Key: In a map, a key refers to the element, for which we want to store information. We can access the value of a key using the key itself like mp[key].

Value: Value refers to the information stored corresponding to a key.

In our example, we want to store the frequency(i.e. How many times an element appears in the array) of each element of the given array. So, here each element of the array will be the key of the map and frequencies will be the value corresponding to the keys. In terms of code, it will look like the following:

Difference between map and unordered_map in C++:

Difference between array hashing and hashing using the map:

Let’s understand the difference considering the array: [1, 2, 3, 1, 3, 2, 12]. Here, the maximum element of the array is 12. So, if we apply array hashing, we need an array of size 13. But if we apply the map data structure, it will be the following:

Note: If we want to fetch the value of a key that does not exist in the map, the map will always return 0 in C++ and null in Java.

Code:

    



#include <bits/stdc++.h>
using namespace std;

int main() {

    int n;
    cin >> n;
    int arr[n];
    for (int i = 0; i < n; i++) {
        cin >> arr[i];
    }

    //precompute:
    map<int, int> mp;
    for (int i = 0; i < n; i++) {
        mp[arr[i]]++;
    }

    // iterate over the map:
    /* for(auto it : mp){
            cout << it.first << "->" << it.second <&