, int value) {
        if(N <m/2) {
            hashTable[key] = value;
            N++;
        } else {
            m=m*2;
            N=0;
            resize(m);
        }
    }

    public void resize(int m) { 
        int[] temp = new int[m];
        for(int i=0; i<hashTable.length; i++) {
            temp[i] = hashTable[i];
        }
        hashTable = new int[m];
        for(int i=0; i<temp.length; i++) {
            hashing(temp[i]);
        }
    }

    public static void main(String[] args) {
        JumpHashing hashT1 = new JumpHashing();

        In file = new In("random.txt");
        while(file.hasNextLine()) {
            int value = Integer.parseInt(file.readLine());
            hashT1.hashing(value);
        }   
        for(int j=0; j<hashT1.hashTable.length; j++) {
            StdOut.println("Key: "+j+" Value: "+hashT1.hashTable[j]);
        }
    }
}

Upvotes: 1

Views: 159

Answers (1)

Always Learning
Always Learning

Reputation: 5581

You end up repeatedly calling resize until memory is used up. The problem is in this function:

    public void resize(int m) { 
        int[] temp = new int[m];  // <-- this is the new double-size of m
        for(int i=0; i<hashTable.length; i++) {
            temp[i] = hashTable[i];
        }
        hashTable = new int[m];
        for(int i=0; i<temp.length; i++) {  // <-- here we go too far
            hashing(temp[i]);
        }
    }

Your second loop goes through the full new 'm' size array, not the original m/2 size array. Half way plus one through the loop your N will be greater than m/2 again and it will call resize every time that happens.

Here's what you should have in that function:

public void resize(int m) {
    int[] oldHash = hashTable;
    hashTable = new int[m];
    for(int i=0; i<oldHash.length; i++) {
        if (oldHash[i] != 0) {     // <-- don't hash empty slots
            hashing(oldHash[i]);
        }
    }
}

This also improves performance because you loop through just once and no more than m/2 times.

Upvotes: 1

Related Questions