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Hashing

Last Updated: Nov 17, 2023
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Contents

Hashing Implementation Details

C

C has nothing native for hashmap We need to write one ourselves.

C++ :

C++ has unordered_map which is implemented via hashing. We will discuss treemaps in C++ as well later, which in practice seem to behave faster in C++.

Declaration
    unordered_map<int, int> A; // declares an empty map. O(1)
Adding an key, value pair :
    A.insert({key, value}); // O(1) time on average
Find the value for key = K:
    if (A.find(K) == A.end()) return null; //  means that K does not exist in A. 
    else return A[K];    // O(1) average case. Rare worst case O(n). 
Size ( number of elements ) of the vector :
    A.size()  // O(1)
Erase from the map :
    if (A.find(K) != A.end()) A.erase(A.find(K));
     OR A.erase(K);
JAVA :

Now we come to one of the most popular data structures in Java, HashMap.

Declaration :
    HashMap<Integer, Integer> A = new HashMap<Integer, Integer>(); // declares an empty map.
Adding an key, value pair :
    A.put(key, value); // O(1) time on average
Find the value for key = K:
    A.get(K) // null if the key K is not present
    A.containsKey(K) tells if the key K is present. 
    // Both operations O(1) average time. O(n) rare worst case
Size ( number of elements ) of the vector :
    A.size()  // O(1)
Erase from the map :
    A.remove(K);
PYTHON

Python has dictionaries which serve the same purpose.

Declaration :
    A = {}
Adding an key, value pair :
    A[key] = value // O(1) average time. O(n) worst case
Find the value for key = K:
    A[K] // O(1) average, O(n) worst
Size ( number of elements ) of the vector :
    len(A) // O(1) 
Erase from the map :
        del A[K] // O(1) average, O(n) worst

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