How to Use the STL Unordered Map Container in C++

1. Use the std::unordered_map Element to Declare an Unordered Map Container in C++
2. Use the contains Member Function to Check if the Given Element Exists in a Map in C++
3. Use the erase Member Function to Remove the Specific Element From a Map in C++
4. Use the insert Member Function to Add New Elements to a Map in C++

This article explains several methods of how to use the STL unordered_map container in C++.

Use the std::unordered_map Element to Declare an Unordered Map Container in C++

The std::unordered_map element implements an associative container of key-value pairs where each key is unique. In contrast to the std::map, the std::unordered_map container does not store the elements in sorted order. Thus, the container is mostly utilized for element lookup, and position does not matter. Moreover, the position of elements over the lifetime of the object is not guaranteed to be fixed. So the programmer should treat the order as undefined.

The following example demonstrates the list initialization of the unordered_map container and then outputs the contents of both maps. Note that the descending order of the m1 elements does not imply they are sorted.

#include <iostream>
#include <map>
#include <unordered_map>

using std::cout;
using std::endl;
using std::string;

template <typename Map>
void printMap(Map& m) {
  for (auto& p : m) cout << p.first << " : " << p.second << ";" << endl;
  cout << endl;
}

int main() {
  std::unordered_map<int, string> m1 = {
      {1, "apple"},
      {2, "banana"},
      {3, "grape"},
      {4, "orange"},
  };

  std::map<int, string> m2 = {
      {1, "apple"},
      {2, "banana"},
      {3, "grape"},
      {4, "orange"},
  };

  printMap(m1);
  printMap(m2);

  return EXIT_SUCCESS;
}

Output:

4 : orange;
3 : grape;
2 : banana;
1 : apple;

1 : apple;
2 : banana;
3 : grape;
4 : orange;

Use the contains Member Function to Check if the Given Element Exists in a Map in C++

The contains member function has been part of the std::unordered_map container since the C++20 update. You can use the function to check if the given element exists in the map. This command accepts the key-value as the only argument and returns true if the key is found. The function has the constant average running time, with the worst case being the linear depending on the size of the container itself.

#include <iostream>
#include <map>
#include <unordered_map>

using std::cout;
using std::endl;
using std::string;

template <typename Map>
void printMap(Map& m) {
  for (auto& p : m) cout << p.first << " : " << p.second << ";" << endl;
  cout << endl;
}

int main() {
  std::unordered_map<int, string> m1 = {
      {1, "apple"},
      {2, "banana"},
      {3, "grape"},
      {4, "orange"},
  };

  for (int x : {1, 2, 3, 4, 5}) {
    if (m1.contains(x)) {
      cout << x << ": Found\n";
    } else {
      cout << x << ": Not found\n";
    }
  }

  return EXIT_SUCCESS;
}

Output:

1: Found
2: Found
3: Found
4: Found
5: Not found

Use the erase Member Function to Remove the Specific Element From a Map in C++

You can utilize the erase member function to remove the given key-value pair from the map. The function has three overloads, the first of which takes the iterator to the map element, which needs to be removed from the container. The second function overload takes two iterators to specify the range, which will be removed from the map. Be aware that the given range must be a valid one within the calling container object. The third overload can take the key value of the element that should be removed.

In the following example, we remove each element that has an even number as the key value.

#include <iostream>
#include <map>
#include <unordered_map>

using std::cout;
using std::endl;
using std::string;

template <typename Map>
void printMap(Map& m) {
  for (auto& p : m) cout << p.first << " : " << p.second << ";" << endl;
  cout << endl;
}

int main() {
  std::unordered_map<int, string> m1 = {
      {1, "apple"},
      {2, "banana"},
      {3, "grape"},
      {4, "orange"},
  };

  for (auto it = m1.begin(); it != m1.end();) {
    if (it->first % 2 == 0)
      it = m1.erase(it);
    else
      ++it;
  }
  printMap(m1);

  return EXIT_SUCCESS;
}

Output:

3 : grape;
1 : apple;

Use the insert Member Function to Add New Elements to a Map in C++

Another core member function of the unordered_map container is the insert function, which you can use to add new elements to the map. The function has multiple overloads, but we utilize the first one that just takes a key-value pair. Note that we also use the std::make_pair function to construct a new pair with the given argument values.

#include <iostream>
#include <map>
#include <unordered_map>
#include <vector>

using std::cout;
using std::endl;
using std::string;
using std::vector;

template <typename Map>
void printMap(Map& m) {
  for (auto& p : m) cout << p.first << " : " << p.second << ";" << endl;
  cout << endl;
}

int main() {
  std::unordered_map<int, string> m1 = {
      {1, "apple"},
      {2, "banana"},
      {3, "grape"},
      {4, "orange"},
  };

  vector<string> vec{"papaya", "olive", "melon"};

  auto count = 0;
  for (const auto& item : vec) {
    m1.insert(std::make_pair(count, item));
  }
  printMap(m1);

  return EXIT_SUCCESS;
}

Output:

0 : papaya;
4 : orange;
3 : grape;
2 : banana;
1 : apple;