Hashmap in C++

Basic Usages

unordered_map<int, int> hashmap;
// 2. insert a new (key, value) pair
hashmap.insert(make_pair(2, 3));
// 3. insert a new (key, value) pair or update the value of existed key
hashmap[1] = 1;
hashmap[1] = 2;
// 4. get the value of a specific key
hashmap[1]
// 5. delete a key
hashmap.erase(2);
// 6. check if a key is in the hash map
if (hashmap.count(2) <= 0);
// 7. get the size of the hash map
hashmap.size() 
// 8. iterate the hash map
for (auto it = hashmap.begin(); it != hashmap.end(); ++it) {
    cout << it->first << "," << it->second << endl;
}
// 9. clear the hash map
hashmap.clear();
// 10. check if the hash map is empty
if (hashmap.empty())

Iterating over list of keys:

for (Type key : keys) {
        if (hashmap.count(key) > 0) {
            if (hashmap[key] satisfies the requirement) {
                return needed_information;
            }
        }
        // Value can be any information you needed (e.g. index)
        hashmap[key] = value;
    }

(to be continued…)

Iterating over map

for (auto& it : myMap) {
        vec.push_back(it);
    }

Sorting map based on value

#include <iostream>
#include <map>
#include <vector>
#include <algorithm>

// Comparator function to sort pairs by second value
bool sortByVal(const std::pair<int, int>& a, const std::pair<int, int>& b) {
    return (a.second < b.second);
}

int main() {
    // Define a map
    std::map<int, int> myMap = {{1, 40}, {2, 30}, {3, 60}, {4, 20}};

    // Copy elements to a vector of pairs
    std::vector<std::pair<int, int>> vec;
    for (auto& it : myMap) {
        vec.push_back(it);
    }

    // Sort the vector by value
    std::sort(vec.begin(), vec.end(), sortByVal);

    // Print the sorted vector
    for (auto& it : vec) {
        std::cout << it.first << ": " << it.second << std::endl;
    }

    return 0;
}

Tips

• When counting frequency, don’t need to check if the hashmap[x] exists.

// don't do this:
if (hashmap.count(x) > 0) hashmap[x]++;
else hashmap[x] = 1;
// do this:
hashmap[x]++;

// same for adding up the frequency:
for (...){
   sum += hashmap[x];
}