Help with hash map

[nimble pier]

Hi, can someone help me with this hash map implementation, I tried this line on the TODO but it shows an error.

    // DEFAULT_CAPACITY has to be a prime number
    private static final int DEFAULT_CAPACITY = 23;
    private static final float DEFAULT_LOAD_FACTOR = 0.5f;
    private static final int CAPACITY_INCREASE_FACTOR = 2;

    private Node<KeyType, DataType>[] map;
    private int size = 0;
    private int capacity;
    private final float loadFactor; // Compression factor

    public HashMap() { this(DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR); }

    public HashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR);
    }

    /** TODO
     * Define capacity, loadFactor and map
     */
    public HashMap(int initialCapacity, float loadFactor) {
        this.loadFactor = Math.abs(loadFactor);
        return new HashMap<Integer, Integer>(initialCapacity, loadFactor); ---> ERROR
    }```

[mystic citrus]

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[deep panther]

you don't return from constructors

[nimble pier]

I also tried this but it does not work, I'm note sure how to define the loadFactor, map and capacity HashMap<Integer, Integer> = new HashMap<Integer, Integer>(initialCapacity, loadFactor);

[deep panther]

you have to use them yourself directly

you have to make the implementation, after all

[nimble pier]

how? by using defining this.map?

[deep panther]

yeah

[nimble pier]

not sure how to define the map, what do I put after the equal? bc when I try this.map; it doesn't work

[deep panther]

well, what should its capacity be?

this is the thing that's holding your data

[nimble pier]

ok thanks

[nimble pier]

        int keyHash = key.hashCode() % capacity;
        return Math.abs(keyHash);
    }
    private boolean needRehash() {
        return size > capacity * loadFactor;
    }
    public int size() {
        return size;
    }
    public int capacity(){
        return capacity;
    }
    public boolean isEmpty() {
        return size == 0;
    } 

/** TODO Worst Case : O(m + n)
     * m = Capacity of the hashmap
     * n = number of elements in the hashmap
     * Increases capacity to the next prime number after capacity * CAPACITY_INCREASE_FACTOR and
     * reassigns all contained values
     */
    private void rehash() {
}``` Do you have any idea to implement the function rehash?

[deep panther]

copy it into a new array

[nimble pier]

  nextPrime( 2 * map.length);``` I tried this, not sure what to do after

[deep panther]

you need to make a new map then copy the old map's contents into there

[nimble pier]

can you help me with code? im not sure how you copy into an old map, here is what i have HashMap<KeyType, DataType>[] oldMap = map; nextPrime( 2 * map.length); for( int i = 0; i < oldMap.length; i++ ) if (oldMap[i] != null) insert(oldMap[i].element);

[deep panther]

copy straight into the new array

[nimble pier]

how do i do that?

[deep panther]

you need to first make a new map, then just do map[i] = oldMap[i]

you aren't using nextPrime there

[nimble pier]

  
  map[i] = oldMap[i];```?

In the instructions of the rehash exercise it said this : /** TODO Worst Case : O(m + n) * m = Capacity of the hashmap * n = number of elements in the hashmap * Increases capacity to the next prime number after capacity * CAPACITY_INCREASE_FACTOR and * reassigns all contained values */

[deep panther]

no, you need to make it a new, larger array

that's what increasing the capacity is

[nimble pier]

it's what I tried to do here nextPrime( 2 * map.length); I doubled the capacity

[deep panther]

does nextPrime set map?

[nimble pier]

no, not sure how to set the new map with a larger array

        HashMap<KeyType, DataType>[] oldMap = map;
        for( int i = 0; i < oldMap.length; i++ )
            map[i] = oldMap[i];
    }``` I have this... what do I modify or erase?

[deep panther]

you need to make a new map, like you did in the constructor

[nimble pier]

ok so I have this ```HashMap<Integer, Integer> newMap = new HashMap<Integer, Integer>;

[deep panther]

no

you don't do new HashMap ever

don't make your structure recursive

your internal map is an array, make that

...have you not done it in the constructor?

[nimble pier]

        this.loadFactor = Math.abs(loadFactor);
        this.capacity = initialCapacity;
    }``` I only had this before

[deep panther]

you need to initialize the map

__initial__Capacity

[nimble pier]

I add this.map to it?

[deep panther]

yes, initialize it to an array of some size

[deep panther]

i wonder what that initial size should be 🤔

[nimble pier]

this.map = new map[initialCapacity]; ??